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Page 1: Mechanical Properties of Dental Materials

1

MECHANICAL PROPERTIES OF DENTAL MATERIALS

Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONTENTS

bull Introductionbull Forcebull Stress

bull Tensilebull Compressivebull Shearbull Flexural

bull Strainbull Elastic amp Plastic Deformationbull Stress ndash Strain Curve

3

bull Mechanical Properties based onElastic Deformationbull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength Propertiesbull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength

shear strength compressivestrength amp flexural strength

4

bull Mechanical Properties based onPlastic Deformationbull Flexural Strengthbull Impact Strengthbull Toughnessbull Fracture toughnessbull Brittlenessbull Ductilitybull Malleabilitybull Hardness

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Hardness testsbull Brinellbull Rockwellbull Vickersbull Knoops

bull Stress Concentration Effects

bull Methods To Minimize Stress Concentration

bull Conclusion

bull References

5

Mechanical Properties of Dental Materials - Dr Nithin Mathew

INTRODUCTION

bull In the oral environment restorative materials and dental appliances are exposed tochemical thermal and mechanical challenges

bull These challenges can cause deformation of materials

bull The mechanical properties of a material define how materials respond tomechanical challenges

bull Mechanical properties are defined by the laws of mechanics

ie It is the physical science that deals with energy forces and their effects on thebodies

6

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull So it is necessary to understand the principles involved in a variety of mechanicalproperties to optimise the clinical service of a material

bull Mechanical properties are measured responses both elastic (reversible on forceremoval) and plastic (irreversible on force removal) of materials under an appliedforce or distribution of forces

bull They are expressed most often in units of stress and strain

7

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull They can represent measurements of

bull Elastic deformation (reversible)bull Proportional limitbull Resiliencebull Modulus of elasticity

bull Plastic deformation (irreversible)bull Percentage of elongation

bull Combination of bothbull Toughnessbull Yield strength

8

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FORCE

bull In physics a force is any influence that causes an object to undergo a certainchange either concerning its movement direction or geometrical construction

bull A force is defined by 3 characteristicsbull Point of applicationbull Magnitudebull Direction of application

bull The SI unit of force is Newton (N)

9

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Occlusal Forces

bull Max occlusal forces 200 ndash 3500N

bull Occlusal forces between adult teeth are highest in the posterior region closest to themandibular hinge axis and decrease from the molar to the incisors

bull Forces on first and second molars vary from 400 to 800N

bull Average on bicuspids cuspids and incisors is about 300 200 and 150N

bull Increase in force from 235 ndash 494N in growing children with an average yearlyincrease of about 22N

10

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS amp STRAIN

Stressbull The force per unit area acting on millions of atoms or molecules in a given

plane of a materialbull Force acting per unit area

bull Unit of measurement is Megapascal (Mpa)

bull Stress is the internal resistance of a material to an external load applied onthat material

Stress (σ)= 119865119865 (119873119873)119860119860 (1198981198982)

11

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Type Of Stress Produced By Examples

Residual Stress

Stress caused within the material during the manufacturing process

During welding

StructuralStress

Stresses produced in the structure during function Weights they support provide the loadings

In abutments of fixed partial denture

Pressure Stress

Induced in vessels containing pressurized materials In dentures during processing under pressure and heat

Flow Stress Force of liquid striking against the wall acts as the load

Molten metal alloy striking the walls of the mould during casting

Thermal Stress

Material is subjected to internal stress due to different temperatures causing varying expansions in the material

Materials that undergo thermal stress such as inlay wax soldering and welding alloys

Fatigue Stress Stress caused due to cyclic rotation of a material Rotary instruments undergo rotational or cyclic fatigue

12

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull By means of the direction of force stresses can be classified as

bull Tensile stressbull Compressive stressbull Shear stressbull Flexural stress

13

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Tensile Stress

bull Tensile stress occurs when 2 sets of forces are directed away from each other in thesame straight line

bull Also when one end is constrained and the other end is subjected to a force awayfrom the constraint

14

bull It is caused by a load that tends to stretch or elongate a body

Mechanical Properties of Dental Materials - Dr Nithin Mathew 15

bull In fixed prosthodontics a sticky candy (Jujube) can be used toremove crowns by means of a tensile force when the patienttries to open the mouth after the candy has mechanicallybonded to opposing teeth or gums

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Compressive Stress

bull Compressive stress occurs when 2 sets of forces are directed towards each other inthe same straight line

bull Also when one end is constrained and the other end is subjected to a force towardsthe constraint

bull It is caused by a load that tends to compress or shorten a body

16

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Shear Stress

bull Shear stress occurs when 2 sets of forces are directed parallel to each other but notalong the same straight line

bull A shear stress tends to resist the sliding of one portion of a body over another

bull Shear stress can also be produced by a twisting ortorsional action on a material

17

Mechanical Properties of Dental Materials - Dr Nithin Mathew 18

bull Eg If a force is applied along the surface of tooth enamel by a sharp-edgedinstrument parallel to the interface between the enamel and the orthodonticbracket the bracket may debond by shear stress failure of the resin luting agent

Mechanical Properties of Dental Materials - Dr Nithin Mathew 19

In the oral cavity shear failure is unlikely to occur due to

1 Many of the brittle materials in restored tooth surfaces generally have roughcurved surfaces

2 The presence of chamfers bevels or changes in curvature of a bonded toothsurface

3 To produce shear failure the applied force must be located immediately adjacentto the interface The farther away from the interface the load is applied more likelythat it is a tensile failure

Mechanical Properties of Dental Materials - Dr Nithin Mathew 20

5 Since the tensile strength of brittle materials is usually well below their shearstrength values tensile failure is more likely to occur

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Flexural Stress (bending)

bull Force per unit area of a material that is subjected to flexural loading (bending)

bull A shear stress tends to resist the sliding of one portion of a body over another

bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component

21

Mechanical Properties of Dental Materials - Dr Nithin Mathew 22

bull Flexural stresses produced in a three-unit fixeddental prosthesis

bull Flexural stresses produced in a two-unit cantileverbridge

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull StrengthStrength of a material is defined as the average level of stress

at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize

23

Mechanical Properties of Dental Materials - Dr Nithin Mathew 24

bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component

bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage

bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture

25

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration

bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact

26

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRAIN

bull Defined as the change in length per unit original length

bull Strain of a material is reported as percentage()

bull Strain may be either elastic plastic or a combination of both elastic and plastic

bull Elastic strain is reversible ie it disappears when force is removed

bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed

Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)

27

Eg

bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2

bull If the wire is 01m long and if it stretches 0001m under the load

bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001

Strain (ε)= (120549120549119862119862)(1198621198620)

= 0001 11989811989801119898119898

= 00001 = 001

Stress (σ)= 200 119873119873

0000002 1198981198982 = 2002

x 106 = 100 MPa

28

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC AND PLASTIC DEFORMATION

29

Elastic Shear Deformation Plastic Shear Deformation

PL = 1020

YS = 1536

UTS = 1625

0

200

400

600

800

1000

1200

1400

1600

1800

0 05 1 15

STRESS ndash STRAIN CURVESt

ress

(Mpa

)

Strain ()

YS (02) = 1536 Mpa

UTS = 1625 Mpa

E = 10200053 = 192 GPa

bull PL Proportional Limit

bull YS Yield Strength

bull UTS Ultimate Tensile Strength

bull E Elastic Modulus

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION

bull Mechanical properties that are measures of the elastic strain of dental materialsincludes

bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio

31

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YOUNGrsquoS MODULUS

bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph

bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain

bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity

32

E = 10200053 = 192 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic modulus of a tensile test specimen can be calculated as

bull By definition

Stress (σ)=119865119865119860119860

Strain (ε)=1205491205491198621198621198621198620

33

F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus

E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862

= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620

34

E = 10200053 = 192 GPa

Steep Line Higher modulus and more rigidity

Flat Line Lower modulus and less rigidity

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Principle of Elastic Recovery

bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain

bull After the force is removed the metal springs back to anamount equal to the total elastic strain

bull A final clearance of 25microm is available for the cement

35

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Impression Material

bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth

bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing

36

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Hookersquos Law

bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress

bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality

bull Hookes law can be expressed by the formula

bull The value of the constant depends on the material and the type of stress

37

119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930

= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930

STRESS ndash STRAIN CURVE Enamel amp Dentin

PL = 235CS = 262

PL = 176

CS = 234

0

50

100

150

200

250

300

Com

pres

sive

Sre

ss( M

pa)

Strain

Enamel Dentin

bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures

bull Enamel - more stiffer andbrittle than dentin

bull But dentin more flexible andtougher

EEnamel = 117 GPaEDentin = 336 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

POISSONrsquoS RATIO

bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions

bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section

bull Under compressive loading there is an increase in thecross-section

39

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio

bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains

bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03

bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio

40

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 2: Mechanical Properties of Dental Materials

MECHANICAL PROPERTIES OF DENTAL MATERIALS

Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONTENTS

bull Introductionbull Forcebull Stress

bull Tensilebull Compressivebull Shearbull Flexural

bull Strainbull Elastic amp Plastic Deformationbull Stress ndash Strain Curve

3

bull Mechanical Properties based onElastic Deformationbull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength Propertiesbull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength

shear strength compressivestrength amp flexural strength

4

bull Mechanical Properties based onPlastic Deformationbull Flexural Strengthbull Impact Strengthbull Toughnessbull Fracture toughnessbull Brittlenessbull Ductilitybull Malleabilitybull Hardness

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Hardness testsbull Brinellbull Rockwellbull Vickersbull Knoops

bull Stress Concentration Effects

bull Methods To Minimize Stress Concentration

bull Conclusion

bull References

5

Mechanical Properties of Dental Materials - Dr Nithin Mathew

INTRODUCTION

bull In the oral environment restorative materials and dental appliances are exposed tochemical thermal and mechanical challenges

bull These challenges can cause deformation of materials

bull The mechanical properties of a material define how materials respond tomechanical challenges

bull Mechanical properties are defined by the laws of mechanics

ie It is the physical science that deals with energy forces and their effects on thebodies

6

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull So it is necessary to understand the principles involved in a variety of mechanicalproperties to optimise the clinical service of a material

bull Mechanical properties are measured responses both elastic (reversible on forceremoval) and plastic (irreversible on force removal) of materials under an appliedforce or distribution of forces

bull They are expressed most often in units of stress and strain

7

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull They can represent measurements of

bull Elastic deformation (reversible)bull Proportional limitbull Resiliencebull Modulus of elasticity

bull Plastic deformation (irreversible)bull Percentage of elongation

bull Combination of bothbull Toughnessbull Yield strength

8

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FORCE

bull In physics a force is any influence that causes an object to undergo a certainchange either concerning its movement direction or geometrical construction

bull A force is defined by 3 characteristicsbull Point of applicationbull Magnitudebull Direction of application

bull The SI unit of force is Newton (N)

9

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Occlusal Forces

bull Max occlusal forces 200 ndash 3500N

bull Occlusal forces between adult teeth are highest in the posterior region closest to themandibular hinge axis and decrease from the molar to the incisors

bull Forces on first and second molars vary from 400 to 800N

bull Average on bicuspids cuspids and incisors is about 300 200 and 150N

bull Increase in force from 235 ndash 494N in growing children with an average yearlyincrease of about 22N

10

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS amp STRAIN

Stressbull The force per unit area acting on millions of atoms or molecules in a given

plane of a materialbull Force acting per unit area

bull Unit of measurement is Megapascal (Mpa)

bull Stress is the internal resistance of a material to an external load applied onthat material

Stress (σ)= 119865119865 (119873119873)119860119860 (1198981198982)

11

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Type Of Stress Produced By Examples

Residual Stress

Stress caused within the material during the manufacturing process

During welding

StructuralStress

Stresses produced in the structure during function Weights they support provide the loadings

In abutments of fixed partial denture

Pressure Stress

Induced in vessels containing pressurized materials In dentures during processing under pressure and heat

Flow Stress Force of liquid striking against the wall acts as the load

Molten metal alloy striking the walls of the mould during casting

Thermal Stress

Material is subjected to internal stress due to different temperatures causing varying expansions in the material

Materials that undergo thermal stress such as inlay wax soldering and welding alloys

Fatigue Stress Stress caused due to cyclic rotation of a material Rotary instruments undergo rotational or cyclic fatigue

12

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull By means of the direction of force stresses can be classified as

bull Tensile stressbull Compressive stressbull Shear stressbull Flexural stress

13

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Tensile Stress

bull Tensile stress occurs when 2 sets of forces are directed away from each other in thesame straight line

bull Also when one end is constrained and the other end is subjected to a force awayfrom the constraint

14

bull It is caused by a load that tends to stretch or elongate a body

Mechanical Properties of Dental Materials - Dr Nithin Mathew 15

bull In fixed prosthodontics a sticky candy (Jujube) can be used toremove crowns by means of a tensile force when the patienttries to open the mouth after the candy has mechanicallybonded to opposing teeth or gums

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Compressive Stress

bull Compressive stress occurs when 2 sets of forces are directed towards each other inthe same straight line

bull Also when one end is constrained and the other end is subjected to a force towardsthe constraint

bull It is caused by a load that tends to compress or shorten a body

16

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Shear Stress

bull Shear stress occurs when 2 sets of forces are directed parallel to each other but notalong the same straight line

bull A shear stress tends to resist the sliding of one portion of a body over another

bull Shear stress can also be produced by a twisting ortorsional action on a material

17

Mechanical Properties of Dental Materials - Dr Nithin Mathew 18

bull Eg If a force is applied along the surface of tooth enamel by a sharp-edgedinstrument parallel to the interface between the enamel and the orthodonticbracket the bracket may debond by shear stress failure of the resin luting agent

Mechanical Properties of Dental Materials - Dr Nithin Mathew 19

In the oral cavity shear failure is unlikely to occur due to

1 Many of the brittle materials in restored tooth surfaces generally have roughcurved surfaces

2 The presence of chamfers bevels or changes in curvature of a bonded toothsurface

3 To produce shear failure the applied force must be located immediately adjacentto the interface The farther away from the interface the load is applied more likelythat it is a tensile failure

Mechanical Properties of Dental Materials - Dr Nithin Mathew 20

5 Since the tensile strength of brittle materials is usually well below their shearstrength values tensile failure is more likely to occur

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Flexural Stress (bending)

bull Force per unit area of a material that is subjected to flexural loading (bending)

bull A shear stress tends to resist the sliding of one portion of a body over another

bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component

21

Mechanical Properties of Dental Materials - Dr Nithin Mathew 22

bull Flexural stresses produced in a three-unit fixeddental prosthesis

bull Flexural stresses produced in a two-unit cantileverbridge

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull StrengthStrength of a material is defined as the average level of stress

at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize

23

Mechanical Properties of Dental Materials - Dr Nithin Mathew 24

bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component

bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage

bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture

25

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration

bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact

26

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRAIN

bull Defined as the change in length per unit original length

bull Strain of a material is reported as percentage()

bull Strain may be either elastic plastic or a combination of both elastic and plastic

bull Elastic strain is reversible ie it disappears when force is removed

bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed

Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)

27

Eg

bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2

bull If the wire is 01m long and if it stretches 0001m under the load

bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001

Strain (ε)= (120549120549119862119862)(1198621198620)

= 0001 11989811989801119898119898

= 00001 = 001

Stress (σ)= 200 119873119873

0000002 1198981198982 = 2002

x 106 = 100 MPa

28

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC AND PLASTIC DEFORMATION

29

Elastic Shear Deformation Plastic Shear Deformation

PL = 1020

YS = 1536

UTS = 1625

0

200

400

600

800

1000

1200

1400

1600

1800

0 05 1 15

STRESS ndash STRAIN CURVESt

ress

(Mpa

)

Strain ()

YS (02) = 1536 Mpa

UTS = 1625 Mpa

E = 10200053 = 192 GPa

bull PL Proportional Limit

bull YS Yield Strength

bull UTS Ultimate Tensile Strength

bull E Elastic Modulus

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION

bull Mechanical properties that are measures of the elastic strain of dental materialsincludes

bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio

31

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YOUNGrsquoS MODULUS

bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph

bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain

bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity

32

E = 10200053 = 192 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic modulus of a tensile test specimen can be calculated as

bull By definition

Stress (σ)=119865119865119860119860

Strain (ε)=1205491205491198621198621198621198620

33

F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus

E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862

= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620

34

E = 10200053 = 192 GPa

Steep Line Higher modulus and more rigidity

Flat Line Lower modulus and less rigidity

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Principle of Elastic Recovery

bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain

bull After the force is removed the metal springs back to anamount equal to the total elastic strain

bull A final clearance of 25microm is available for the cement

35

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Impression Material

bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth

bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing

36

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Hookersquos Law

bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress

bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality

bull Hookes law can be expressed by the formula

bull The value of the constant depends on the material and the type of stress

37

119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930

= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930

STRESS ndash STRAIN CURVE Enamel amp Dentin

PL = 235CS = 262

PL = 176

CS = 234

0

50

100

150

200

250

300

Com

pres

sive

Sre

ss( M

pa)

Strain

Enamel Dentin

bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures

bull Enamel - more stiffer andbrittle than dentin

bull But dentin more flexible andtougher

EEnamel = 117 GPaEDentin = 336 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

POISSONrsquoS RATIO

bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions

bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section

bull Under compressive loading there is an increase in thecross-section

39

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio

bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains

bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03

bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio

40

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 3: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONTENTS

bull Introductionbull Forcebull Stress

bull Tensilebull Compressivebull Shearbull Flexural

bull Strainbull Elastic amp Plastic Deformationbull Stress ndash Strain Curve

3

bull Mechanical Properties based onElastic Deformationbull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength Propertiesbull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength

shear strength compressivestrength amp flexural strength

4

bull Mechanical Properties based onPlastic Deformationbull Flexural Strengthbull Impact Strengthbull Toughnessbull Fracture toughnessbull Brittlenessbull Ductilitybull Malleabilitybull Hardness

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Hardness testsbull Brinellbull Rockwellbull Vickersbull Knoops

bull Stress Concentration Effects

bull Methods To Minimize Stress Concentration

bull Conclusion

bull References

5

Mechanical Properties of Dental Materials - Dr Nithin Mathew

INTRODUCTION

bull In the oral environment restorative materials and dental appliances are exposed tochemical thermal and mechanical challenges

bull These challenges can cause deformation of materials

bull The mechanical properties of a material define how materials respond tomechanical challenges

bull Mechanical properties are defined by the laws of mechanics

ie It is the physical science that deals with energy forces and their effects on thebodies

6

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull So it is necessary to understand the principles involved in a variety of mechanicalproperties to optimise the clinical service of a material

bull Mechanical properties are measured responses both elastic (reversible on forceremoval) and plastic (irreversible on force removal) of materials under an appliedforce or distribution of forces

bull They are expressed most often in units of stress and strain

7

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull They can represent measurements of

bull Elastic deformation (reversible)bull Proportional limitbull Resiliencebull Modulus of elasticity

bull Plastic deformation (irreversible)bull Percentage of elongation

bull Combination of bothbull Toughnessbull Yield strength

8

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FORCE

bull In physics a force is any influence that causes an object to undergo a certainchange either concerning its movement direction or geometrical construction

bull A force is defined by 3 characteristicsbull Point of applicationbull Magnitudebull Direction of application

bull The SI unit of force is Newton (N)

9

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Occlusal Forces

bull Max occlusal forces 200 ndash 3500N

bull Occlusal forces between adult teeth are highest in the posterior region closest to themandibular hinge axis and decrease from the molar to the incisors

bull Forces on first and second molars vary from 400 to 800N

bull Average on bicuspids cuspids and incisors is about 300 200 and 150N

bull Increase in force from 235 ndash 494N in growing children with an average yearlyincrease of about 22N

10

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS amp STRAIN

Stressbull The force per unit area acting on millions of atoms or molecules in a given

plane of a materialbull Force acting per unit area

bull Unit of measurement is Megapascal (Mpa)

bull Stress is the internal resistance of a material to an external load applied onthat material

Stress (σ)= 119865119865 (119873119873)119860119860 (1198981198982)

11

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Type Of Stress Produced By Examples

Residual Stress

Stress caused within the material during the manufacturing process

During welding

StructuralStress

Stresses produced in the structure during function Weights they support provide the loadings

In abutments of fixed partial denture

Pressure Stress

Induced in vessels containing pressurized materials In dentures during processing under pressure and heat

Flow Stress Force of liquid striking against the wall acts as the load

Molten metal alloy striking the walls of the mould during casting

Thermal Stress

Material is subjected to internal stress due to different temperatures causing varying expansions in the material

Materials that undergo thermal stress such as inlay wax soldering and welding alloys

Fatigue Stress Stress caused due to cyclic rotation of a material Rotary instruments undergo rotational or cyclic fatigue

12

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull By means of the direction of force stresses can be classified as

bull Tensile stressbull Compressive stressbull Shear stressbull Flexural stress

13

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Tensile Stress

bull Tensile stress occurs when 2 sets of forces are directed away from each other in thesame straight line

bull Also when one end is constrained and the other end is subjected to a force awayfrom the constraint

14

bull It is caused by a load that tends to stretch or elongate a body

Mechanical Properties of Dental Materials - Dr Nithin Mathew 15

bull In fixed prosthodontics a sticky candy (Jujube) can be used toremove crowns by means of a tensile force when the patienttries to open the mouth after the candy has mechanicallybonded to opposing teeth or gums

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Compressive Stress

bull Compressive stress occurs when 2 sets of forces are directed towards each other inthe same straight line

bull Also when one end is constrained and the other end is subjected to a force towardsthe constraint

bull It is caused by a load that tends to compress or shorten a body

16

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Shear Stress

bull Shear stress occurs when 2 sets of forces are directed parallel to each other but notalong the same straight line

bull A shear stress tends to resist the sliding of one portion of a body over another

bull Shear stress can also be produced by a twisting ortorsional action on a material

17

Mechanical Properties of Dental Materials - Dr Nithin Mathew 18

bull Eg If a force is applied along the surface of tooth enamel by a sharp-edgedinstrument parallel to the interface between the enamel and the orthodonticbracket the bracket may debond by shear stress failure of the resin luting agent

Mechanical Properties of Dental Materials - Dr Nithin Mathew 19

In the oral cavity shear failure is unlikely to occur due to

1 Many of the brittle materials in restored tooth surfaces generally have roughcurved surfaces

2 The presence of chamfers bevels or changes in curvature of a bonded toothsurface

3 To produce shear failure the applied force must be located immediately adjacentto the interface The farther away from the interface the load is applied more likelythat it is a tensile failure

Mechanical Properties of Dental Materials - Dr Nithin Mathew 20

5 Since the tensile strength of brittle materials is usually well below their shearstrength values tensile failure is more likely to occur

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Flexural Stress (bending)

bull Force per unit area of a material that is subjected to flexural loading (bending)

bull A shear stress tends to resist the sliding of one portion of a body over another

bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component

21

Mechanical Properties of Dental Materials - Dr Nithin Mathew 22

bull Flexural stresses produced in a three-unit fixeddental prosthesis

bull Flexural stresses produced in a two-unit cantileverbridge

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull StrengthStrength of a material is defined as the average level of stress

at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize

23

Mechanical Properties of Dental Materials - Dr Nithin Mathew 24

bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component

bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage

bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture

25

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration

bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact

26

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRAIN

bull Defined as the change in length per unit original length

bull Strain of a material is reported as percentage()

bull Strain may be either elastic plastic or a combination of both elastic and plastic

bull Elastic strain is reversible ie it disappears when force is removed

bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed

Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)

27

Eg

bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2

bull If the wire is 01m long and if it stretches 0001m under the load

bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001

Strain (ε)= (120549120549119862119862)(1198621198620)

= 0001 11989811989801119898119898

= 00001 = 001

Stress (σ)= 200 119873119873

0000002 1198981198982 = 2002

x 106 = 100 MPa

28

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC AND PLASTIC DEFORMATION

29

Elastic Shear Deformation Plastic Shear Deformation

PL = 1020

YS = 1536

UTS = 1625

0

200

400

600

800

1000

1200

1400

1600

1800

0 05 1 15

STRESS ndash STRAIN CURVESt

ress

(Mpa

)

Strain ()

YS (02) = 1536 Mpa

UTS = 1625 Mpa

E = 10200053 = 192 GPa

bull PL Proportional Limit

bull YS Yield Strength

bull UTS Ultimate Tensile Strength

bull E Elastic Modulus

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION

bull Mechanical properties that are measures of the elastic strain of dental materialsincludes

bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio

31

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YOUNGrsquoS MODULUS

bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph

bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain

bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity

32

E = 10200053 = 192 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic modulus of a tensile test specimen can be calculated as

bull By definition

Stress (σ)=119865119865119860119860

Strain (ε)=1205491205491198621198621198621198620

33

F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus

E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862

= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620

34

E = 10200053 = 192 GPa

Steep Line Higher modulus and more rigidity

Flat Line Lower modulus and less rigidity

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Principle of Elastic Recovery

bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain

bull After the force is removed the metal springs back to anamount equal to the total elastic strain

bull A final clearance of 25microm is available for the cement

35

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Impression Material

bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth

bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing

36

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Hookersquos Law

bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress

bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality

bull Hookes law can be expressed by the formula

bull The value of the constant depends on the material and the type of stress

37

119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930

= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930

STRESS ndash STRAIN CURVE Enamel amp Dentin

PL = 235CS = 262

PL = 176

CS = 234

0

50

100

150

200

250

300

Com

pres

sive

Sre

ss( M

pa)

Strain

Enamel Dentin

bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures

bull Enamel - more stiffer andbrittle than dentin

bull But dentin more flexible andtougher

EEnamel = 117 GPaEDentin = 336 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

POISSONrsquoS RATIO

bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions

bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section

bull Under compressive loading there is an increase in thecross-section

39

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio

bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains

bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03

bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio

40

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 4: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength Propertiesbull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength

shear strength compressivestrength amp flexural strength

4

bull Mechanical Properties based onPlastic Deformationbull Flexural Strengthbull Impact Strengthbull Toughnessbull Fracture toughnessbull Brittlenessbull Ductilitybull Malleabilitybull Hardness

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Hardness testsbull Brinellbull Rockwellbull Vickersbull Knoops

bull Stress Concentration Effects

bull Methods To Minimize Stress Concentration

bull Conclusion

bull References

5

Mechanical Properties of Dental Materials - Dr Nithin Mathew

INTRODUCTION

bull In the oral environment restorative materials and dental appliances are exposed tochemical thermal and mechanical challenges

bull These challenges can cause deformation of materials

bull The mechanical properties of a material define how materials respond tomechanical challenges

bull Mechanical properties are defined by the laws of mechanics

ie It is the physical science that deals with energy forces and their effects on thebodies

6

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull So it is necessary to understand the principles involved in a variety of mechanicalproperties to optimise the clinical service of a material

bull Mechanical properties are measured responses both elastic (reversible on forceremoval) and plastic (irreversible on force removal) of materials under an appliedforce or distribution of forces

bull They are expressed most often in units of stress and strain

7

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull They can represent measurements of

bull Elastic deformation (reversible)bull Proportional limitbull Resiliencebull Modulus of elasticity

bull Plastic deformation (irreversible)bull Percentage of elongation

bull Combination of bothbull Toughnessbull Yield strength

8

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FORCE

bull In physics a force is any influence that causes an object to undergo a certainchange either concerning its movement direction or geometrical construction

bull A force is defined by 3 characteristicsbull Point of applicationbull Magnitudebull Direction of application

bull The SI unit of force is Newton (N)

9

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Occlusal Forces

bull Max occlusal forces 200 ndash 3500N

bull Occlusal forces between adult teeth are highest in the posterior region closest to themandibular hinge axis and decrease from the molar to the incisors

bull Forces on first and second molars vary from 400 to 800N

bull Average on bicuspids cuspids and incisors is about 300 200 and 150N

bull Increase in force from 235 ndash 494N in growing children with an average yearlyincrease of about 22N

10

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS amp STRAIN

Stressbull The force per unit area acting on millions of atoms or molecules in a given

plane of a materialbull Force acting per unit area

bull Unit of measurement is Megapascal (Mpa)

bull Stress is the internal resistance of a material to an external load applied onthat material

Stress (σ)= 119865119865 (119873119873)119860119860 (1198981198982)

11

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Type Of Stress Produced By Examples

Residual Stress

Stress caused within the material during the manufacturing process

During welding

StructuralStress

Stresses produced in the structure during function Weights they support provide the loadings

In abutments of fixed partial denture

Pressure Stress

Induced in vessels containing pressurized materials In dentures during processing under pressure and heat

Flow Stress Force of liquid striking against the wall acts as the load

Molten metal alloy striking the walls of the mould during casting

Thermal Stress

Material is subjected to internal stress due to different temperatures causing varying expansions in the material

Materials that undergo thermal stress such as inlay wax soldering and welding alloys

Fatigue Stress Stress caused due to cyclic rotation of a material Rotary instruments undergo rotational or cyclic fatigue

12

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull By means of the direction of force stresses can be classified as

bull Tensile stressbull Compressive stressbull Shear stressbull Flexural stress

13

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Tensile Stress

bull Tensile stress occurs when 2 sets of forces are directed away from each other in thesame straight line

bull Also when one end is constrained and the other end is subjected to a force awayfrom the constraint

14

bull It is caused by a load that tends to stretch or elongate a body

Mechanical Properties of Dental Materials - Dr Nithin Mathew 15

bull In fixed prosthodontics a sticky candy (Jujube) can be used toremove crowns by means of a tensile force when the patienttries to open the mouth after the candy has mechanicallybonded to opposing teeth or gums

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Compressive Stress

bull Compressive stress occurs when 2 sets of forces are directed towards each other inthe same straight line

bull Also when one end is constrained and the other end is subjected to a force towardsthe constraint

bull It is caused by a load that tends to compress or shorten a body

16

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Shear Stress

bull Shear stress occurs when 2 sets of forces are directed parallel to each other but notalong the same straight line

bull A shear stress tends to resist the sliding of one portion of a body over another

bull Shear stress can also be produced by a twisting ortorsional action on a material

17

Mechanical Properties of Dental Materials - Dr Nithin Mathew 18

bull Eg If a force is applied along the surface of tooth enamel by a sharp-edgedinstrument parallel to the interface between the enamel and the orthodonticbracket the bracket may debond by shear stress failure of the resin luting agent

Mechanical Properties of Dental Materials - Dr Nithin Mathew 19

In the oral cavity shear failure is unlikely to occur due to

1 Many of the brittle materials in restored tooth surfaces generally have roughcurved surfaces

2 The presence of chamfers bevels or changes in curvature of a bonded toothsurface

3 To produce shear failure the applied force must be located immediately adjacentto the interface The farther away from the interface the load is applied more likelythat it is a tensile failure

Mechanical Properties of Dental Materials - Dr Nithin Mathew 20

5 Since the tensile strength of brittle materials is usually well below their shearstrength values tensile failure is more likely to occur

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Flexural Stress (bending)

bull Force per unit area of a material that is subjected to flexural loading (bending)

bull A shear stress tends to resist the sliding of one portion of a body over another

bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component

21

Mechanical Properties of Dental Materials - Dr Nithin Mathew 22

bull Flexural stresses produced in a three-unit fixeddental prosthesis

bull Flexural stresses produced in a two-unit cantileverbridge

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull StrengthStrength of a material is defined as the average level of stress

at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize

23

Mechanical Properties of Dental Materials - Dr Nithin Mathew 24

bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component

bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage

bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture

25

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration

bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact

26

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRAIN

bull Defined as the change in length per unit original length

bull Strain of a material is reported as percentage()

bull Strain may be either elastic plastic or a combination of both elastic and plastic

bull Elastic strain is reversible ie it disappears when force is removed

bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed

Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)

27

Eg

bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2

bull If the wire is 01m long and if it stretches 0001m under the load

bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001

Strain (ε)= (120549120549119862119862)(1198621198620)

= 0001 11989811989801119898119898

= 00001 = 001

Stress (σ)= 200 119873119873

0000002 1198981198982 = 2002

x 106 = 100 MPa

28

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC AND PLASTIC DEFORMATION

29

Elastic Shear Deformation Plastic Shear Deformation

PL = 1020

YS = 1536

UTS = 1625

0

200

400

600

800

1000

1200

1400

1600

1800

0 05 1 15

STRESS ndash STRAIN CURVESt

ress

(Mpa

)

Strain ()

YS (02) = 1536 Mpa

UTS = 1625 Mpa

E = 10200053 = 192 GPa

bull PL Proportional Limit

bull YS Yield Strength

bull UTS Ultimate Tensile Strength

bull E Elastic Modulus

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION

bull Mechanical properties that are measures of the elastic strain of dental materialsincludes

bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio

31

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YOUNGrsquoS MODULUS

bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph

bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain

bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity

32

E = 10200053 = 192 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic modulus of a tensile test specimen can be calculated as

bull By definition

Stress (σ)=119865119865119860119860

Strain (ε)=1205491205491198621198621198621198620

33

F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus

E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862

= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620

34

E = 10200053 = 192 GPa

Steep Line Higher modulus and more rigidity

Flat Line Lower modulus and less rigidity

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Principle of Elastic Recovery

bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain

bull After the force is removed the metal springs back to anamount equal to the total elastic strain

bull A final clearance of 25microm is available for the cement

35

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Impression Material

bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth

bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing

36

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Hookersquos Law

bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress

bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality

bull Hookes law can be expressed by the formula

bull The value of the constant depends on the material and the type of stress

37

119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930

= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930

STRESS ndash STRAIN CURVE Enamel amp Dentin

PL = 235CS = 262

PL = 176

CS = 234

0

50

100

150

200

250

300

Com

pres

sive

Sre

ss( M

pa)

Strain

Enamel Dentin

bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures

bull Enamel - more stiffer andbrittle than dentin

bull But dentin more flexible andtougher

EEnamel = 117 GPaEDentin = 336 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

POISSONrsquoS RATIO

bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions

bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section

bull Under compressive loading there is an increase in thecross-section

39

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio

bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains

bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03

bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio

40

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 5: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Hardness testsbull Brinellbull Rockwellbull Vickersbull Knoops

bull Stress Concentration Effects

bull Methods To Minimize Stress Concentration

bull Conclusion

bull References

5

Mechanical Properties of Dental Materials - Dr Nithin Mathew

INTRODUCTION

bull In the oral environment restorative materials and dental appliances are exposed tochemical thermal and mechanical challenges

bull These challenges can cause deformation of materials

bull The mechanical properties of a material define how materials respond tomechanical challenges

bull Mechanical properties are defined by the laws of mechanics

ie It is the physical science that deals with energy forces and their effects on thebodies

6

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull So it is necessary to understand the principles involved in a variety of mechanicalproperties to optimise the clinical service of a material

bull Mechanical properties are measured responses both elastic (reversible on forceremoval) and plastic (irreversible on force removal) of materials under an appliedforce or distribution of forces

bull They are expressed most often in units of stress and strain

7

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull They can represent measurements of

bull Elastic deformation (reversible)bull Proportional limitbull Resiliencebull Modulus of elasticity

bull Plastic deformation (irreversible)bull Percentage of elongation

bull Combination of bothbull Toughnessbull Yield strength

8

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FORCE

bull In physics a force is any influence that causes an object to undergo a certainchange either concerning its movement direction or geometrical construction

bull A force is defined by 3 characteristicsbull Point of applicationbull Magnitudebull Direction of application

bull The SI unit of force is Newton (N)

9

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Occlusal Forces

bull Max occlusal forces 200 ndash 3500N

bull Occlusal forces between adult teeth are highest in the posterior region closest to themandibular hinge axis and decrease from the molar to the incisors

bull Forces on first and second molars vary from 400 to 800N

bull Average on bicuspids cuspids and incisors is about 300 200 and 150N

bull Increase in force from 235 ndash 494N in growing children with an average yearlyincrease of about 22N

10

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS amp STRAIN

Stressbull The force per unit area acting on millions of atoms or molecules in a given

plane of a materialbull Force acting per unit area

bull Unit of measurement is Megapascal (Mpa)

bull Stress is the internal resistance of a material to an external load applied onthat material

Stress (σ)= 119865119865 (119873119873)119860119860 (1198981198982)

11

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Type Of Stress Produced By Examples

Residual Stress

Stress caused within the material during the manufacturing process

During welding

StructuralStress

Stresses produced in the structure during function Weights they support provide the loadings

In abutments of fixed partial denture

Pressure Stress

Induced in vessels containing pressurized materials In dentures during processing under pressure and heat

Flow Stress Force of liquid striking against the wall acts as the load

Molten metal alloy striking the walls of the mould during casting

Thermal Stress

Material is subjected to internal stress due to different temperatures causing varying expansions in the material

Materials that undergo thermal stress such as inlay wax soldering and welding alloys

Fatigue Stress Stress caused due to cyclic rotation of a material Rotary instruments undergo rotational or cyclic fatigue

12

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull By means of the direction of force stresses can be classified as

bull Tensile stressbull Compressive stressbull Shear stressbull Flexural stress

13

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Tensile Stress

bull Tensile stress occurs when 2 sets of forces are directed away from each other in thesame straight line

bull Also when one end is constrained and the other end is subjected to a force awayfrom the constraint

14

bull It is caused by a load that tends to stretch or elongate a body

Mechanical Properties of Dental Materials - Dr Nithin Mathew 15

bull In fixed prosthodontics a sticky candy (Jujube) can be used toremove crowns by means of a tensile force when the patienttries to open the mouth after the candy has mechanicallybonded to opposing teeth or gums

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Compressive Stress

bull Compressive stress occurs when 2 sets of forces are directed towards each other inthe same straight line

bull Also when one end is constrained and the other end is subjected to a force towardsthe constraint

bull It is caused by a load that tends to compress or shorten a body

16

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Shear Stress

bull Shear stress occurs when 2 sets of forces are directed parallel to each other but notalong the same straight line

bull A shear stress tends to resist the sliding of one portion of a body over another

bull Shear stress can also be produced by a twisting ortorsional action on a material

17

Mechanical Properties of Dental Materials - Dr Nithin Mathew 18

bull Eg If a force is applied along the surface of tooth enamel by a sharp-edgedinstrument parallel to the interface between the enamel and the orthodonticbracket the bracket may debond by shear stress failure of the resin luting agent

Mechanical Properties of Dental Materials - Dr Nithin Mathew 19

In the oral cavity shear failure is unlikely to occur due to

1 Many of the brittle materials in restored tooth surfaces generally have roughcurved surfaces

2 The presence of chamfers bevels or changes in curvature of a bonded toothsurface

3 To produce shear failure the applied force must be located immediately adjacentto the interface The farther away from the interface the load is applied more likelythat it is a tensile failure

Mechanical Properties of Dental Materials - Dr Nithin Mathew 20

5 Since the tensile strength of brittle materials is usually well below their shearstrength values tensile failure is more likely to occur

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Flexural Stress (bending)

bull Force per unit area of a material that is subjected to flexural loading (bending)

bull A shear stress tends to resist the sliding of one portion of a body over another

bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component

21

Mechanical Properties of Dental Materials - Dr Nithin Mathew 22

bull Flexural stresses produced in a three-unit fixeddental prosthesis

bull Flexural stresses produced in a two-unit cantileverbridge

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull StrengthStrength of a material is defined as the average level of stress

at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize

23

Mechanical Properties of Dental Materials - Dr Nithin Mathew 24

bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component

bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage

bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture

25

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration

bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact

26

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRAIN

bull Defined as the change in length per unit original length

bull Strain of a material is reported as percentage()

bull Strain may be either elastic plastic or a combination of both elastic and plastic

bull Elastic strain is reversible ie it disappears when force is removed

bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed

Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)

27

Eg

bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2

bull If the wire is 01m long and if it stretches 0001m under the load

bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001

Strain (ε)= (120549120549119862119862)(1198621198620)

= 0001 11989811989801119898119898

= 00001 = 001

Stress (σ)= 200 119873119873

0000002 1198981198982 = 2002

x 106 = 100 MPa

28

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC AND PLASTIC DEFORMATION

29

Elastic Shear Deformation Plastic Shear Deformation

PL = 1020

YS = 1536

UTS = 1625

0

200

400

600

800

1000

1200

1400

1600

1800

0 05 1 15

STRESS ndash STRAIN CURVESt

ress

(Mpa

)

Strain ()

YS (02) = 1536 Mpa

UTS = 1625 Mpa

E = 10200053 = 192 GPa

bull PL Proportional Limit

bull YS Yield Strength

bull UTS Ultimate Tensile Strength

bull E Elastic Modulus

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION

bull Mechanical properties that are measures of the elastic strain of dental materialsincludes

bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio

31

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YOUNGrsquoS MODULUS

bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph

bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain

bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity

32

E = 10200053 = 192 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic modulus of a tensile test specimen can be calculated as

bull By definition

Stress (σ)=119865119865119860119860

Strain (ε)=1205491205491198621198621198621198620

33

F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus

E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862

= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620

34

E = 10200053 = 192 GPa

Steep Line Higher modulus and more rigidity

Flat Line Lower modulus and less rigidity

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Principle of Elastic Recovery

bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain

bull After the force is removed the metal springs back to anamount equal to the total elastic strain

bull A final clearance of 25microm is available for the cement

35

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Impression Material

bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth

bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing

36

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Hookersquos Law

bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress

bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality

bull Hookes law can be expressed by the formula

bull The value of the constant depends on the material and the type of stress

37

119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930

= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930

STRESS ndash STRAIN CURVE Enamel amp Dentin

PL = 235CS = 262

PL = 176

CS = 234

0

50

100

150

200

250

300

Com

pres

sive

Sre

ss( M

pa)

Strain

Enamel Dentin

bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures

bull Enamel - more stiffer andbrittle than dentin

bull But dentin more flexible andtougher

EEnamel = 117 GPaEDentin = 336 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

POISSONrsquoS RATIO

bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions

bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section

bull Under compressive loading there is an increase in thecross-section

39

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio

bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains

bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03

bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio

40

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 6: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

INTRODUCTION

bull In the oral environment restorative materials and dental appliances are exposed tochemical thermal and mechanical challenges

bull These challenges can cause deformation of materials

bull The mechanical properties of a material define how materials respond tomechanical challenges

bull Mechanical properties are defined by the laws of mechanics

ie It is the physical science that deals with energy forces and their effects on thebodies

6

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull So it is necessary to understand the principles involved in a variety of mechanicalproperties to optimise the clinical service of a material

bull Mechanical properties are measured responses both elastic (reversible on forceremoval) and plastic (irreversible on force removal) of materials under an appliedforce or distribution of forces

bull They are expressed most often in units of stress and strain

7

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull They can represent measurements of

bull Elastic deformation (reversible)bull Proportional limitbull Resiliencebull Modulus of elasticity

bull Plastic deformation (irreversible)bull Percentage of elongation

bull Combination of bothbull Toughnessbull Yield strength

8

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FORCE

bull In physics a force is any influence that causes an object to undergo a certainchange either concerning its movement direction or geometrical construction

bull A force is defined by 3 characteristicsbull Point of applicationbull Magnitudebull Direction of application

bull The SI unit of force is Newton (N)

9

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Occlusal Forces

bull Max occlusal forces 200 ndash 3500N

bull Occlusal forces between adult teeth are highest in the posterior region closest to themandibular hinge axis and decrease from the molar to the incisors

bull Forces on first and second molars vary from 400 to 800N

bull Average on bicuspids cuspids and incisors is about 300 200 and 150N

bull Increase in force from 235 ndash 494N in growing children with an average yearlyincrease of about 22N

10

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS amp STRAIN

Stressbull The force per unit area acting on millions of atoms or molecules in a given

plane of a materialbull Force acting per unit area

bull Unit of measurement is Megapascal (Mpa)

bull Stress is the internal resistance of a material to an external load applied onthat material

Stress (σ)= 119865119865 (119873119873)119860119860 (1198981198982)

11

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Type Of Stress Produced By Examples

Residual Stress

Stress caused within the material during the manufacturing process

During welding

StructuralStress

Stresses produced in the structure during function Weights they support provide the loadings

In abutments of fixed partial denture

Pressure Stress

Induced in vessels containing pressurized materials In dentures during processing under pressure and heat

Flow Stress Force of liquid striking against the wall acts as the load

Molten metal alloy striking the walls of the mould during casting

Thermal Stress

Material is subjected to internal stress due to different temperatures causing varying expansions in the material

Materials that undergo thermal stress such as inlay wax soldering and welding alloys

Fatigue Stress Stress caused due to cyclic rotation of a material Rotary instruments undergo rotational or cyclic fatigue

12

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull By means of the direction of force stresses can be classified as

bull Tensile stressbull Compressive stressbull Shear stressbull Flexural stress

13

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Tensile Stress

bull Tensile stress occurs when 2 sets of forces are directed away from each other in thesame straight line

bull Also when one end is constrained and the other end is subjected to a force awayfrom the constraint

14

bull It is caused by a load that tends to stretch or elongate a body

Mechanical Properties of Dental Materials - Dr Nithin Mathew 15

bull In fixed prosthodontics a sticky candy (Jujube) can be used toremove crowns by means of a tensile force when the patienttries to open the mouth after the candy has mechanicallybonded to opposing teeth or gums

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Compressive Stress

bull Compressive stress occurs when 2 sets of forces are directed towards each other inthe same straight line

bull Also when one end is constrained and the other end is subjected to a force towardsthe constraint

bull It is caused by a load that tends to compress or shorten a body

16

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Shear Stress

bull Shear stress occurs when 2 sets of forces are directed parallel to each other but notalong the same straight line

bull A shear stress tends to resist the sliding of one portion of a body over another

bull Shear stress can also be produced by a twisting ortorsional action on a material

17

Mechanical Properties of Dental Materials - Dr Nithin Mathew 18

bull Eg If a force is applied along the surface of tooth enamel by a sharp-edgedinstrument parallel to the interface between the enamel and the orthodonticbracket the bracket may debond by shear stress failure of the resin luting agent

Mechanical Properties of Dental Materials - Dr Nithin Mathew 19

In the oral cavity shear failure is unlikely to occur due to

1 Many of the brittle materials in restored tooth surfaces generally have roughcurved surfaces

2 The presence of chamfers bevels or changes in curvature of a bonded toothsurface

3 To produce shear failure the applied force must be located immediately adjacentto the interface The farther away from the interface the load is applied more likelythat it is a tensile failure

Mechanical Properties of Dental Materials - Dr Nithin Mathew 20

5 Since the tensile strength of brittle materials is usually well below their shearstrength values tensile failure is more likely to occur

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Flexural Stress (bending)

bull Force per unit area of a material that is subjected to flexural loading (bending)

bull A shear stress tends to resist the sliding of one portion of a body over another

bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component

21

Mechanical Properties of Dental Materials - Dr Nithin Mathew 22

bull Flexural stresses produced in a three-unit fixeddental prosthesis

bull Flexural stresses produced in a two-unit cantileverbridge

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull StrengthStrength of a material is defined as the average level of stress

at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize

23

Mechanical Properties of Dental Materials - Dr Nithin Mathew 24

bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component

bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage

bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture

25

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration

bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact

26

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRAIN

bull Defined as the change in length per unit original length

bull Strain of a material is reported as percentage()

bull Strain may be either elastic plastic or a combination of both elastic and plastic

bull Elastic strain is reversible ie it disappears when force is removed

bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed

Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)

27

Eg

bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2

bull If the wire is 01m long and if it stretches 0001m under the load

bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001

Strain (ε)= (120549120549119862119862)(1198621198620)

= 0001 11989811989801119898119898

= 00001 = 001

Stress (σ)= 200 119873119873

0000002 1198981198982 = 2002

x 106 = 100 MPa

28

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC AND PLASTIC DEFORMATION

29

Elastic Shear Deformation Plastic Shear Deformation

PL = 1020

YS = 1536

UTS = 1625

0

200

400

600

800

1000

1200

1400

1600

1800

0 05 1 15

STRESS ndash STRAIN CURVESt

ress

(Mpa

)

Strain ()

YS (02) = 1536 Mpa

UTS = 1625 Mpa

E = 10200053 = 192 GPa

bull PL Proportional Limit

bull YS Yield Strength

bull UTS Ultimate Tensile Strength

bull E Elastic Modulus

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION

bull Mechanical properties that are measures of the elastic strain of dental materialsincludes

bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio

31

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YOUNGrsquoS MODULUS

bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph

bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain

bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity

32

E = 10200053 = 192 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic modulus of a tensile test specimen can be calculated as

bull By definition

Stress (σ)=119865119865119860119860

Strain (ε)=1205491205491198621198621198621198620

33

F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus

E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862

= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620

34

E = 10200053 = 192 GPa

Steep Line Higher modulus and more rigidity

Flat Line Lower modulus and less rigidity

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Principle of Elastic Recovery

bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain

bull After the force is removed the metal springs back to anamount equal to the total elastic strain

bull A final clearance of 25microm is available for the cement

35

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Impression Material

bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth

bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing

36

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Hookersquos Law

bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress

bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality

bull Hookes law can be expressed by the formula

bull The value of the constant depends on the material and the type of stress

37

119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930

= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930

STRESS ndash STRAIN CURVE Enamel amp Dentin

PL = 235CS = 262

PL = 176

CS = 234

0

50

100

150

200

250

300

Com

pres

sive

Sre

ss( M

pa)

Strain

Enamel Dentin

bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures

bull Enamel - more stiffer andbrittle than dentin

bull But dentin more flexible andtougher

EEnamel = 117 GPaEDentin = 336 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

POISSONrsquoS RATIO

bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions

bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section

bull Under compressive loading there is an increase in thecross-section

39

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio

bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains

bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03

bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio

40

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 7: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull So it is necessary to understand the principles involved in a variety of mechanicalproperties to optimise the clinical service of a material

bull Mechanical properties are measured responses both elastic (reversible on forceremoval) and plastic (irreversible on force removal) of materials under an appliedforce or distribution of forces

bull They are expressed most often in units of stress and strain

7

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull They can represent measurements of

bull Elastic deformation (reversible)bull Proportional limitbull Resiliencebull Modulus of elasticity

bull Plastic deformation (irreversible)bull Percentage of elongation

bull Combination of bothbull Toughnessbull Yield strength

8

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FORCE

bull In physics a force is any influence that causes an object to undergo a certainchange either concerning its movement direction or geometrical construction

bull A force is defined by 3 characteristicsbull Point of applicationbull Magnitudebull Direction of application

bull The SI unit of force is Newton (N)

9

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Occlusal Forces

bull Max occlusal forces 200 ndash 3500N

bull Occlusal forces between adult teeth are highest in the posterior region closest to themandibular hinge axis and decrease from the molar to the incisors

bull Forces on first and second molars vary from 400 to 800N

bull Average on bicuspids cuspids and incisors is about 300 200 and 150N

bull Increase in force from 235 ndash 494N in growing children with an average yearlyincrease of about 22N

10

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS amp STRAIN

Stressbull The force per unit area acting on millions of atoms or molecules in a given

plane of a materialbull Force acting per unit area

bull Unit of measurement is Megapascal (Mpa)

bull Stress is the internal resistance of a material to an external load applied onthat material

Stress (σ)= 119865119865 (119873119873)119860119860 (1198981198982)

11

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Type Of Stress Produced By Examples

Residual Stress

Stress caused within the material during the manufacturing process

During welding

StructuralStress

Stresses produced in the structure during function Weights they support provide the loadings

In abutments of fixed partial denture

Pressure Stress

Induced in vessels containing pressurized materials In dentures during processing under pressure and heat

Flow Stress Force of liquid striking against the wall acts as the load

Molten metal alloy striking the walls of the mould during casting

Thermal Stress

Material is subjected to internal stress due to different temperatures causing varying expansions in the material

Materials that undergo thermal stress such as inlay wax soldering and welding alloys

Fatigue Stress Stress caused due to cyclic rotation of a material Rotary instruments undergo rotational or cyclic fatigue

12

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull By means of the direction of force stresses can be classified as

bull Tensile stressbull Compressive stressbull Shear stressbull Flexural stress

13

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Tensile Stress

bull Tensile stress occurs when 2 sets of forces are directed away from each other in thesame straight line

bull Also when one end is constrained and the other end is subjected to a force awayfrom the constraint

14

bull It is caused by a load that tends to stretch or elongate a body

Mechanical Properties of Dental Materials - Dr Nithin Mathew 15

bull In fixed prosthodontics a sticky candy (Jujube) can be used toremove crowns by means of a tensile force when the patienttries to open the mouth after the candy has mechanicallybonded to opposing teeth or gums

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Compressive Stress

bull Compressive stress occurs when 2 sets of forces are directed towards each other inthe same straight line

bull Also when one end is constrained and the other end is subjected to a force towardsthe constraint

bull It is caused by a load that tends to compress or shorten a body

16

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Shear Stress

bull Shear stress occurs when 2 sets of forces are directed parallel to each other but notalong the same straight line

bull A shear stress tends to resist the sliding of one portion of a body over another

bull Shear stress can also be produced by a twisting ortorsional action on a material

17

Mechanical Properties of Dental Materials - Dr Nithin Mathew 18

bull Eg If a force is applied along the surface of tooth enamel by a sharp-edgedinstrument parallel to the interface between the enamel and the orthodonticbracket the bracket may debond by shear stress failure of the resin luting agent

Mechanical Properties of Dental Materials - Dr Nithin Mathew 19

In the oral cavity shear failure is unlikely to occur due to

1 Many of the brittle materials in restored tooth surfaces generally have roughcurved surfaces

2 The presence of chamfers bevels or changes in curvature of a bonded toothsurface

3 To produce shear failure the applied force must be located immediately adjacentto the interface The farther away from the interface the load is applied more likelythat it is a tensile failure

Mechanical Properties of Dental Materials - Dr Nithin Mathew 20

5 Since the tensile strength of brittle materials is usually well below their shearstrength values tensile failure is more likely to occur

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Flexural Stress (bending)

bull Force per unit area of a material that is subjected to flexural loading (bending)

bull A shear stress tends to resist the sliding of one portion of a body over another

bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component

21

Mechanical Properties of Dental Materials - Dr Nithin Mathew 22

bull Flexural stresses produced in a three-unit fixeddental prosthesis

bull Flexural stresses produced in a two-unit cantileverbridge

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull StrengthStrength of a material is defined as the average level of stress

at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize

23

Mechanical Properties of Dental Materials - Dr Nithin Mathew 24

bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component

bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage

bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture

25

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration

bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact

26

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRAIN

bull Defined as the change in length per unit original length

bull Strain of a material is reported as percentage()

bull Strain may be either elastic plastic or a combination of both elastic and plastic

bull Elastic strain is reversible ie it disappears when force is removed

bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed

Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)

27

Eg

bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2

bull If the wire is 01m long and if it stretches 0001m under the load

bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001

Strain (ε)= (120549120549119862119862)(1198621198620)

= 0001 11989811989801119898119898

= 00001 = 001

Stress (σ)= 200 119873119873

0000002 1198981198982 = 2002

x 106 = 100 MPa

28

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC AND PLASTIC DEFORMATION

29

Elastic Shear Deformation Plastic Shear Deformation

PL = 1020

YS = 1536

UTS = 1625

0

200

400

600

800

1000

1200

1400

1600

1800

0 05 1 15

STRESS ndash STRAIN CURVESt

ress

(Mpa

)

Strain ()

YS (02) = 1536 Mpa

UTS = 1625 Mpa

E = 10200053 = 192 GPa

bull PL Proportional Limit

bull YS Yield Strength

bull UTS Ultimate Tensile Strength

bull E Elastic Modulus

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION

bull Mechanical properties that are measures of the elastic strain of dental materialsincludes

bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio

31

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YOUNGrsquoS MODULUS

bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph

bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain

bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity

32

E = 10200053 = 192 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic modulus of a tensile test specimen can be calculated as

bull By definition

Stress (σ)=119865119865119860119860

Strain (ε)=1205491205491198621198621198621198620

33

F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus

E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862

= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620

34

E = 10200053 = 192 GPa

Steep Line Higher modulus and more rigidity

Flat Line Lower modulus and less rigidity

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Principle of Elastic Recovery

bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain

bull After the force is removed the metal springs back to anamount equal to the total elastic strain

bull A final clearance of 25microm is available for the cement

35

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Impression Material

bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth

bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing

36

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Hookersquos Law

bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress

bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality

bull Hookes law can be expressed by the formula

bull The value of the constant depends on the material and the type of stress

37

119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930

= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930

STRESS ndash STRAIN CURVE Enamel amp Dentin

PL = 235CS = 262

PL = 176

CS = 234

0

50

100

150

200

250

300

Com

pres

sive

Sre

ss( M

pa)

Strain

Enamel Dentin

bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures

bull Enamel - more stiffer andbrittle than dentin

bull But dentin more flexible andtougher

EEnamel = 117 GPaEDentin = 336 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

POISSONrsquoS RATIO

bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions

bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section

bull Under compressive loading there is an increase in thecross-section

39

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio

bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains

bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03

bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio

40

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 8: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull They can represent measurements of

bull Elastic deformation (reversible)bull Proportional limitbull Resiliencebull Modulus of elasticity

bull Plastic deformation (irreversible)bull Percentage of elongation

bull Combination of bothbull Toughnessbull Yield strength

8

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FORCE

bull In physics a force is any influence that causes an object to undergo a certainchange either concerning its movement direction or geometrical construction

bull A force is defined by 3 characteristicsbull Point of applicationbull Magnitudebull Direction of application

bull The SI unit of force is Newton (N)

9

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Occlusal Forces

bull Max occlusal forces 200 ndash 3500N

bull Occlusal forces between adult teeth are highest in the posterior region closest to themandibular hinge axis and decrease from the molar to the incisors

bull Forces on first and second molars vary from 400 to 800N

bull Average on bicuspids cuspids and incisors is about 300 200 and 150N

bull Increase in force from 235 ndash 494N in growing children with an average yearlyincrease of about 22N

10

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS amp STRAIN

Stressbull The force per unit area acting on millions of atoms or molecules in a given

plane of a materialbull Force acting per unit area

bull Unit of measurement is Megapascal (Mpa)

bull Stress is the internal resistance of a material to an external load applied onthat material

Stress (σ)= 119865119865 (119873119873)119860119860 (1198981198982)

11

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Type Of Stress Produced By Examples

Residual Stress

Stress caused within the material during the manufacturing process

During welding

StructuralStress

Stresses produced in the structure during function Weights they support provide the loadings

In abutments of fixed partial denture

Pressure Stress

Induced in vessels containing pressurized materials In dentures during processing under pressure and heat

Flow Stress Force of liquid striking against the wall acts as the load

Molten metal alloy striking the walls of the mould during casting

Thermal Stress

Material is subjected to internal stress due to different temperatures causing varying expansions in the material

Materials that undergo thermal stress such as inlay wax soldering and welding alloys

Fatigue Stress Stress caused due to cyclic rotation of a material Rotary instruments undergo rotational or cyclic fatigue

12

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull By means of the direction of force stresses can be classified as

bull Tensile stressbull Compressive stressbull Shear stressbull Flexural stress

13

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Tensile Stress

bull Tensile stress occurs when 2 sets of forces are directed away from each other in thesame straight line

bull Also when one end is constrained and the other end is subjected to a force awayfrom the constraint

14

bull It is caused by a load that tends to stretch or elongate a body

Mechanical Properties of Dental Materials - Dr Nithin Mathew 15

bull In fixed prosthodontics a sticky candy (Jujube) can be used toremove crowns by means of a tensile force when the patienttries to open the mouth after the candy has mechanicallybonded to opposing teeth or gums

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Compressive Stress

bull Compressive stress occurs when 2 sets of forces are directed towards each other inthe same straight line

bull Also when one end is constrained and the other end is subjected to a force towardsthe constraint

bull It is caused by a load that tends to compress or shorten a body

16

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Shear Stress

bull Shear stress occurs when 2 sets of forces are directed parallel to each other but notalong the same straight line

bull A shear stress tends to resist the sliding of one portion of a body over another

bull Shear stress can also be produced by a twisting ortorsional action on a material

17

Mechanical Properties of Dental Materials - Dr Nithin Mathew 18

bull Eg If a force is applied along the surface of tooth enamel by a sharp-edgedinstrument parallel to the interface between the enamel and the orthodonticbracket the bracket may debond by shear stress failure of the resin luting agent

Mechanical Properties of Dental Materials - Dr Nithin Mathew 19

In the oral cavity shear failure is unlikely to occur due to

1 Many of the brittle materials in restored tooth surfaces generally have roughcurved surfaces

2 The presence of chamfers bevels or changes in curvature of a bonded toothsurface

3 To produce shear failure the applied force must be located immediately adjacentto the interface The farther away from the interface the load is applied more likelythat it is a tensile failure

Mechanical Properties of Dental Materials - Dr Nithin Mathew 20

5 Since the tensile strength of brittle materials is usually well below their shearstrength values tensile failure is more likely to occur

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Flexural Stress (bending)

bull Force per unit area of a material that is subjected to flexural loading (bending)

bull A shear stress tends to resist the sliding of one portion of a body over another

bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component

21

Mechanical Properties of Dental Materials - Dr Nithin Mathew 22

bull Flexural stresses produced in a three-unit fixeddental prosthesis

bull Flexural stresses produced in a two-unit cantileverbridge

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull StrengthStrength of a material is defined as the average level of stress

at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize

23

Mechanical Properties of Dental Materials - Dr Nithin Mathew 24

bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component

bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage

bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture

25

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration

bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact

26

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRAIN

bull Defined as the change in length per unit original length

bull Strain of a material is reported as percentage()

bull Strain may be either elastic plastic or a combination of both elastic and plastic

bull Elastic strain is reversible ie it disappears when force is removed

bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed

Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)

27

Eg

bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2

bull If the wire is 01m long and if it stretches 0001m under the load

bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001

Strain (ε)= (120549120549119862119862)(1198621198620)

= 0001 11989811989801119898119898

= 00001 = 001

Stress (σ)= 200 119873119873

0000002 1198981198982 = 2002

x 106 = 100 MPa

28

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC AND PLASTIC DEFORMATION

29

Elastic Shear Deformation Plastic Shear Deformation

PL = 1020

YS = 1536

UTS = 1625

0

200

400

600

800

1000

1200

1400

1600

1800

0 05 1 15

STRESS ndash STRAIN CURVESt

ress

(Mpa

)

Strain ()

YS (02) = 1536 Mpa

UTS = 1625 Mpa

E = 10200053 = 192 GPa

bull PL Proportional Limit

bull YS Yield Strength

bull UTS Ultimate Tensile Strength

bull E Elastic Modulus

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION

bull Mechanical properties that are measures of the elastic strain of dental materialsincludes

bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio

31

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YOUNGrsquoS MODULUS

bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph

bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain

bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity

32

E = 10200053 = 192 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic modulus of a tensile test specimen can be calculated as

bull By definition

Stress (σ)=119865119865119860119860

Strain (ε)=1205491205491198621198621198621198620

33

F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus

E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862

= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620

34

E = 10200053 = 192 GPa

Steep Line Higher modulus and more rigidity

Flat Line Lower modulus and less rigidity

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Principle of Elastic Recovery

bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain

bull After the force is removed the metal springs back to anamount equal to the total elastic strain

bull A final clearance of 25microm is available for the cement

35

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Impression Material

bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth

bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing

36

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Hookersquos Law

bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress

bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality

bull Hookes law can be expressed by the formula

bull The value of the constant depends on the material and the type of stress

37

119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930

= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930

STRESS ndash STRAIN CURVE Enamel amp Dentin

PL = 235CS = 262

PL = 176

CS = 234

0

50

100

150

200

250

300

Com

pres

sive

Sre

ss( M

pa)

Strain

Enamel Dentin

bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures

bull Enamel - more stiffer andbrittle than dentin

bull But dentin more flexible andtougher

EEnamel = 117 GPaEDentin = 336 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

POISSONrsquoS RATIO

bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions

bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section

bull Under compressive loading there is an increase in thecross-section

39

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio

bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains

bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03

bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio

40

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 9: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FORCE

bull In physics a force is any influence that causes an object to undergo a certainchange either concerning its movement direction or geometrical construction

bull A force is defined by 3 characteristicsbull Point of applicationbull Magnitudebull Direction of application

bull The SI unit of force is Newton (N)

9

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Occlusal Forces

bull Max occlusal forces 200 ndash 3500N

bull Occlusal forces between adult teeth are highest in the posterior region closest to themandibular hinge axis and decrease from the molar to the incisors

bull Forces on first and second molars vary from 400 to 800N

bull Average on bicuspids cuspids and incisors is about 300 200 and 150N

bull Increase in force from 235 ndash 494N in growing children with an average yearlyincrease of about 22N

10

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS amp STRAIN

Stressbull The force per unit area acting on millions of atoms or molecules in a given

plane of a materialbull Force acting per unit area

bull Unit of measurement is Megapascal (Mpa)

bull Stress is the internal resistance of a material to an external load applied onthat material

Stress (σ)= 119865119865 (119873119873)119860119860 (1198981198982)

11

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Type Of Stress Produced By Examples

Residual Stress

Stress caused within the material during the manufacturing process

During welding

StructuralStress

Stresses produced in the structure during function Weights they support provide the loadings

In abutments of fixed partial denture

Pressure Stress

Induced in vessels containing pressurized materials In dentures during processing under pressure and heat

Flow Stress Force of liquid striking against the wall acts as the load

Molten metal alloy striking the walls of the mould during casting

Thermal Stress

Material is subjected to internal stress due to different temperatures causing varying expansions in the material

Materials that undergo thermal stress such as inlay wax soldering and welding alloys

Fatigue Stress Stress caused due to cyclic rotation of a material Rotary instruments undergo rotational or cyclic fatigue

12

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull By means of the direction of force stresses can be classified as

bull Tensile stressbull Compressive stressbull Shear stressbull Flexural stress

13

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Tensile Stress

bull Tensile stress occurs when 2 sets of forces are directed away from each other in thesame straight line

bull Also when one end is constrained and the other end is subjected to a force awayfrom the constraint

14

bull It is caused by a load that tends to stretch or elongate a body

Mechanical Properties of Dental Materials - Dr Nithin Mathew 15

bull In fixed prosthodontics a sticky candy (Jujube) can be used toremove crowns by means of a tensile force when the patienttries to open the mouth after the candy has mechanicallybonded to opposing teeth or gums

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Compressive Stress

bull Compressive stress occurs when 2 sets of forces are directed towards each other inthe same straight line

bull Also when one end is constrained and the other end is subjected to a force towardsthe constraint

bull It is caused by a load that tends to compress or shorten a body

16

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Shear Stress

bull Shear stress occurs when 2 sets of forces are directed parallel to each other but notalong the same straight line

bull A shear stress tends to resist the sliding of one portion of a body over another

bull Shear stress can also be produced by a twisting ortorsional action on a material

17

Mechanical Properties of Dental Materials - Dr Nithin Mathew 18

bull Eg If a force is applied along the surface of tooth enamel by a sharp-edgedinstrument parallel to the interface between the enamel and the orthodonticbracket the bracket may debond by shear stress failure of the resin luting agent

Mechanical Properties of Dental Materials - Dr Nithin Mathew 19

In the oral cavity shear failure is unlikely to occur due to

1 Many of the brittle materials in restored tooth surfaces generally have roughcurved surfaces

2 The presence of chamfers bevels or changes in curvature of a bonded toothsurface

3 To produce shear failure the applied force must be located immediately adjacentto the interface The farther away from the interface the load is applied more likelythat it is a tensile failure

Mechanical Properties of Dental Materials - Dr Nithin Mathew 20

5 Since the tensile strength of brittle materials is usually well below their shearstrength values tensile failure is more likely to occur

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Flexural Stress (bending)

bull Force per unit area of a material that is subjected to flexural loading (bending)

bull A shear stress tends to resist the sliding of one portion of a body over another

bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component

21

Mechanical Properties of Dental Materials - Dr Nithin Mathew 22

bull Flexural stresses produced in a three-unit fixeddental prosthesis

bull Flexural stresses produced in a two-unit cantileverbridge

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull StrengthStrength of a material is defined as the average level of stress

at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize

23

Mechanical Properties of Dental Materials - Dr Nithin Mathew 24

bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component

bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage

bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture

25

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration

bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact

26

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRAIN

bull Defined as the change in length per unit original length

bull Strain of a material is reported as percentage()

bull Strain may be either elastic plastic or a combination of both elastic and plastic

bull Elastic strain is reversible ie it disappears when force is removed

bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed

Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)

27

Eg

bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2

bull If the wire is 01m long and if it stretches 0001m under the load

bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001

Strain (ε)= (120549120549119862119862)(1198621198620)

= 0001 11989811989801119898119898

= 00001 = 001

Stress (σ)= 200 119873119873

0000002 1198981198982 = 2002

x 106 = 100 MPa

28

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC AND PLASTIC DEFORMATION

29

Elastic Shear Deformation Plastic Shear Deformation

PL = 1020

YS = 1536

UTS = 1625

0

200

400

600

800

1000

1200

1400

1600

1800

0 05 1 15

STRESS ndash STRAIN CURVESt

ress

(Mpa

)

Strain ()

YS (02) = 1536 Mpa

UTS = 1625 Mpa

E = 10200053 = 192 GPa

bull PL Proportional Limit

bull YS Yield Strength

bull UTS Ultimate Tensile Strength

bull E Elastic Modulus

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION

bull Mechanical properties that are measures of the elastic strain of dental materialsincludes

bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio

31

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YOUNGrsquoS MODULUS

bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph

bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain

bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity

32

E = 10200053 = 192 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic modulus of a tensile test specimen can be calculated as

bull By definition

Stress (σ)=119865119865119860119860

Strain (ε)=1205491205491198621198621198621198620

33

F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus

E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862

= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620

34

E = 10200053 = 192 GPa

Steep Line Higher modulus and more rigidity

Flat Line Lower modulus and less rigidity

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Principle of Elastic Recovery

bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain

bull After the force is removed the metal springs back to anamount equal to the total elastic strain

bull A final clearance of 25microm is available for the cement

35

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Impression Material

bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth

bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing

36

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Hookersquos Law

bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress

bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality

bull Hookes law can be expressed by the formula

bull The value of the constant depends on the material and the type of stress

37

119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930

= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930

STRESS ndash STRAIN CURVE Enamel amp Dentin

PL = 235CS = 262

PL = 176

CS = 234

0

50

100

150

200

250

300

Com

pres

sive

Sre

ss( M

pa)

Strain

Enamel Dentin

bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures

bull Enamel - more stiffer andbrittle than dentin

bull But dentin more flexible andtougher

EEnamel = 117 GPaEDentin = 336 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

POISSONrsquoS RATIO

bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions

bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section

bull Under compressive loading there is an increase in thecross-section

39

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio

bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains

bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03

bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio

40

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 10: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Occlusal Forces

bull Max occlusal forces 200 ndash 3500N

bull Occlusal forces between adult teeth are highest in the posterior region closest to themandibular hinge axis and decrease from the molar to the incisors

bull Forces on first and second molars vary from 400 to 800N

bull Average on bicuspids cuspids and incisors is about 300 200 and 150N

bull Increase in force from 235 ndash 494N in growing children with an average yearlyincrease of about 22N

10

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS amp STRAIN

Stressbull The force per unit area acting on millions of atoms or molecules in a given

plane of a materialbull Force acting per unit area

bull Unit of measurement is Megapascal (Mpa)

bull Stress is the internal resistance of a material to an external load applied onthat material

Stress (σ)= 119865119865 (119873119873)119860119860 (1198981198982)

11

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Type Of Stress Produced By Examples

Residual Stress

Stress caused within the material during the manufacturing process

During welding

StructuralStress

Stresses produced in the structure during function Weights they support provide the loadings

In abutments of fixed partial denture

Pressure Stress

Induced in vessels containing pressurized materials In dentures during processing under pressure and heat

Flow Stress Force of liquid striking against the wall acts as the load

Molten metal alloy striking the walls of the mould during casting

Thermal Stress

Material is subjected to internal stress due to different temperatures causing varying expansions in the material

Materials that undergo thermal stress such as inlay wax soldering and welding alloys

Fatigue Stress Stress caused due to cyclic rotation of a material Rotary instruments undergo rotational or cyclic fatigue

12

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull By means of the direction of force stresses can be classified as

bull Tensile stressbull Compressive stressbull Shear stressbull Flexural stress

13

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Tensile Stress

bull Tensile stress occurs when 2 sets of forces are directed away from each other in thesame straight line

bull Also when one end is constrained and the other end is subjected to a force awayfrom the constraint

14

bull It is caused by a load that tends to stretch or elongate a body

Mechanical Properties of Dental Materials - Dr Nithin Mathew 15

bull In fixed prosthodontics a sticky candy (Jujube) can be used toremove crowns by means of a tensile force when the patienttries to open the mouth after the candy has mechanicallybonded to opposing teeth or gums

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Compressive Stress

bull Compressive stress occurs when 2 sets of forces are directed towards each other inthe same straight line

bull Also when one end is constrained and the other end is subjected to a force towardsthe constraint

bull It is caused by a load that tends to compress or shorten a body

16

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Shear Stress

bull Shear stress occurs when 2 sets of forces are directed parallel to each other but notalong the same straight line

bull A shear stress tends to resist the sliding of one portion of a body over another

bull Shear stress can also be produced by a twisting ortorsional action on a material

17

Mechanical Properties of Dental Materials - Dr Nithin Mathew 18

bull Eg If a force is applied along the surface of tooth enamel by a sharp-edgedinstrument parallel to the interface between the enamel and the orthodonticbracket the bracket may debond by shear stress failure of the resin luting agent

Mechanical Properties of Dental Materials - Dr Nithin Mathew 19

In the oral cavity shear failure is unlikely to occur due to

1 Many of the brittle materials in restored tooth surfaces generally have roughcurved surfaces

2 The presence of chamfers bevels or changes in curvature of a bonded toothsurface

3 To produce shear failure the applied force must be located immediately adjacentto the interface The farther away from the interface the load is applied more likelythat it is a tensile failure

Mechanical Properties of Dental Materials - Dr Nithin Mathew 20

5 Since the tensile strength of brittle materials is usually well below their shearstrength values tensile failure is more likely to occur

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Flexural Stress (bending)

bull Force per unit area of a material that is subjected to flexural loading (bending)

bull A shear stress tends to resist the sliding of one portion of a body over another

bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component

21

Mechanical Properties of Dental Materials - Dr Nithin Mathew 22

bull Flexural stresses produced in a three-unit fixeddental prosthesis

bull Flexural stresses produced in a two-unit cantileverbridge

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull StrengthStrength of a material is defined as the average level of stress

at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize

23

Mechanical Properties of Dental Materials - Dr Nithin Mathew 24

bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component

bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage

bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture

25

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration

bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact

26

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRAIN

bull Defined as the change in length per unit original length

bull Strain of a material is reported as percentage()

bull Strain may be either elastic plastic or a combination of both elastic and plastic

bull Elastic strain is reversible ie it disappears when force is removed

bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed

Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)

27

Eg

bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2

bull If the wire is 01m long and if it stretches 0001m under the load

bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001

Strain (ε)= (120549120549119862119862)(1198621198620)

= 0001 11989811989801119898119898

= 00001 = 001

Stress (σ)= 200 119873119873

0000002 1198981198982 = 2002

x 106 = 100 MPa

28

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC AND PLASTIC DEFORMATION

29

Elastic Shear Deformation Plastic Shear Deformation

PL = 1020

YS = 1536

UTS = 1625

0

200

400

600

800

1000

1200

1400

1600

1800

0 05 1 15

STRESS ndash STRAIN CURVESt

ress

(Mpa

)

Strain ()

YS (02) = 1536 Mpa

UTS = 1625 Mpa

E = 10200053 = 192 GPa

bull PL Proportional Limit

bull YS Yield Strength

bull UTS Ultimate Tensile Strength

bull E Elastic Modulus

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION

bull Mechanical properties that are measures of the elastic strain of dental materialsincludes

bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio

31

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YOUNGrsquoS MODULUS

bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph

bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain

bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity

32

E = 10200053 = 192 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic modulus of a tensile test specimen can be calculated as

bull By definition

Stress (σ)=119865119865119860119860

Strain (ε)=1205491205491198621198621198621198620

33

F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus

E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862

= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620

34

E = 10200053 = 192 GPa

Steep Line Higher modulus and more rigidity

Flat Line Lower modulus and less rigidity

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Principle of Elastic Recovery

bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain

bull After the force is removed the metal springs back to anamount equal to the total elastic strain

bull A final clearance of 25microm is available for the cement

35

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Impression Material

bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth

bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing

36

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Hookersquos Law

bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress

bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality

bull Hookes law can be expressed by the formula

bull The value of the constant depends on the material and the type of stress

37

119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930

= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930

STRESS ndash STRAIN CURVE Enamel amp Dentin

PL = 235CS = 262

PL = 176

CS = 234

0

50

100

150

200

250

300

Com

pres

sive

Sre

ss( M

pa)

Strain

Enamel Dentin

bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures

bull Enamel - more stiffer andbrittle than dentin

bull But dentin more flexible andtougher

EEnamel = 117 GPaEDentin = 336 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

POISSONrsquoS RATIO

bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions

bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section

bull Under compressive loading there is an increase in thecross-section

39

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio

bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains

bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03

bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio

40

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 11: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS amp STRAIN

Stressbull The force per unit area acting on millions of atoms or molecules in a given

plane of a materialbull Force acting per unit area

bull Unit of measurement is Megapascal (Mpa)

bull Stress is the internal resistance of a material to an external load applied onthat material

Stress (σ)= 119865119865 (119873119873)119860119860 (1198981198982)

11

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Type Of Stress Produced By Examples

Residual Stress

Stress caused within the material during the manufacturing process

During welding

StructuralStress

Stresses produced in the structure during function Weights they support provide the loadings

In abutments of fixed partial denture

Pressure Stress

Induced in vessels containing pressurized materials In dentures during processing under pressure and heat

Flow Stress Force of liquid striking against the wall acts as the load

Molten metal alloy striking the walls of the mould during casting

Thermal Stress

Material is subjected to internal stress due to different temperatures causing varying expansions in the material

Materials that undergo thermal stress such as inlay wax soldering and welding alloys

Fatigue Stress Stress caused due to cyclic rotation of a material Rotary instruments undergo rotational or cyclic fatigue

12

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull By means of the direction of force stresses can be classified as

bull Tensile stressbull Compressive stressbull Shear stressbull Flexural stress

13

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Tensile Stress

bull Tensile stress occurs when 2 sets of forces are directed away from each other in thesame straight line

bull Also when one end is constrained and the other end is subjected to a force awayfrom the constraint

14

bull It is caused by a load that tends to stretch or elongate a body

Mechanical Properties of Dental Materials - Dr Nithin Mathew 15

bull In fixed prosthodontics a sticky candy (Jujube) can be used toremove crowns by means of a tensile force when the patienttries to open the mouth after the candy has mechanicallybonded to opposing teeth or gums

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Compressive Stress

bull Compressive stress occurs when 2 sets of forces are directed towards each other inthe same straight line

bull Also when one end is constrained and the other end is subjected to a force towardsthe constraint

bull It is caused by a load that tends to compress or shorten a body

16

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Shear Stress

bull Shear stress occurs when 2 sets of forces are directed parallel to each other but notalong the same straight line

bull A shear stress tends to resist the sliding of one portion of a body over another

bull Shear stress can also be produced by a twisting ortorsional action on a material

17

Mechanical Properties of Dental Materials - Dr Nithin Mathew 18

bull Eg If a force is applied along the surface of tooth enamel by a sharp-edgedinstrument parallel to the interface between the enamel and the orthodonticbracket the bracket may debond by shear stress failure of the resin luting agent

Mechanical Properties of Dental Materials - Dr Nithin Mathew 19

In the oral cavity shear failure is unlikely to occur due to

1 Many of the brittle materials in restored tooth surfaces generally have roughcurved surfaces

2 The presence of chamfers bevels or changes in curvature of a bonded toothsurface

3 To produce shear failure the applied force must be located immediately adjacentto the interface The farther away from the interface the load is applied more likelythat it is a tensile failure

Mechanical Properties of Dental Materials - Dr Nithin Mathew 20

5 Since the tensile strength of brittle materials is usually well below their shearstrength values tensile failure is more likely to occur

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Flexural Stress (bending)

bull Force per unit area of a material that is subjected to flexural loading (bending)

bull A shear stress tends to resist the sliding of one portion of a body over another

bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component

21

Mechanical Properties of Dental Materials - Dr Nithin Mathew 22

bull Flexural stresses produced in a three-unit fixeddental prosthesis

bull Flexural stresses produced in a two-unit cantileverbridge

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull StrengthStrength of a material is defined as the average level of stress

at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize

23

Mechanical Properties of Dental Materials - Dr Nithin Mathew 24

bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component

bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage

bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture

25

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration

bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact

26

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRAIN

bull Defined as the change in length per unit original length

bull Strain of a material is reported as percentage()

bull Strain may be either elastic plastic or a combination of both elastic and plastic

bull Elastic strain is reversible ie it disappears when force is removed

bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed

Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)

27

Eg

bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2

bull If the wire is 01m long and if it stretches 0001m under the load

bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001

Strain (ε)= (120549120549119862119862)(1198621198620)

= 0001 11989811989801119898119898

= 00001 = 001

Stress (σ)= 200 119873119873

0000002 1198981198982 = 2002

x 106 = 100 MPa

28

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC AND PLASTIC DEFORMATION

29

Elastic Shear Deformation Plastic Shear Deformation

PL = 1020

YS = 1536

UTS = 1625

0

200

400

600

800

1000

1200

1400

1600

1800

0 05 1 15

STRESS ndash STRAIN CURVESt

ress

(Mpa

)

Strain ()

YS (02) = 1536 Mpa

UTS = 1625 Mpa

E = 10200053 = 192 GPa

bull PL Proportional Limit

bull YS Yield Strength

bull UTS Ultimate Tensile Strength

bull E Elastic Modulus

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION

bull Mechanical properties that are measures of the elastic strain of dental materialsincludes

bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio

31

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YOUNGrsquoS MODULUS

bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph

bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain

bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity

32

E = 10200053 = 192 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic modulus of a tensile test specimen can be calculated as

bull By definition

Stress (σ)=119865119865119860119860

Strain (ε)=1205491205491198621198621198621198620

33

F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus

E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862

= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620

34

E = 10200053 = 192 GPa

Steep Line Higher modulus and more rigidity

Flat Line Lower modulus and less rigidity

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Principle of Elastic Recovery

bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain

bull After the force is removed the metal springs back to anamount equal to the total elastic strain

bull A final clearance of 25microm is available for the cement

35

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Impression Material

bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth

bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing

36

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Hookersquos Law

bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress

bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality

bull Hookes law can be expressed by the formula

bull The value of the constant depends on the material and the type of stress

37

119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930

= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930

STRESS ndash STRAIN CURVE Enamel amp Dentin

PL = 235CS = 262

PL = 176

CS = 234

0

50

100

150

200

250

300

Com

pres

sive

Sre

ss( M

pa)

Strain

Enamel Dentin

bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures

bull Enamel - more stiffer andbrittle than dentin

bull But dentin more flexible andtougher

EEnamel = 117 GPaEDentin = 336 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

POISSONrsquoS RATIO

bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions

bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section

bull Under compressive loading there is an increase in thecross-section

39

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio

bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains

bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03

bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio

40

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 12: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Type Of Stress Produced By Examples

Residual Stress

Stress caused within the material during the manufacturing process

During welding

StructuralStress

Stresses produced in the structure during function Weights they support provide the loadings

In abutments of fixed partial denture

Pressure Stress

Induced in vessels containing pressurized materials In dentures during processing under pressure and heat

Flow Stress Force of liquid striking against the wall acts as the load

Molten metal alloy striking the walls of the mould during casting

Thermal Stress

Material is subjected to internal stress due to different temperatures causing varying expansions in the material

Materials that undergo thermal stress such as inlay wax soldering and welding alloys

Fatigue Stress Stress caused due to cyclic rotation of a material Rotary instruments undergo rotational or cyclic fatigue

12

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull By means of the direction of force stresses can be classified as

bull Tensile stressbull Compressive stressbull Shear stressbull Flexural stress

13

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Tensile Stress

bull Tensile stress occurs when 2 sets of forces are directed away from each other in thesame straight line

bull Also when one end is constrained and the other end is subjected to a force awayfrom the constraint

14

bull It is caused by a load that tends to stretch or elongate a body

Mechanical Properties of Dental Materials - Dr Nithin Mathew 15

bull In fixed prosthodontics a sticky candy (Jujube) can be used toremove crowns by means of a tensile force when the patienttries to open the mouth after the candy has mechanicallybonded to opposing teeth or gums

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Compressive Stress

bull Compressive stress occurs when 2 sets of forces are directed towards each other inthe same straight line

bull Also when one end is constrained and the other end is subjected to a force towardsthe constraint

bull It is caused by a load that tends to compress or shorten a body

16

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Shear Stress

bull Shear stress occurs when 2 sets of forces are directed parallel to each other but notalong the same straight line

bull A shear stress tends to resist the sliding of one portion of a body over another

bull Shear stress can also be produced by a twisting ortorsional action on a material

17

Mechanical Properties of Dental Materials - Dr Nithin Mathew 18

bull Eg If a force is applied along the surface of tooth enamel by a sharp-edgedinstrument parallel to the interface between the enamel and the orthodonticbracket the bracket may debond by shear stress failure of the resin luting agent

Mechanical Properties of Dental Materials - Dr Nithin Mathew 19

In the oral cavity shear failure is unlikely to occur due to

1 Many of the brittle materials in restored tooth surfaces generally have roughcurved surfaces

2 The presence of chamfers bevels or changes in curvature of a bonded toothsurface

3 To produce shear failure the applied force must be located immediately adjacentto the interface The farther away from the interface the load is applied more likelythat it is a tensile failure

Mechanical Properties of Dental Materials - Dr Nithin Mathew 20

5 Since the tensile strength of brittle materials is usually well below their shearstrength values tensile failure is more likely to occur

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Flexural Stress (bending)

bull Force per unit area of a material that is subjected to flexural loading (bending)

bull A shear stress tends to resist the sliding of one portion of a body over another

bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component

21

Mechanical Properties of Dental Materials - Dr Nithin Mathew 22

bull Flexural stresses produced in a three-unit fixeddental prosthesis

bull Flexural stresses produced in a two-unit cantileverbridge

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull StrengthStrength of a material is defined as the average level of stress

at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize

23

Mechanical Properties of Dental Materials - Dr Nithin Mathew 24

bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component

bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage

bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture

25

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration

bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact

26

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRAIN

bull Defined as the change in length per unit original length

bull Strain of a material is reported as percentage()

bull Strain may be either elastic plastic or a combination of both elastic and plastic

bull Elastic strain is reversible ie it disappears when force is removed

bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed

Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)

27

Eg

bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2

bull If the wire is 01m long and if it stretches 0001m under the load

bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001

Strain (ε)= (120549120549119862119862)(1198621198620)

= 0001 11989811989801119898119898

= 00001 = 001

Stress (σ)= 200 119873119873

0000002 1198981198982 = 2002

x 106 = 100 MPa

28

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC AND PLASTIC DEFORMATION

29

Elastic Shear Deformation Plastic Shear Deformation

PL = 1020

YS = 1536

UTS = 1625

0

200

400

600

800

1000

1200

1400

1600

1800

0 05 1 15

STRESS ndash STRAIN CURVESt

ress

(Mpa

)

Strain ()

YS (02) = 1536 Mpa

UTS = 1625 Mpa

E = 10200053 = 192 GPa

bull PL Proportional Limit

bull YS Yield Strength

bull UTS Ultimate Tensile Strength

bull E Elastic Modulus

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION

bull Mechanical properties that are measures of the elastic strain of dental materialsincludes

bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio

31

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YOUNGrsquoS MODULUS

bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph

bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain

bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity

32

E = 10200053 = 192 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic modulus of a tensile test specimen can be calculated as

bull By definition

Stress (σ)=119865119865119860119860

Strain (ε)=1205491205491198621198621198621198620

33

F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus

E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862

= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620

34

E = 10200053 = 192 GPa

Steep Line Higher modulus and more rigidity

Flat Line Lower modulus and less rigidity

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Principle of Elastic Recovery

bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain

bull After the force is removed the metal springs back to anamount equal to the total elastic strain

bull A final clearance of 25microm is available for the cement

35

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Impression Material

bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth

bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing

36

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Hookersquos Law

bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress

bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality

bull Hookes law can be expressed by the formula

bull The value of the constant depends on the material and the type of stress

37

119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930

= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930

STRESS ndash STRAIN CURVE Enamel amp Dentin

PL = 235CS = 262

PL = 176

CS = 234

0

50

100

150

200

250

300

Com

pres

sive

Sre

ss( M

pa)

Strain

Enamel Dentin

bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures

bull Enamel - more stiffer andbrittle than dentin

bull But dentin more flexible andtougher

EEnamel = 117 GPaEDentin = 336 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

POISSONrsquoS RATIO

bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions

bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section

bull Under compressive loading there is an increase in thecross-section

39

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio

bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains

bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03

bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio

40

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 13: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull By means of the direction of force stresses can be classified as

bull Tensile stressbull Compressive stressbull Shear stressbull Flexural stress

13

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Tensile Stress

bull Tensile stress occurs when 2 sets of forces are directed away from each other in thesame straight line

bull Also when one end is constrained and the other end is subjected to a force awayfrom the constraint

14

bull It is caused by a load that tends to stretch or elongate a body

Mechanical Properties of Dental Materials - Dr Nithin Mathew 15

bull In fixed prosthodontics a sticky candy (Jujube) can be used toremove crowns by means of a tensile force when the patienttries to open the mouth after the candy has mechanicallybonded to opposing teeth or gums

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Compressive Stress

bull Compressive stress occurs when 2 sets of forces are directed towards each other inthe same straight line

bull Also when one end is constrained and the other end is subjected to a force towardsthe constraint

bull It is caused by a load that tends to compress or shorten a body

16

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Shear Stress

bull Shear stress occurs when 2 sets of forces are directed parallel to each other but notalong the same straight line

bull A shear stress tends to resist the sliding of one portion of a body over another

bull Shear stress can also be produced by a twisting ortorsional action on a material

17

Mechanical Properties of Dental Materials - Dr Nithin Mathew 18

bull Eg If a force is applied along the surface of tooth enamel by a sharp-edgedinstrument parallel to the interface between the enamel and the orthodonticbracket the bracket may debond by shear stress failure of the resin luting agent

Mechanical Properties of Dental Materials - Dr Nithin Mathew 19

In the oral cavity shear failure is unlikely to occur due to

1 Many of the brittle materials in restored tooth surfaces generally have roughcurved surfaces

2 The presence of chamfers bevels or changes in curvature of a bonded toothsurface

3 To produce shear failure the applied force must be located immediately adjacentto the interface The farther away from the interface the load is applied more likelythat it is a tensile failure

Mechanical Properties of Dental Materials - Dr Nithin Mathew 20

5 Since the tensile strength of brittle materials is usually well below their shearstrength values tensile failure is more likely to occur

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Flexural Stress (bending)

bull Force per unit area of a material that is subjected to flexural loading (bending)

bull A shear stress tends to resist the sliding of one portion of a body over another

bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component

21

Mechanical Properties of Dental Materials - Dr Nithin Mathew 22

bull Flexural stresses produced in a three-unit fixeddental prosthesis

bull Flexural stresses produced in a two-unit cantileverbridge

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull StrengthStrength of a material is defined as the average level of stress

at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize

23

Mechanical Properties of Dental Materials - Dr Nithin Mathew 24

bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component

bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage

bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture

25

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration

bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact

26

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRAIN

bull Defined as the change in length per unit original length

bull Strain of a material is reported as percentage()

bull Strain may be either elastic plastic or a combination of both elastic and plastic

bull Elastic strain is reversible ie it disappears when force is removed

bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed

Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)

27

Eg

bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2

bull If the wire is 01m long and if it stretches 0001m under the load

bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001

Strain (ε)= (120549120549119862119862)(1198621198620)

= 0001 11989811989801119898119898

= 00001 = 001

Stress (σ)= 200 119873119873

0000002 1198981198982 = 2002

x 106 = 100 MPa

28

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC AND PLASTIC DEFORMATION

29

Elastic Shear Deformation Plastic Shear Deformation

PL = 1020

YS = 1536

UTS = 1625

0

200

400

600

800

1000

1200

1400

1600

1800

0 05 1 15

STRESS ndash STRAIN CURVESt

ress

(Mpa

)

Strain ()

YS (02) = 1536 Mpa

UTS = 1625 Mpa

E = 10200053 = 192 GPa

bull PL Proportional Limit

bull YS Yield Strength

bull UTS Ultimate Tensile Strength

bull E Elastic Modulus

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION

bull Mechanical properties that are measures of the elastic strain of dental materialsincludes

bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio

31

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YOUNGrsquoS MODULUS

bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph

bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain

bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity

32

E = 10200053 = 192 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic modulus of a tensile test specimen can be calculated as

bull By definition

Stress (σ)=119865119865119860119860

Strain (ε)=1205491205491198621198621198621198620

33

F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus

E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862

= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620

34

E = 10200053 = 192 GPa

Steep Line Higher modulus and more rigidity

Flat Line Lower modulus and less rigidity

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Principle of Elastic Recovery

bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain

bull After the force is removed the metal springs back to anamount equal to the total elastic strain

bull A final clearance of 25microm is available for the cement

35

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Impression Material

bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth

bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing

36

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Hookersquos Law

bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress

bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality

bull Hookes law can be expressed by the formula

bull The value of the constant depends on the material and the type of stress

37

119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930

= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930

STRESS ndash STRAIN CURVE Enamel amp Dentin

PL = 235CS = 262

PL = 176

CS = 234

0

50

100

150

200

250

300

Com

pres

sive

Sre

ss( M

pa)

Strain

Enamel Dentin

bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures

bull Enamel - more stiffer andbrittle than dentin

bull But dentin more flexible andtougher

EEnamel = 117 GPaEDentin = 336 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

POISSONrsquoS RATIO

bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions

bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section

bull Under compressive loading there is an increase in thecross-section

39

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio

bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains

bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03

bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio

40

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 14: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Tensile Stress

bull Tensile stress occurs when 2 sets of forces are directed away from each other in thesame straight line

bull Also when one end is constrained and the other end is subjected to a force awayfrom the constraint

14

bull It is caused by a load that tends to stretch or elongate a body

Mechanical Properties of Dental Materials - Dr Nithin Mathew 15

bull In fixed prosthodontics a sticky candy (Jujube) can be used toremove crowns by means of a tensile force when the patienttries to open the mouth after the candy has mechanicallybonded to opposing teeth or gums

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Compressive Stress

bull Compressive stress occurs when 2 sets of forces are directed towards each other inthe same straight line

bull Also when one end is constrained and the other end is subjected to a force towardsthe constraint

bull It is caused by a load that tends to compress or shorten a body

16

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Shear Stress

bull Shear stress occurs when 2 sets of forces are directed parallel to each other but notalong the same straight line

bull A shear stress tends to resist the sliding of one portion of a body over another

bull Shear stress can also be produced by a twisting ortorsional action on a material

17

Mechanical Properties of Dental Materials - Dr Nithin Mathew 18

bull Eg If a force is applied along the surface of tooth enamel by a sharp-edgedinstrument parallel to the interface between the enamel and the orthodonticbracket the bracket may debond by shear stress failure of the resin luting agent

Mechanical Properties of Dental Materials - Dr Nithin Mathew 19

In the oral cavity shear failure is unlikely to occur due to

1 Many of the brittle materials in restored tooth surfaces generally have roughcurved surfaces

2 The presence of chamfers bevels or changes in curvature of a bonded toothsurface

3 To produce shear failure the applied force must be located immediately adjacentto the interface The farther away from the interface the load is applied more likelythat it is a tensile failure

Mechanical Properties of Dental Materials - Dr Nithin Mathew 20

5 Since the tensile strength of brittle materials is usually well below their shearstrength values tensile failure is more likely to occur

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Flexural Stress (bending)

bull Force per unit area of a material that is subjected to flexural loading (bending)

bull A shear stress tends to resist the sliding of one portion of a body over another

bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component

21

Mechanical Properties of Dental Materials - Dr Nithin Mathew 22

bull Flexural stresses produced in a three-unit fixeddental prosthesis

bull Flexural stresses produced in a two-unit cantileverbridge

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull StrengthStrength of a material is defined as the average level of stress

at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize

23

Mechanical Properties of Dental Materials - Dr Nithin Mathew 24

bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component

bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage

bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture

25

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration

bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact

26

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRAIN

bull Defined as the change in length per unit original length

bull Strain of a material is reported as percentage()

bull Strain may be either elastic plastic or a combination of both elastic and plastic

bull Elastic strain is reversible ie it disappears when force is removed

bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed

Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)

27

Eg

bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2

bull If the wire is 01m long and if it stretches 0001m under the load

bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001

Strain (ε)= (120549120549119862119862)(1198621198620)

= 0001 11989811989801119898119898

= 00001 = 001

Stress (σ)= 200 119873119873

0000002 1198981198982 = 2002

x 106 = 100 MPa

28

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC AND PLASTIC DEFORMATION

29

Elastic Shear Deformation Plastic Shear Deformation

PL = 1020

YS = 1536

UTS = 1625

0

200

400

600

800

1000

1200

1400

1600

1800

0 05 1 15

STRESS ndash STRAIN CURVESt

ress

(Mpa

)

Strain ()

YS (02) = 1536 Mpa

UTS = 1625 Mpa

E = 10200053 = 192 GPa

bull PL Proportional Limit

bull YS Yield Strength

bull UTS Ultimate Tensile Strength

bull E Elastic Modulus

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION

bull Mechanical properties that are measures of the elastic strain of dental materialsincludes

bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio

31

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YOUNGrsquoS MODULUS

bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph

bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain

bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity

32

E = 10200053 = 192 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic modulus of a tensile test specimen can be calculated as

bull By definition

Stress (σ)=119865119865119860119860

Strain (ε)=1205491205491198621198621198621198620

33

F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus

E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862

= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620

34

E = 10200053 = 192 GPa

Steep Line Higher modulus and more rigidity

Flat Line Lower modulus and less rigidity

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Principle of Elastic Recovery

bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain

bull After the force is removed the metal springs back to anamount equal to the total elastic strain

bull A final clearance of 25microm is available for the cement

35

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Impression Material

bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth

bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing

36

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Hookersquos Law

bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress

bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality

bull Hookes law can be expressed by the formula

bull The value of the constant depends on the material and the type of stress

37

119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930

= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930

STRESS ndash STRAIN CURVE Enamel amp Dentin

PL = 235CS = 262

PL = 176

CS = 234

0

50

100

150

200

250

300

Com

pres

sive

Sre

ss( M

pa)

Strain

Enamel Dentin

bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures

bull Enamel - more stiffer andbrittle than dentin

bull But dentin more flexible andtougher

EEnamel = 117 GPaEDentin = 336 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

POISSONrsquoS RATIO

bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions

bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section

bull Under compressive loading there is an increase in thecross-section

39

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio

bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains

bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03

bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio

40

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 15: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew 15

bull In fixed prosthodontics a sticky candy (Jujube) can be used toremove crowns by means of a tensile force when the patienttries to open the mouth after the candy has mechanicallybonded to opposing teeth or gums

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Compressive Stress

bull Compressive stress occurs when 2 sets of forces are directed towards each other inthe same straight line

bull Also when one end is constrained and the other end is subjected to a force towardsthe constraint

bull It is caused by a load that tends to compress or shorten a body

16

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Shear Stress

bull Shear stress occurs when 2 sets of forces are directed parallel to each other but notalong the same straight line

bull A shear stress tends to resist the sliding of one portion of a body over another

bull Shear stress can also be produced by a twisting ortorsional action on a material

17

Mechanical Properties of Dental Materials - Dr Nithin Mathew 18

bull Eg If a force is applied along the surface of tooth enamel by a sharp-edgedinstrument parallel to the interface between the enamel and the orthodonticbracket the bracket may debond by shear stress failure of the resin luting agent

Mechanical Properties of Dental Materials - Dr Nithin Mathew 19

In the oral cavity shear failure is unlikely to occur due to

1 Many of the brittle materials in restored tooth surfaces generally have roughcurved surfaces

2 The presence of chamfers bevels or changes in curvature of a bonded toothsurface

3 To produce shear failure the applied force must be located immediately adjacentto the interface The farther away from the interface the load is applied more likelythat it is a tensile failure

Mechanical Properties of Dental Materials - Dr Nithin Mathew 20

5 Since the tensile strength of brittle materials is usually well below their shearstrength values tensile failure is more likely to occur

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Flexural Stress (bending)

bull Force per unit area of a material that is subjected to flexural loading (bending)

bull A shear stress tends to resist the sliding of one portion of a body over another

bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component

21

Mechanical Properties of Dental Materials - Dr Nithin Mathew 22

bull Flexural stresses produced in a three-unit fixeddental prosthesis

bull Flexural stresses produced in a two-unit cantileverbridge

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull StrengthStrength of a material is defined as the average level of stress

at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize

23

Mechanical Properties of Dental Materials - Dr Nithin Mathew 24

bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component

bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage

bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture

25

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration

bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact

26

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRAIN

bull Defined as the change in length per unit original length

bull Strain of a material is reported as percentage()

bull Strain may be either elastic plastic or a combination of both elastic and plastic

bull Elastic strain is reversible ie it disappears when force is removed

bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed

Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)

27

Eg

bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2

bull If the wire is 01m long and if it stretches 0001m under the load

bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001

Strain (ε)= (120549120549119862119862)(1198621198620)

= 0001 11989811989801119898119898

= 00001 = 001

Stress (σ)= 200 119873119873

0000002 1198981198982 = 2002

x 106 = 100 MPa

28

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC AND PLASTIC DEFORMATION

29

Elastic Shear Deformation Plastic Shear Deformation

PL = 1020

YS = 1536

UTS = 1625

0

200

400

600

800

1000

1200

1400

1600

1800

0 05 1 15

STRESS ndash STRAIN CURVESt

ress

(Mpa

)

Strain ()

YS (02) = 1536 Mpa

UTS = 1625 Mpa

E = 10200053 = 192 GPa

bull PL Proportional Limit

bull YS Yield Strength

bull UTS Ultimate Tensile Strength

bull E Elastic Modulus

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION

bull Mechanical properties that are measures of the elastic strain of dental materialsincludes

bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio

31

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YOUNGrsquoS MODULUS

bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph

bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain

bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity

32

E = 10200053 = 192 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic modulus of a tensile test specimen can be calculated as

bull By definition

Stress (σ)=119865119865119860119860

Strain (ε)=1205491205491198621198621198621198620

33

F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus

E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862

= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620

34

E = 10200053 = 192 GPa

Steep Line Higher modulus and more rigidity

Flat Line Lower modulus and less rigidity

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Principle of Elastic Recovery

bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain

bull After the force is removed the metal springs back to anamount equal to the total elastic strain

bull A final clearance of 25microm is available for the cement

35

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Impression Material

bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth

bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing

36

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Hookersquos Law

bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress

bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality

bull Hookes law can be expressed by the formula

bull The value of the constant depends on the material and the type of stress

37

119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930

= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930

STRESS ndash STRAIN CURVE Enamel amp Dentin

PL = 235CS = 262

PL = 176

CS = 234

0

50

100

150

200

250

300

Com

pres

sive

Sre

ss( M

pa)

Strain

Enamel Dentin

bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures

bull Enamel - more stiffer andbrittle than dentin

bull But dentin more flexible andtougher

EEnamel = 117 GPaEDentin = 336 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

POISSONrsquoS RATIO

bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions

bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section

bull Under compressive loading there is an increase in thecross-section

39

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio

bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains

bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03

bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio

40

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 16: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Compressive Stress

bull Compressive stress occurs when 2 sets of forces are directed towards each other inthe same straight line

bull Also when one end is constrained and the other end is subjected to a force towardsthe constraint

bull It is caused by a load that tends to compress or shorten a body

16

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Shear Stress

bull Shear stress occurs when 2 sets of forces are directed parallel to each other but notalong the same straight line

bull A shear stress tends to resist the sliding of one portion of a body over another

bull Shear stress can also be produced by a twisting ortorsional action on a material

17

Mechanical Properties of Dental Materials - Dr Nithin Mathew 18

bull Eg If a force is applied along the surface of tooth enamel by a sharp-edgedinstrument parallel to the interface between the enamel and the orthodonticbracket the bracket may debond by shear stress failure of the resin luting agent

Mechanical Properties of Dental Materials - Dr Nithin Mathew 19

In the oral cavity shear failure is unlikely to occur due to

1 Many of the brittle materials in restored tooth surfaces generally have roughcurved surfaces

2 The presence of chamfers bevels or changes in curvature of a bonded toothsurface

3 To produce shear failure the applied force must be located immediately adjacentto the interface The farther away from the interface the load is applied more likelythat it is a tensile failure

Mechanical Properties of Dental Materials - Dr Nithin Mathew 20

5 Since the tensile strength of brittle materials is usually well below their shearstrength values tensile failure is more likely to occur

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Flexural Stress (bending)

bull Force per unit area of a material that is subjected to flexural loading (bending)

bull A shear stress tends to resist the sliding of one portion of a body over another

bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component

21

Mechanical Properties of Dental Materials - Dr Nithin Mathew 22

bull Flexural stresses produced in a three-unit fixeddental prosthesis

bull Flexural stresses produced in a two-unit cantileverbridge

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull StrengthStrength of a material is defined as the average level of stress

at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize

23

Mechanical Properties of Dental Materials - Dr Nithin Mathew 24

bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component

bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage

bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture

25

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration

bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact

26

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRAIN

bull Defined as the change in length per unit original length

bull Strain of a material is reported as percentage()

bull Strain may be either elastic plastic or a combination of both elastic and plastic

bull Elastic strain is reversible ie it disappears when force is removed

bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed

Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)

27

Eg

bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2

bull If the wire is 01m long and if it stretches 0001m under the load

bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001

Strain (ε)= (120549120549119862119862)(1198621198620)

= 0001 11989811989801119898119898

= 00001 = 001

Stress (σ)= 200 119873119873

0000002 1198981198982 = 2002

x 106 = 100 MPa

28

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC AND PLASTIC DEFORMATION

29

Elastic Shear Deformation Plastic Shear Deformation

PL = 1020

YS = 1536

UTS = 1625

0

200

400

600

800

1000

1200

1400

1600

1800

0 05 1 15

STRESS ndash STRAIN CURVESt

ress

(Mpa

)

Strain ()

YS (02) = 1536 Mpa

UTS = 1625 Mpa

E = 10200053 = 192 GPa

bull PL Proportional Limit

bull YS Yield Strength

bull UTS Ultimate Tensile Strength

bull E Elastic Modulus

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION

bull Mechanical properties that are measures of the elastic strain of dental materialsincludes

bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio

31

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YOUNGrsquoS MODULUS

bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph

bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain

bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity

32

E = 10200053 = 192 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic modulus of a tensile test specimen can be calculated as

bull By definition

Stress (σ)=119865119865119860119860

Strain (ε)=1205491205491198621198621198621198620

33

F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus

E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862

= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620

34

E = 10200053 = 192 GPa

Steep Line Higher modulus and more rigidity

Flat Line Lower modulus and less rigidity

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Principle of Elastic Recovery

bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain

bull After the force is removed the metal springs back to anamount equal to the total elastic strain

bull A final clearance of 25microm is available for the cement

35

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Impression Material

bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth

bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing

36

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Hookersquos Law

bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress

bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality

bull Hookes law can be expressed by the formula

bull The value of the constant depends on the material and the type of stress

37

119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930

= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930

STRESS ndash STRAIN CURVE Enamel amp Dentin

PL = 235CS = 262

PL = 176

CS = 234

0

50

100

150

200

250

300

Com

pres

sive

Sre

ss( M

pa)

Strain

Enamel Dentin

bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures

bull Enamel - more stiffer andbrittle than dentin

bull But dentin more flexible andtougher

EEnamel = 117 GPaEDentin = 336 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

POISSONrsquoS RATIO

bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions

bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section

bull Under compressive loading there is an increase in thecross-section

39

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio

bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains

bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03

bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio

40

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 17: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Shear Stress

bull Shear stress occurs when 2 sets of forces are directed parallel to each other but notalong the same straight line

bull A shear stress tends to resist the sliding of one portion of a body over another

bull Shear stress can also be produced by a twisting ortorsional action on a material

17

Mechanical Properties of Dental Materials - Dr Nithin Mathew 18

bull Eg If a force is applied along the surface of tooth enamel by a sharp-edgedinstrument parallel to the interface between the enamel and the orthodonticbracket the bracket may debond by shear stress failure of the resin luting agent

Mechanical Properties of Dental Materials - Dr Nithin Mathew 19

In the oral cavity shear failure is unlikely to occur due to

1 Many of the brittle materials in restored tooth surfaces generally have roughcurved surfaces

2 The presence of chamfers bevels or changes in curvature of a bonded toothsurface

3 To produce shear failure the applied force must be located immediately adjacentto the interface The farther away from the interface the load is applied more likelythat it is a tensile failure

Mechanical Properties of Dental Materials - Dr Nithin Mathew 20

5 Since the tensile strength of brittle materials is usually well below their shearstrength values tensile failure is more likely to occur

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Flexural Stress (bending)

bull Force per unit area of a material that is subjected to flexural loading (bending)

bull A shear stress tends to resist the sliding of one portion of a body over another

bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component

21

Mechanical Properties of Dental Materials - Dr Nithin Mathew 22

bull Flexural stresses produced in a three-unit fixeddental prosthesis

bull Flexural stresses produced in a two-unit cantileverbridge

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull StrengthStrength of a material is defined as the average level of stress

at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize

23

Mechanical Properties of Dental Materials - Dr Nithin Mathew 24

bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component

bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage

bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture

25

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration

bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact

26

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRAIN

bull Defined as the change in length per unit original length

bull Strain of a material is reported as percentage()

bull Strain may be either elastic plastic or a combination of both elastic and plastic

bull Elastic strain is reversible ie it disappears when force is removed

bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed

Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)

27

Eg

bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2

bull If the wire is 01m long and if it stretches 0001m under the load

bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001

Strain (ε)= (120549120549119862119862)(1198621198620)

= 0001 11989811989801119898119898

= 00001 = 001

Stress (σ)= 200 119873119873

0000002 1198981198982 = 2002

x 106 = 100 MPa

28

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC AND PLASTIC DEFORMATION

29

Elastic Shear Deformation Plastic Shear Deformation

PL = 1020

YS = 1536

UTS = 1625

0

200

400

600

800

1000

1200

1400

1600

1800

0 05 1 15

STRESS ndash STRAIN CURVESt

ress

(Mpa

)

Strain ()

YS (02) = 1536 Mpa

UTS = 1625 Mpa

E = 10200053 = 192 GPa

bull PL Proportional Limit

bull YS Yield Strength

bull UTS Ultimate Tensile Strength

bull E Elastic Modulus

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION

bull Mechanical properties that are measures of the elastic strain of dental materialsincludes

bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio

31

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YOUNGrsquoS MODULUS

bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph

bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain

bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity

32

E = 10200053 = 192 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic modulus of a tensile test specimen can be calculated as

bull By definition

Stress (σ)=119865119865119860119860

Strain (ε)=1205491205491198621198621198621198620

33

F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus

E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862

= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620

34

E = 10200053 = 192 GPa

Steep Line Higher modulus and more rigidity

Flat Line Lower modulus and less rigidity

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Principle of Elastic Recovery

bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain

bull After the force is removed the metal springs back to anamount equal to the total elastic strain

bull A final clearance of 25microm is available for the cement

35

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Impression Material

bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth

bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing

36

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Hookersquos Law

bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress

bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality

bull Hookes law can be expressed by the formula

bull The value of the constant depends on the material and the type of stress

37

119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930

= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930

STRESS ndash STRAIN CURVE Enamel amp Dentin

PL = 235CS = 262

PL = 176

CS = 234

0

50

100

150

200

250

300

Com

pres

sive

Sre

ss( M

pa)

Strain

Enamel Dentin

bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures

bull Enamel - more stiffer andbrittle than dentin

bull But dentin more flexible andtougher

EEnamel = 117 GPaEDentin = 336 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

POISSONrsquoS RATIO

bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions

bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section

bull Under compressive loading there is an increase in thecross-section

39

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio

bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains

bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03

bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio

40

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 18: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew 18

bull Eg If a force is applied along the surface of tooth enamel by a sharp-edgedinstrument parallel to the interface between the enamel and the orthodonticbracket the bracket may debond by shear stress failure of the resin luting agent

Mechanical Properties of Dental Materials - Dr Nithin Mathew 19

In the oral cavity shear failure is unlikely to occur due to

1 Many of the brittle materials in restored tooth surfaces generally have roughcurved surfaces

2 The presence of chamfers bevels or changes in curvature of a bonded toothsurface

3 To produce shear failure the applied force must be located immediately adjacentto the interface The farther away from the interface the load is applied more likelythat it is a tensile failure

Mechanical Properties of Dental Materials - Dr Nithin Mathew 20

5 Since the tensile strength of brittle materials is usually well below their shearstrength values tensile failure is more likely to occur

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Flexural Stress (bending)

bull Force per unit area of a material that is subjected to flexural loading (bending)

bull A shear stress tends to resist the sliding of one portion of a body over another

bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component

21

Mechanical Properties of Dental Materials - Dr Nithin Mathew 22

bull Flexural stresses produced in a three-unit fixeddental prosthesis

bull Flexural stresses produced in a two-unit cantileverbridge

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull StrengthStrength of a material is defined as the average level of stress

at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize

23

Mechanical Properties of Dental Materials - Dr Nithin Mathew 24

bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component

bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage

bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture

25

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration

bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact

26

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRAIN

bull Defined as the change in length per unit original length

bull Strain of a material is reported as percentage()

bull Strain may be either elastic plastic or a combination of both elastic and plastic

bull Elastic strain is reversible ie it disappears when force is removed

bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed

Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)

27

Eg

bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2

bull If the wire is 01m long and if it stretches 0001m under the load

bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001

Strain (ε)= (120549120549119862119862)(1198621198620)

= 0001 11989811989801119898119898

= 00001 = 001

Stress (σ)= 200 119873119873

0000002 1198981198982 = 2002

x 106 = 100 MPa

28

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC AND PLASTIC DEFORMATION

29

Elastic Shear Deformation Plastic Shear Deformation

PL = 1020

YS = 1536

UTS = 1625

0

200

400

600

800

1000

1200

1400

1600

1800

0 05 1 15

STRESS ndash STRAIN CURVESt

ress

(Mpa

)

Strain ()

YS (02) = 1536 Mpa

UTS = 1625 Mpa

E = 10200053 = 192 GPa

bull PL Proportional Limit

bull YS Yield Strength

bull UTS Ultimate Tensile Strength

bull E Elastic Modulus

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION

bull Mechanical properties that are measures of the elastic strain of dental materialsincludes

bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio

31

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YOUNGrsquoS MODULUS

bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph

bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain

bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity

32

E = 10200053 = 192 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic modulus of a tensile test specimen can be calculated as

bull By definition

Stress (σ)=119865119865119860119860

Strain (ε)=1205491205491198621198621198621198620

33

F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus

E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862

= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620

34

E = 10200053 = 192 GPa

Steep Line Higher modulus and more rigidity

Flat Line Lower modulus and less rigidity

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Principle of Elastic Recovery

bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain

bull After the force is removed the metal springs back to anamount equal to the total elastic strain

bull A final clearance of 25microm is available for the cement

35

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Impression Material

bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth

bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing

36

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Hookersquos Law

bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress

bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality

bull Hookes law can be expressed by the formula

bull The value of the constant depends on the material and the type of stress

37

119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930

= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930

STRESS ndash STRAIN CURVE Enamel amp Dentin

PL = 235CS = 262

PL = 176

CS = 234

0

50

100

150

200

250

300

Com

pres

sive

Sre

ss( M

pa)

Strain

Enamel Dentin

bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures

bull Enamel - more stiffer andbrittle than dentin

bull But dentin more flexible andtougher

EEnamel = 117 GPaEDentin = 336 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

POISSONrsquoS RATIO

bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions

bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section

bull Under compressive loading there is an increase in thecross-section

39

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio

bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains

bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03

bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio

40

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 19: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew 19

In the oral cavity shear failure is unlikely to occur due to

1 Many of the brittle materials in restored tooth surfaces generally have roughcurved surfaces

2 The presence of chamfers bevels or changes in curvature of a bonded toothsurface

3 To produce shear failure the applied force must be located immediately adjacentto the interface The farther away from the interface the load is applied more likelythat it is a tensile failure

Mechanical Properties of Dental Materials - Dr Nithin Mathew 20

5 Since the tensile strength of brittle materials is usually well below their shearstrength values tensile failure is more likely to occur

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Flexural Stress (bending)

bull Force per unit area of a material that is subjected to flexural loading (bending)

bull A shear stress tends to resist the sliding of one portion of a body over another

bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component

21

Mechanical Properties of Dental Materials - Dr Nithin Mathew 22

bull Flexural stresses produced in a three-unit fixeddental prosthesis

bull Flexural stresses produced in a two-unit cantileverbridge

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull StrengthStrength of a material is defined as the average level of stress

at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize

23

Mechanical Properties of Dental Materials - Dr Nithin Mathew 24

bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component

bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage

bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture

25

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration

bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact

26

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRAIN

bull Defined as the change in length per unit original length

bull Strain of a material is reported as percentage()

bull Strain may be either elastic plastic or a combination of both elastic and plastic

bull Elastic strain is reversible ie it disappears when force is removed

bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed

Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)

27

Eg

bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2

bull If the wire is 01m long and if it stretches 0001m under the load

bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001

Strain (ε)= (120549120549119862119862)(1198621198620)

= 0001 11989811989801119898119898

= 00001 = 001

Stress (σ)= 200 119873119873

0000002 1198981198982 = 2002

x 106 = 100 MPa

28

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC AND PLASTIC DEFORMATION

29

Elastic Shear Deformation Plastic Shear Deformation

PL = 1020

YS = 1536

UTS = 1625

0

200

400

600

800

1000

1200

1400

1600

1800

0 05 1 15

STRESS ndash STRAIN CURVESt

ress

(Mpa

)

Strain ()

YS (02) = 1536 Mpa

UTS = 1625 Mpa

E = 10200053 = 192 GPa

bull PL Proportional Limit

bull YS Yield Strength

bull UTS Ultimate Tensile Strength

bull E Elastic Modulus

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION

bull Mechanical properties that are measures of the elastic strain of dental materialsincludes

bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio

31

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YOUNGrsquoS MODULUS

bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph

bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain

bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity

32

E = 10200053 = 192 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic modulus of a tensile test specimen can be calculated as

bull By definition

Stress (σ)=119865119865119860119860

Strain (ε)=1205491205491198621198621198621198620

33

F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus

E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862

= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620

34

E = 10200053 = 192 GPa

Steep Line Higher modulus and more rigidity

Flat Line Lower modulus and less rigidity

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Principle of Elastic Recovery

bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain

bull After the force is removed the metal springs back to anamount equal to the total elastic strain

bull A final clearance of 25microm is available for the cement

35

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Impression Material

bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth

bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing

36

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Hookersquos Law

bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress

bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality

bull Hookes law can be expressed by the formula

bull The value of the constant depends on the material and the type of stress

37

119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930

= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930

STRESS ndash STRAIN CURVE Enamel amp Dentin

PL = 235CS = 262

PL = 176

CS = 234

0

50

100

150

200

250

300

Com

pres

sive

Sre

ss( M

pa)

Strain

Enamel Dentin

bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures

bull Enamel - more stiffer andbrittle than dentin

bull But dentin more flexible andtougher

EEnamel = 117 GPaEDentin = 336 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

POISSONrsquoS RATIO

bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions

bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section

bull Under compressive loading there is an increase in thecross-section

39

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio

bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains

bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03

bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio

40

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 20: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew 20

5 Since the tensile strength of brittle materials is usually well below their shearstrength values tensile failure is more likely to occur

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Flexural Stress (bending)

bull Force per unit area of a material that is subjected to flexural loading (bending)

bull A shear stress tends to resist the sliding of one portion of a body over another

bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component

21

Mechanical Properties of Dental Materials - Dr Nithin Mathew 22

bull Flexural stresses produced in a three-unit fixeddental prosthesis

bull Flexural stresses produced in a two-unit cantileverbridge

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull StrengthStrength of a material is defined as the average level of stress

at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize

23

Mechanical Properties of Dental Materials - Dr Nithin Mathew 24

bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component

bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage

bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture

25

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration

bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact

26

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRAIN

bull Defined as the change in length per unit original length

bull Strain of a material is reported as percentage()

bull Strain may be either elastic plastic or a combination of both elastic and plastic

bull Elastic strain is reversible ie it disappears when force is removed

bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed

Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)

27

Eg

bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2

bull If the wire is 01m long and if it stretches 0001m under the load

bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001

Strain (ε)= (120549120549119862119862)(1198621198620)

= 0001 11989811989801119898119898

= 00001 = 001

Stress (σ)= 200 119873119873

0000002 1198981198982 = 2002

x 106 = 100 MPa

28

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC AND PLASTIC DEFORMATION

29

Elastic Shear Deformation Plastic Shear Deformation

PL = 1020

YS = 1536

UTS = 1625

0

200

400

600

800

1000

1200

1400

1600

1800

0 05 1 15

STRESS ndash STRAIN CURVESt

ress

(Mpa

)

Strain ()

YS (02) = 1536 Mpa

UTS = 1625 Mpa

E = 10200053 = 192 GPa

bull PL Proportional Limit

bull YS Yield Strength

bull UTS Ultimate Tensile Strength

bull E Elastic Modulus

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION

bull Mechanical properties that are measures of the elastic strain of dental materialsincludes

bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio

31

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YOUNGrsquoS MODULUS

bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph

bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain

bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity

32

E = 10200053 = 192 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic modulus of a tensile test specimen can be calculated as

bull By definition

Stress (σ)=119865119865119860119860

Strain (ε)=1205491205491198621198621198621198620

33

F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus

E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862

= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620

34

E = 10200053 = 192 GPa

Steep Line Higher modulus and more rigidity

Flat Line Lower modulus and less rigidity

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Principle of Elastic Recovery

bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain

bull After the force is removed the metal springs back to anamount equal to the total elastic strain

bull A final clearance of 25microm is available for the cement

35

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Impression Material

bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth

bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing

36

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Hookersquos Law

bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress

bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality

bull Hookes law can be expressed by the formula

bull The value of the constant depends on the material and the type of stress

37

119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930

= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930

STRESS ndash STRAIN CURVE Enamel amp Dentin

PL = 235CS = 262

PL = 176

CS = 234

0

50

100

150

200

250

300

Com

pres

sive

Sre

ss( M

pa)

Strain

Enamel Dentin

bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures

bull Enamel - more stiffer andbrittle than dentin

bull But dentin more flexible andtougher

EEnamel = 117 GPaEDentin = 336 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

POISSONrsquoS RATIO

bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions

bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section

bull Under compressive loading there is an increase in thecross-section

39

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio

bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains

bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03

bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio

40

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 21: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Flexural Stress (bending)

bull Force per unit area of a material that is subjected to flexural loading (bending)

bull A shear stress tends to resist the sliding of one portion of a body over another

bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component

21

Mechanical Properties of Dental Materials - Dr Nithin Mathew 22

bull Flexural stresses produced in a three-unit fixeddental prosthesis

bull Flexural stresses produced in a two-unit cantileverbridge

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull StrengthStrength of a material is defined as the average level of stress

at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize

23

Mechanical Properties of Dental Materials - Dr Nithin Mathew 24

bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component

bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage

bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture

25

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration

bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact

26

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRAIN

bull Defined as the change in length per unit original length

bull Strain of a material is reported as percentage()

bull Strain may be either elastic plastic or a combination of both elastic and plastic

bull Elastic strain is reversible ie it disappears when force is removed

bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed

Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)

27

Eg

bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2

bull If the wire is 01m long and if it stretches 0001m under the load

bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001

Strain (ε)= (120549120549119862119862)(1198621198620)

= 0001 11989811989801119898119898

= 00001 = 001

Stress (σ)= 200 119873119873

0000002 1198981198982 = 2002

x 106 = 100 MPa

28

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC AND PLASTIC DEFORMATION

29

Elastic Shear Deformation Plastic Shear Deformation

PL = 1020

YS = 1536

UTS = 1625

0

200

400

600

800

1000

1200

1400

1600

1800

0 05 1 15

STRESS ndash STRAIN CURVESt

ress

(Mpa

)

Strain ()

YS (02) = 1536 Mpa

UTS = 1625 Mpa

E = 10200053 = 192 GPa

bull PL Proportional Limit

bull YS Yield Strength

bull UTS Ultimate Tensile Strength

bull E Elastic Modulus

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION

bull Mechanical properties that are measures of the elastic strain of dental materialsincludes

bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio

31

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YOUNGrsquoS MODULUS

bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph

bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain

bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity

32

E = 10200053 = 192 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic modulus of a tensile test specimen can be calculated as

bull By definition

Stress (σ)=119865119865119860119860

Strain (ε)=1205491205491198621198621198621198620

33

F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus

E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862

= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620

34

E = 10200053 = 192 GPa

Steep Line Higher modulus and more rigidity

Flat Line Lower modulus and less rigidity

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Principle of Elastic Recovery

bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain

bull After the force is removed the metal springs back to anamount equal to the total elastic strain

bull A final clearance of 25microm is available for the cement

35

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Impression Material

bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth

bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing

36

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Hookersquos Law

bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress

bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality

bull Hookes law can be expressed by the formula

bull The value of the constant depends on the material and the type of stress

37

119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930

= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930

STRESS ndash STRAIN CURVE Enamel amp Dentin

PL = 235CS = 262

PL = 176

CS = 234

0

50

100

150

200

250

300

Com

pres

sive

Sre

ss( M

pa)

Strain

Enamel Dentin

bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures

bull Enamel - more stiffer andbrittle than dentin

bull But dentin more flexible andtougher

EEnamel = 117 GPaEDentin = 336 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

POISSONrsquoS RATIO

bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions

bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section

bull Under compressive loading there is an increase in thecross-section

39

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio

bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains

bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03

bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio

40

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 22: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew 22

bull Flexural stresses produced in a three-unit fixeddental prosthesis

bull Flexural stresses produced in a two-unit cantileverbridge

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull StrengthStrength of a material is defined as the average level of stress

at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize

23

Mechanical Properties of Dental Materials - Dr Nithin Mathew 24

bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component

bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage

bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture

25

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration

bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact

26

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRAIN

bull Defined as the change in length per unit original length

bull Strain of a material is reported as percentage()

bull Strain may be either elastic plastic or a combination of both elastic and plastic

bull Elastic strain is reversible ie it disappears when force is removed

bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed

Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)

27

Eg

bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2

bull If the wire is 01m long and if it stretches 0001m under the load

bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001

Strain (ε)= (120549120549119862119862)(1198621198620)

= 0001 11989811989801119898119898

= 00001 = 001

Stress (σ)= 200 119873119873

0000002 1198981198982 = 2002

x 106 = 100 MPa

28

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC AND PLASTIC DEFORMATION

29

Elastic Shear Deformation Plastic Shear Deformation

PL = 1020

YS = 1536

UTS = 1625

0

200

400

600

800

1000

1200

1400

1600

1800

0 05 1 15

STRESS ndash STRAIN CURVESt

ress

(Mpa

)

Strain ()

YS (02) = 1536 Mpa

UTS = 1625 Mpa

E = 10200053 = 192 GPa

bull PL Proportional Limit

bull YS Yield Strength

bull UTS Ultimate Tensile Strength

bull E Elastic Modulus

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION

bull Mechanical properties that are measures of the elastic strain of dental materialsincludes

bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio

31

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YOUNGrsquoS MODULUS

bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph

bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain

bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity

32

E = 10200053 = 192 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic modulus of a tensile test specimen can be calculated as

bull By definition

Stress (σ)=119865119865119860119860

Strain (ε)=1205491205491198621198621198621198620

33

F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus

E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862

= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620

34

E = 10200053 = 192 GPa

Steep Line Higher modulus and more rigidity

Flat Line Lower modulus and less rigidity

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Principle of Elastic Recovery

bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain

bull After the force is removed the metal springs back to anamount equal to the total elastic strain

bull A final clearance of 25microm is available for the cement

35

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Impression Material

bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth

bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing

36

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Hookersquos Law

bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress

bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality

bull Hookes law can be expressed by the formula

bull The value of the constant depends on the material and the type of stress

37

119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930

= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930

STRESS ndash STRAIN CURVE Enamel amp Dentin

PL = 235CS = 262

PL = 176

CS = 234

0

50

100

150

200

250

300

Com

pres

sive

Sre

ss( M

pa)

Strain

Enamel Dentin

bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures

bull Enamel - more stiffer andbrittle than dentin

bull But dentin more flexible andtougher

EEnamel = 117 GPaEDentin = 336 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

POISSONrsquoS RATIO

bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions

bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section

bull Under compressive loading there is an increase in thecross-section

39

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio

bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains

bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03

bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio

40

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 23: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull StrengthStrength of a material is defined as the average level of stress

at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize

23

Mechanical Properties of Dental Materials - Dr Nithin Mathew 24

bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component

bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage

bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture

25

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration

bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact

26

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRAIN

bull Defined as the change in length per unit original length

bull Strain of a material is reported as percentage()

bull Strain may be either elastic plastic or a combination of both elastic and plastic

bull Elastic strain is reversible ie it disappears when force is removed

bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed

Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)

27

Eg

bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2

bull If the wire is 01m long and if it stretches 0001m under the load

bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001

Strain (ε)= (120549120549119862119862)(1198621198620)

= 0001 11989811989801119898119898

= 00001 = 001

Stress (σ)= 200 119873119873

0000002 1198981198982 = 2002

x 106 = 100 MPa

28

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC AND PLASTIC DEFORMATION

29

Elastic Shear Deformation Plastic Shear Deformation

PL = 1020

YS = 1536

UTS = 1625

0

200

400

600

800

1000

1200

1400

1600

1800

0 05 1 15

STRESS ndash STRAIN CURVESt

ress

(Mpa

)

Strain ()

YS (02) = 1536 Mpa

UTS = 1625 Mpa

E = 10200053 = 192 GPa

bull PL Proportional Limit

bull YS Yield Strength

bull UTS Ultimate Tensile Strength

bull E Elastic Modulus

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION

bull Mechanical properties that are measures of the elastic strain of dental materialsincludes

bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio

31

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YOUNGrsquoS MODULUS

bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph

bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain

bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity

32

E = 10200053 = 192 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic modulus of a tensile test specimen can be calculated as

bull By definition

Stress (σ)=119865119865119860119860

Strain (ε)=1205491205491198621198621198621198620

33

F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus

E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862

= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620

34

E = 10200053 = 192 GPa

Steep Line Higher modulus and more rigidity

Flat Line Lower modulus and less rigidity

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Principle of Elastic Recovery

bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain

bull After the force is removed the metal springs back to anamount equal to the total elastic strain

bull A final clearance of 25microm is available for the cement

35

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Impression Material

bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth

bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing

36

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Hookersquos Law

bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress

bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality

bull Hookes law can be expressed by the formula

bull The value of the constant depends on the material and the type of stress

37

119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930

= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930

STRESS ndash STRAIN CURVE Enamel amp Dentin

PL = 235CS = 262

PL = 176

CS = 234

0

50

100

150

200

250

300

Com

pres

sive

Sre

ss( M

pa)

Strain

Enamel Dentin

bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures

bull Enamel - more stiffer andbrittle than dentin

bull But dentin more flexible andtougher

EEnamel = 117 GPaEDentin = 336 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

POISSONrsquoS RATIO

bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions

bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section

bull Under compressive loading there is an increase in thecross-section

39

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio

bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains

bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03

bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio

40

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 24: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew 24

bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component

bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage

bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture

25

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration

bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact

26

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRAIN

bull Defined as the change in length per unit original length

bull Strain of a material is reported as percentage()

bull Strain may be either elastic plastic or a combination of both elastic and plastic

bull Elastic strain is reversible ie it disappears when force is removed

bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed

Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)

27

Eg

bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2

bull If the wire is 01m long and if it stretches 0001m under the load

bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001

Strain (ε)= (120549120549119862119862)(1198621198620)

= 0001 11989811989801119898119898

= 00001 = 001

Stress (σ)= 200 119873119873

0000002 1198981198982 = 2002

x 106 = 100 MPa

28

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC AND PLASTIC DEFORMATION

29

Elastic Shear Deformation Plastic Shear Deformation

PL = 1020

YS = 1536

UTS = 1625

0

200

400

600

800

1000

1200

1400

1600

1800

0 05 1 15

STRESS ndash STRAIN CURVESt

ress

(Mpa

)

Strain ()

YS (02) = 1536 Mpa

UTS = 1625 Mpa

E = 10200053 = 192 GPa

bull PL Proportional Limit

bull YS Yield Strength

bull UTS Ultimate Tensile Strength

bull E Elastic Modulus

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION

bull Mechanical properties that are measures of the elastic strain of dental materialsincludes

bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio

31

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YOUNGrsquoS MODULUS

bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph

bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain

bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity

32

E = 10200053 = 192 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic modulus of a tensile test specimen can be calculated as

bull By definition

Stress (σ)=119865119865119860119860

Strain (ε)=1205491205491198621198621198621198620

33

F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus

E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862

= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620

34

E = 10200053 = 192 GPa

Steep Line Higher modulus and more rigidity

Flat Line Lower modulus and less rigidity

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Principle of Elastic Recovery

bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain

bull After the force is removed the metal springs back to anamount equal to the total elastic strain

bull A final clearance of 25microm is available for the cement

35

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Impression Material

bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth

bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing

36

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Hookersquos Law

bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress

bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality

bull Hookes law can be expressed by the formula

bull The value of the constant depends on the material and the type of stress

37

119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930

= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930

STRESS ndash STRAIN CURVE Enamel amp Dentin

PL = 235CS = 262

PL = 176

CS = 234

0

50

100

150

200

250

300

Com

pres

sive

Sre

ss( M

pa)

Strain

Enamel Dentin

bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures

bull Enamel - more stiffer andbrittle than dentin

bull But dentin more flexible andtougher

EEnamel = 117 GPaEDentin = 336 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

POISSONrsquoS RATIO

bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions

bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section

bull Under compressive loading there is an increase in thecross-section

39

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio

bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains

bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03

bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio

40

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 25: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage

bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture

25

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration

bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact

26

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRAIN

bull Defined as the change in length per unit original length

bull Strain of a material is reported as percentage()

bull Strain may be either elastic plastic or a combination of both elastic and plastic

bull Elastic strain is reversible ie it disappears when force is removed

bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed

Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)

27

Eg

bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2

bull If the wire is 01m long and if it stretches 0001m under the load

bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001

Strain (ε)= (120549120549119862119862)(1198621198620)

= 0001 11989811989801119898119898

= 00001 = 001

Stress (σ)= 200 119873119873

0000002 1198981198982 = 2002

x 106 = 100 MPa

28

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC AND PLASTIC DEFORMATION

29

Elastic Shear Deformation Plastic Shear Deformation

PL = 1020

YS = 1536

UTS = 1625

0

200

400

600

800

1000

1200

1400

1600

1800

0 05 1 15

STRESS ndash STRAIN CURVESt

ress

(Mpa

)

Strain ()

YS (02) = 1536 Mpa

UTS = 1625 Mpa

E = 10200053 = 192 GPa

bull PL Proportional Limit

bull YS Yield Strength

bull UTS Ultimate Tensile Strength

bull E Elastic Modulus

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION

bull Mechanical properties that are measures of the elastic strain of dental materialsincludes

bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio

31

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YOUNGrsquoS MODULUS

bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph

bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain

bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity

32

E = 10200053 = 192 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic modulus of a tensile test specimen can be calculated as

bull By definition

Stress (σ)=119865119865119860119860

Strain (ε)=1205491205491198621198621198621198620

33

F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus

E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862

= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620

34

E = 10200053 = 192 GPa

Steep Line Higher modulus and more rigidity

Flat Line Lower modulus and less rigidity

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Principle of Elastic Recovery

bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain

bull After the force is removed the metal springs back to anamount equal to the total elastic strain

bull A final clearance of 25microm is available for the cement

35

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Impression Material

bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth

bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing

36

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Hookersquos Law

bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress

bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality

bull Hookes law can be expressed by the formula

bull The value of the constant depends on the material and the type of stress

37

119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930

= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930

STRESS ndash STRAIN CURVE Enamel amp Dentin

PL = 235CS = 262

PL = 176

CS = 234

0

50

100

150

200

250

300

Com

pres

sive

Sre

ss( M

pa)

Strain

Enamel Dentin

bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures

bull Enamel - more stiffer andbrittle than dentin

bull But dentin more flexible andtougher

EEnamel = 117 GPaEDentin = 336 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

POISSONrsquoS RATIO

bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions

bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section

bull Under compressive loading there is an increase in thecross-section

39

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio

bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains

bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03

bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio

40

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 26: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration

bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact

26

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRAIN

bull Defined as the change in length per unit original length

bull Strain of a material is reported as percentage()

bull Strain may be either elastic plastic or a combination of both elastic and plastic

bull Elastic strain is reversible ie it disappears when force is removed

bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed

Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)

27

Eg

bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2

bull If the wire is 01m long and if it stretches 0001m under the load

bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001

Strain (ε)= (120549120549119862119862)(1198621198620)

= 0001 11989811989801119898119898

= 00001 = 001

Stress (σ)= 200 119873119873

0000002 1198981198982 = 2002

x 106 = 100 MPa

28

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC AND PLASTIC DEFORMATION

29

Elastic Shear Deformation Plastic Shear Deformation

PL = 1020

YS = 1536

UTS = 1625

0

200

400

600

800

1000

1200

1400

1600

1800

0 05 1 15

STRESS ndash STRAIN CURVESt

ress

(Mpa

)

Strain ()

YS (02) = 1536 Mpa

UTS = 1625 Mpa

E = 10200053 = 192 GPa

bull PL Proportional Limit

bull YS Yield Strength

bull UTS Ultimate Tensile Strength

bull E Elastic Modulus

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION

bull Mechanical properties that are measures of the elastic strain of dental materialsincludes

bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio

31

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YOUNGrsquoS MODULUS

bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph

bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain

bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity

32

E = 10200053 = 192 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic modulus of a tensile test specimen can be calculated as

bull By definition

Stress (σ)=119865119865119860119860

Strain (ε)=1205491205491198621198621198621198620

33

F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus

E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862

= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620

34

E = 10200053 = 192 GPa

Steep Line Higher modulus and more rigidity

Flat Line Lower modulus and less rigidity

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Principle of Elastic Recovery

bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain

bull After the force is removed the metal springs back to anamount equal to the total elastic strain

bull A final clearance of 25microm is available for the cement

35

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Impression Material

bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth

bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing

36

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Hookersquos Law

bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress

bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality

bull Hookes law can be expressed by the formula

bull The value of the constant depends on the material and the type of stress

37

119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930

= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930

STRESS ndash STRAIN CURVE Enamel amp Dentin

PL = 235CS = 262

PL = 176

CS = 234

0

50

100

150

200

250

300

Com

pres

sive

Sre

ss( M

pa)

Strain

Enamel Dentin

bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures

bull Enamel - more stiffer andbrittle than dentin

bull But dentin more flexible andtougher

EEnamel = 117 GPaEDentin = 336 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

POISSONrsquoS RATIO

bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions

bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section

bull Under compressive loading there is an increase in thecross-section

39

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio

bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains

bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03

bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio

40

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 27: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRAIN

bull Defined as the change in length per unit original length

bull Strain of a material is reported as percentage()

bull Strain may be either elastic plastic or a combination of both elastic and plastic

bull Elastic strain is reversible ie it disappears when force is removed

bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed

Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)

27

Eg

bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2

bull If the wire is 01m long and if it stretches 0001m under the load

bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001

Strain (ε)= (120549120549119862119862)(1198621198620)

= 0001 11989811989801119898119898

= 00001 = 001

Stress (σ)= 200 119873119873

0000002 1198981198982 = 2002

x 106 = 100 MPa

28

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC AND PLASTIC DEFORMATION

29

Elastic Shear Deformation Plastic Shear Deformation

PL = 1020

YS = 1536

UTS = 1625

0

200

400

600

800

1000

1200

1400

1600

1800

0 05 1 15

STRESS ndash STRAIN CURVESt

ress

(Mpa

)

Strain ()

YS (02) = 1536 Mpa

UTS = 1625 Mpa

E = 10200053 = 192 GPa

bull PL Proportional Limit

bull YS Yield Strength

bull UTS Ultimate Tensile Strength

bull E Elastic Modulus

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION

bull Mechanical properties that are measures of the elastic strain of dental materialsincludes

bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio

31

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YOUNGrsquoS MODULUS

bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph

bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain

bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity

32

E = 10200053 = 192 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic modulus of a tensile test specimen can be calculated as

bull By definition

Stress (σ)=119865119865119860119860

Strain (ε)=1205491205491198621198621198621198620

33

F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus

E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862

= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620

34

E = 10200053 = 192 GPa

Steep Line Higher modulus and more rigidity

Flat Line Lower modulus and less rigidity

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Principle of Elastic Recovery

bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain

bull After the force is removed the metal springs back to anamount equal to the total elastic strain

bull A final clearance of 25microm is available for the cement

35

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Impression Material

bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth

bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing

36

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Hookersquos Law

bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress

bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality

bull Hookes law can be expressed by the formula

bull The value of the constant depends on the material and the type of stress

37

119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930

= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930

STRESS ndash STRAIN CURVE Enamel amp Dentin

PL = 235CS = 262

PL = 176

CS = 234

0

50

100

150

200

250

300

Com

pres

sive

Sre

ss( M

pa)

Strain

Enamel Dentin

bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures

bull Enamel - more stiffer andbrittle than dentin

bull But dentin more flexible andtougher

EEnamel = 117 GPaEDentin = 336 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

POISSONrsquoS RATIO

bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions

bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section

bull Under compressive loading there is an increase in thecross-section

39

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio

bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains

bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03

bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio

40

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 28: Mechanical Properties of Dental Materials

Eg

bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2

bull If the wire is 01m long and if it stretches 0001m under the load

bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001

Strain (ε)= (120549120549119862119862)(1198621198620)

= 0001 11989811989801119898119898

= 00001 = 001

Stress (σ)= 200 119873119873

0000002 1198981198982 = 2002

x 106 = 100 MPa

28

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC AND PLASTIC DEFORMATION

29

Elastic Shear Deformation Plastic Shear Deformation

PL = 1020

YS = 1536

UTS = 1625

0

200

400

600

800

1000

1200

1400

1600

1800

0 05 1 15

STRESS ndash STRAIN CURVESt

ress

(Mpa

)

Strain ()

YS (02) = 1536 Mpa

UTS = 1625 Mpa

E = 10200053 = 192 GPa

bull PL Proportional Limit

bull YS Yield Strength

bull UTS Ultimate Tensile Strength

bull E Elastic Modulus

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION

bull Mechanical properties that are measures of the elastic strain of dental materialsincludes

bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio

31

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YOUNGrsquoS MODULUS

bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph

bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain

bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity

32

E = 10200053 = 192 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic modulus of a tensile test specimen can be calculated as

bull By definition

Stress (σ)=119865119865119860119860

Strain (ε)=1205491205491198621198621198621198620

33

F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus

E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862

= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620

34

E = 10200053 = 192 GPa

Steep Line Higher modulus and more rigidity

Flat Line Lower modulus and less rigidity

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Principle of Elastic Recovery

bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain

bull After the force is removed the metal springs back to anamount equal to the total elastic strain

bull A final clearance of 25microm is available for the cement

35

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Impression Material

bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth

bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing

36

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Hookersquos Law

bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress

bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality

bull Hookes law can be expressed by the formula

bull The value of the constant depends on the material and the type of stress

37

119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930

= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930

STRESS ndash STRAIN CURVE Enamel amp Dentin

PL = 235CS = 262

PL = 176

CS = 234

0

50

100

150

200

250

300

Com

pres

sive

Sre

ss( M

pa)

Strain

Enamel Dentin

bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures

bull Enamel - more stiffer andbrittle than dentin

bull But dentin more flexible andtougher

EEnamel = 117 GPaEDentin = 336 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

POISSONrsquoS RATIO

bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions

bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section

bull Under compressive loading there is an increase in thecross-section

39

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio

bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains

bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03

bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio

40

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 29: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC AND PLASTIC DEFORMATION

29

Elastic Shear Deformation Plastic Shear Deformation

PL = 1020

YS = 1536

UTS = 1625

0

200

400

600

800

1000

1200

1400

1600

1800

0 05 1 15

STRESS ndash STRAIN CURVESt

ress

(Mpa

)

Strain ()

YS (02) = 1536 Mpa

UTS = 1625 Mpa

E = 10200053 = 192 GPa

bull PL Proportional Limit

bull YS Yield Strength

bull UTS Ultimate Tensile Strength

bull E Elastic Modulus

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION

bull Mechanical properties that are measures of the elastic strain of dental materialsincludes

bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio

31

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YOUNGrsquoS MODULUS

bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph

bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain

bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity

32

E = 10200053 = 192 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic modulus of a tensile test specimen can be calculated as

bull By definition

Stress (σ)=119865119865119860119860

Strain (ε)=1205491205491198621198621198621198620

33

F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus

E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862

= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620

34

E = 10200053 = 192 GPa

Steep Line Higher modulus and more rigidity

Flat Line Lower modulus and less rigidity

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Principle of Elastic Recovery

bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain

bull After the force is removed the metal springs back to anamount equal to the total elastic strain

bull A final clearance of 25microm is available for the cement

35

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Impression Material

bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth

bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing

36

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Hookersquos Law

bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress

bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality

bull Hookes law can be expressed by the formula

bull The value of the constant depends on the material and the type of stress

37

119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930

= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930

STRESS ndash STRAIN CURVE Enamel amp Dentin

PL = 235CS = 262

PL = 176

CS = 234

0

50

100

150

200

250

300

Com

pres

sive

Sre

ss( M

pa)

Strain

Enamel Dentin

bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures

bull Enamel - more stiffer andbrittle than dentin

bull But dentin more flexible andtougher

EEnamel = 117 GPaEDentin = 336 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

POISSONrsquoS RATIO

bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions

bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section

bull Under compressive loading there is an increase in thecross-section

39

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio

bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains

bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03

bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio

40

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 30: Mechanical Properties of Dental Materials

PL = 1020

YS = 1536

UTS = 1625

0

200

400

600

800

1000

1200

1400

1600

1800

0 05 1 15

STRESS ndash STRAIN CURVESt

ress

(Mpa

)

Strain ()

YS (02) = 1536 Mpa

UTS = 1625 Mpa

E = 10200053 = 192 GPa

bull PL Proportional Limit

bull YS Yield Strength

bull UTS Ultimate Tensile Strength

bull E Elastic Modulus

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION

bull Mechanical properties that are measures of the elastic strain of dental materialsincludes

bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio

31

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YOUNGrsquoS MODULUS

bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph

bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain

bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity

32

E = 10200053 = 192 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic modulus of a tensile test specimen can be calculated as

bull By definition

Stress (σ)=119865119865119860119860

Strain (ε)=1205491205491198621198621198621198620

33

F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus

E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862

= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620

34

E = 10200053 = 192 GPa

Steep Line Higher modulus and more rigidity

Flat Line Lower modulus and less rigidity

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Principle of Elastic Recovery

bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain

bull After the force is removed the metal springs back to anamount equal to the total elastic strain

bull A final clearance of 25microm is available for the cement

35

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Impression Material

bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth

bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing

36

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Hookersquos Law

bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress

bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality

bull Hookes law can be expressed by the formula

bull The value of the constant depends on the material and the type of stress

37

119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930

= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930

STRESS ndash STRAIN CURVE Enamel amp Dentin

PL = 235CS = 262

PL = 176

CS = 234

0

50

100

150

200

250

300

Com

pres

sive

Sre

ss( M

pa)

Strain

Enamel Dentin

bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures

bull Enamel - more stiffer andbrittle than dentin

bull But dentin more flexible andtougher

EEnamel = 117 GPaEDentin = 336 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

POISSONrsquoS RATIO

bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions

bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section

bull Under compressive loading there is an increase in thecross-section

39

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio

bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains

bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03

bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio

40

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 31: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION

bull Mechanical properties that are measures of the elastic strain of dental materialsincludes

bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio

31

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YOUNGrsquoS MODULUS

bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph

bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain

bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity

32

E = 10200053 = 192 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic modulus of a tensile test specimen can be calculated as

bull By definition

Stress (σ)=119865119865119860119860

Strain (ε)=1205491205491198621198621198621198620

33

F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus

E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862

= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620

34

E = 10200053 = 192 GPa

Steep Line Higher modulus and more rigidity

Flat Line Lower modulus and less rigidity

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Principle of Elastic Recovery

bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain

bull After the force is removed the metal springs back to anamount equal to the total elastic strain

bull A final clearance of 25microm is available for the cement

35

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Impression Material

bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth

bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing

36

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Hookersquos Law

bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress

bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality

bull Hookes law can be expressed by the formula

bull The value of the constant depends on the material and the type of stress

37

119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930

= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930

STRESS ndash STRAIN CURVE Enamel amp Dentin

PL = 235CS = 262

PL = 176

CS = 234

0

50

100

150

200

250

300

Com

pres

sive

Sre

ss( M

pa)

Strain

Enamel Dentin

bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures

bull Enamel - more stiffer andbrittle than dentin

bull But dentin more flexible andtougher

EEnamel = 117 GPaEDentin = 336 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

POISSONrsquoS RATIO

bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions

bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section

bull Under compressive loading there is an increase in thecross-section

39

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio

bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains

bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03

bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio

40

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 32: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YOUNGrsquoS MODULUS

bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph

bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain

bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity

32

E = 10200053 = 192 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic modulus of a tensile test specimen can be calculated as

bull By definition

Stress (σ)=119865119865119860119860

Strain (ε)=1205491205491198621198621198621198620

33

F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus

E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862

= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620

34

E = 10200053 = 192 GPa

Steep Line Higher modulus and more rigidity

Flat Line Lower modulus and less rigidity

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Principle of Elastic Recovery

bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain

bull After the force is removed the metal springs back to anamount equal to the total elastic strain

bull A final clearance of 25microm is available for the cement

35

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Impression Material

bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth

bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing

36

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Hookersquos Law

bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress

bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality

bull Hookes law can be expressed by the formula

bull The value of the constant depends on the material and the type of stress

37

119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930

= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930

STRESS ndash STRAIN CURVE Enamel amp Dentin

PL = 235CS = 262

PL = 176

CS = 234

0

50

100

150

200

250

300

Com

pres

sive

Sre

ss( M

pa)

Strain

Enamel Dentin

bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures

bull Enamel - more stiffer andbrittle than dentin

bull But dentin more flexible andtougher

EEnamel = 117 GPaEDentin = 336 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

POISSONrsquoS RATIO

bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions

bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section

bull Under compressive loading there is an increase in thecross-section

39

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio

bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains

bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03

bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio

40

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 33: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Elastic modulus of a tensile test specimen can be calculated as

bull By definition

Stress (σ)=119865119865119860119860

Strain (ε)=1205491205491198621198621198621198620

33

F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus

E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862

= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620

34

E = 10200053 = 192 GPa

Steep Line Higher modulus and more rigidity

Flat Line Lower modulus and less rigidity

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Principle of Elastic Recovery

bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain

bull After the force is removed the metal springs back to anamount equal to the total elastic strain

bull A final clearance of 25microm is available for the cement

35

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Impression Material

bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth

bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing

36

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Hookersquos Law

bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress

bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality

bull Hookes law can be expressed by the formula

bull The value of the constant depends on the material and the type of stress

37

119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930

= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930

STRESS ndash STRAIN CURVE Enamel amp Dentin

PL = 235CS = 262

PL = 176

CS = 234

0

50

100

150

200

250

300

Com

pres

sive

Sre

ss( M

pa)

Strain

Enamel Dentin

bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures

bull Enamel - more stiffer andbrittle than dentin

bull But dentin more flexible andtougher

EEnamel = 117 GPaEDentin = 336 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

POISSONrsquoS RATIO

bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions

bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section

bull Under compressive loading there is an increase in thecross-section

39

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio

bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains

bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03

bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio

40

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 34: Mechanical Properties of Dental Materials

34

E = 10200053 = 192 GPa

Steep Line Higher modulus and more rigidity

Flat Line Lower modulus and less rigidity

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Principle of Elastic Recovery

bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain

bull After the force is removed the metal springs back to anamount equal to the total elastic strain

bull A final clearance of 25microm is available for the cement

35

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Impression Material

bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth

bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing

36

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Hookersquos Law

bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress

bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality

bull Hookes law can be expressed by the formula

bull The value of the constant depends on the material and the type of stress

37

119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930

= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930

STRESS ndash STRAIN CURVE Enamel amp Dentin

PL = 235CS = 262

PL = 176

CS = 234

0

50

100

150

200

250

300

Com

pres

sive

Sre

ss( M

pa)

Strain

Enamel Dentin

bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures

bull Enamel - more stiffer andbrittle than dentin

bull But dentin more flexible andtougher

EEnamel = 117 GPaEDentin = 336 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

POISSONrsquoS RATIO

bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions

bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section

bull Under compressive loading there is an increase in thecross-section

39

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio

bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains

bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03

bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio

40

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 35: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Principle of Elastic Recovery

bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain

bull After the force is removed the metal springs back to anamount equal to the total elastic strain

bull A final clearance of 25microm is available for the cement

35

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Impression Material

bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth

bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing

36

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Hookersquos Law

bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress

bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality

bull Hookes law can be expressed by the formula

bull The value of the constant depends on the material and the type of stress

37

119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930

= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930

STRESS ndash STRAIN CURVE Enamel amp Dentin

PL = 235CS = 262

PL = 176

CS = 234

0

50

100

150

200

250

300

Com

pres

sive

Sre

ss( M

pa)

Strain

Enamel Dentin

bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures

bull Enamel - more stiffer andbrittle than dentin

bull But dentin more flexible andtougher

EEnamel = 117 GPaEDentin = 336 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

POISSONrsquoS RATIO

bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions

bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section

bull Under compressive loading there is an increase in thecross-section

39

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio

bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains

bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03

bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio

40

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 36: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Eg Impression Material

bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth

bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing

36

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Hookersquos Law

bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress

bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality

bull Hookes law can be expressed by the formula

bull The value of the constant depends on the material and the type of stress

37

119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930

= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930

STRESS ndash STRAIN CURVE Enamel amp Dentin

PL = 235CS = 262

PL = 176

CS = 234

0

50

100

150

200

250

300

Com

pres

sive

Sre

ss( M

pa)

Strain

Enamel Dentin

bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures

bull Enamel - more stiffer andbrittle than dentin

bull But dentin more flexible andtougher

EEnamel = 117 GPaEDentin = 336 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

POISSONrsquoS RATIO

bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions

bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section

bull Under compressive loading there is an increase in thecross-section

39

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio

bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains

bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03

bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio

40

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 37: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Hookersquos Law

bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress

bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality

bull Hookes law can be expressed by the formula

bull The value of the constant depends on the material and the type of stress

37

119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930

= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930

STRESS ndash STRAIN CURVE Enamel amp Dentin

PL = 235CS = 262

PL = 176

CS = 234

0

50

100

150

200

250

300

Com

pres

sive

Sre

ss( M

pa)

Strain

Enamel Dentin

bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures

bull Enamel - more stiffer andbrittle than dentin

bull But dentin more flexible andtougher

EEnamel = 117 GPaEDentin = 336 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

POISSONrsquoS RATIO

bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions

bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section

bull Under compressive loading there is an increase in thecross-section

39

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio

bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains

bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03

bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio

40

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 38: Mechanical Properties of Dental Materials

STRESS ndash STRAIN CURVE Enamel amp Dentin

PL = 235CS = 262

PL = 176

CS = 234

0

50

100

150

200

250

300

Com

pres

sive

Sre

ss( M

pa)

Strain

Enamel Dentin

bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures

bull Enamel - more stiffer andbrittle than dentin

bull But dentin more flexible andtougher

EEnamel = 117 GPaEDentin = 336 GPa

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Mechanical Properties of Dental Materials - Dr Nithin Mathew

POISSONrsquoS RATIO

bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions

bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section

bull Under compressive loading there is an increase in thecross-section

39

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio

bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains

bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03

bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio

40

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 39: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

POISSONrsquoS RATIO

bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions

bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section

bull Under compressive loading there is an increase in thecross-section

39

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio

bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains

bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03

bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio

40

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 40: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio

bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains

bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03

bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio

40

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 41: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DYNAMIC YOUNGrsquoS MODULUS

bull Elastic modulus can be measured by a dynamic method as well as a static method

bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers

41

bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 42: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material

bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)

bull The value of 025 to 030 for Poissonrsquos ratio is typical

bull Therefore the shear modulus is usually 38 of the elastic modulus value

42

G = 1198641198642(1+119907119907)

= 1198641198642(1+03)

= 038E

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 43: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXIBILITY

bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit

bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement

43

bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed

bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 44: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance

bull Here a spring is often bent a considerable distance underthe influence of a small stress

bull In this case the structure is said to possess the propertyof flexibility

44

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 45: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

RESILIENCE

bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen

45

bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 46: Mechanical Properties of Dental Materials

bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph

bull ie The material with the longer elastic area has the higher resilience

46

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 47: Mechanical Properties of Dental Materials

bull When a dental restoration is deformed during mastication it absorbs energy

bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed

bull ie only elastic energy is stored in it

bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced

47

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 48: Mechanical Properties of Dental Materials

bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)

bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament

bull The pain stimulus causes the force to be decreased andinduced stress to be reduced

bull This prevent damage to teeth or restoration

48

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 49: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRENGTH PROPERTIES

bull Strength can be defined as the

bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)

ORbull Stress at the point of fracture (ultimate strength)

49

bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 50: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Strength of a material can be described by one or more of the following properties

bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural

strength

50

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 51: Mechanical Properties of Dental Materials

PROPORTIONAL LIMIT

bull Defined as the magnitude of elastic stress above which plastic deformation occurs

51

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull As stress is increased the strain is alsoincreased

bull Initial 0 ndash A portion shows that stress islinearly proportional to strain

bull As strain is doubled stress is also doubled

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 52: Mechanical Properties of Dental Materials

bull After the point A stress is no longer linearly proportional to strain

bull Here the value of stress at A is known as the proportional limit

52

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line

bull ie stress is linearly proportional to strain

bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 53: Mechanical Properties of Dental Materials

bull The material is elastic in nature below the proportional limit

53

A

B C D

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stre

ss (M

pa)

StrainElastic Plastic

bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 54: Mechanical Properties of Dental Materials

bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation

54

bull The connectors of partial dentures should have highproportional limit

bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 55: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ELASTIC LIMIT

bull Defined as the maximum stress that a material will withstand without permanentdeformation

bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure

55

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 56: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

YIELD STRENGTH

bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain

bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred

bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner

bull The amount of permanent strain is referred to as the percent offset

56

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 57: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset

57

02

02 offset

bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength

bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 58: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins

bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed

58

bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 59: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted

59

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 60: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ULTIMATE STRENGTH

bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension

bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression

bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen

60

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 61: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION

bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains

bull Thus the object will not return to its original shape when the force is removed

bull It remains bent stretched or compressed ie it becomes plastically deformed

61

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 62: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

COLD WORKING (Strain HardeningWork Hardening)

bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases

bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment

62

bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 63: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading

bull Also known as modulus of rupture

bull It is a strength test of a bar supported at each endunder a static load

63

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 64: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FLEXURAL STRENGTH

bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as

64

σ = 3PL2 wt2

σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 65: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs

bull This phenomenon is called fatigue failure

65

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 66: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

IMPACT STRENGTH

bull Defined as the energy required to fracture a material under an impact force

66

bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 67: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull A moving object possesses a known kinetic energy

bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner

bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve

67

bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces

bull A material with low elastic modulus and low tensile strength has low impactresistance

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 68: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Elastic Modulus (Gpa)

Tensile Strength (Mpa)

Composite 17 30 ndash 90

Porcelain 40 50 ndash 100

Amalgam 21 27 ndash 55

Alumina ceramic 350 ndash 418 120

Acrylic 35 60

68

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 69: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

TOUGHNESS

bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing

bull Measured as the total area under a plot of the tensile stressvs strain

bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material

bull Toughness increases with increase in strength and ductility

bull ie Greater the strength and higher the ductility the greateris the toughness

69

TOUGHNESS

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 70: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

FRACTURE TOUGHNESS

bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress

70

bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw

bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material

bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 71: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Material Fracture Toughness

Enamel 07 ndash 13

Dentin 31

Amalgam 13 ndash 16

Ceramic 12 ndash 30

Composite 14 ndash 23

Porcelain 09 ndash 10

71

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 72: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRITTLENESS

bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs

bull Eg amalgam ceramics composites are brittle at oral temperatures

bull They sustain nolittle plastic strain before they fracture

bull ie a brittle material fractures at or near its proportional limit

bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential

72

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 73: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

DUCTILITY

bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture

bull Eg a metal can be drawn readily into long thin wire is said tobe ductile

bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing

bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve

73

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 74: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

Methods to determine ductility are

bull Percent elongation after fracture

bull Reduction in area of tensile test specimens

bull Maximum number of bends performed in a cold bend test

74

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 75: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

MALLEABILITY

bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet

bull Gold is the most ductile and malleable pure metal followed by silver

75

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 76: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

HARDNESS

bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force

bull In mineralogy the relative hardness of a material is based on its ability to resistscratching

76

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 77: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew 77

CLASSIFICATION OF HARDNESS TEST

Method of Application Size of the indenter Amount of load applied to the indenter

Static Loading ndash slowly applied

Macro-indentation- Large indenter tip

Macrohardness- ˃ 1kg load

Dynamic Loading ndash rapidly applied

Micro-indentation- Small indenter tip

Microhardness- lt 1kg load

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 78: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Various hardness tests include

bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test

bull Selection of the test should be done on the basis of the material being tested

78

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 79: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry

bull Related to the proportional limit and ultimate tensile strength

bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load

79

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 80: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

BRINELL TEST

bull Methodbull A hardened steel ball is pressed under a specified load

into a polished surface of the material to be tested

bull Load remains in contact with the material for a fixedtime of 30s

bull After 30s it is then removed and the indentationdiameter is measured

80

bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 81: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values

bull Thus for a given load the smaller the indentation the larger is the number andharder is the material

81

ADVANTAGES DISADVANTAGESBest suited for testing ductile materials

Hardness of cold-worked materialsare difficult to measure

Not suitable for brittle materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 82: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

ROCKWELL HARDNESS TEST

bull This test was developed as a rapid method for hardness determinations

bull Here instead of a steel ball a conical diamond point is used

bull The depth of the penetration is directly measured by a dial gauge on the instrument

bull This test is not suitable for testing brittle materials

82

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 83: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

The value is the Rockwell HardnessNumber (RHN)

83

ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation

Not suitable for brittle materials

Rapid testing time

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 84: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

VICKERrsquoS HARDNESS TEST

bull This test uses a square based pyramidalindenter

bull The impression obtained on the material isa square

bull The method is similar to Knooprsquos andBrinell tests

84

bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)

bull The lengths of the diagonals of the indentations aremeasured and averaged

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 85: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull Used for testing dental casting gold alloys as well as toothstructure

bull Suitable for determining the hardness of brittle materials

85

ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 86: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

KNOOPrsquoS HARDNESS TEST

bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod

bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N

bull This test is designed so that varying loads may be applied to the indentinginstrument

bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested

86

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 87: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured

bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)

87

ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets

Material requires highly polished surface

Longer time required to complete the test

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 88: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew 88

MATERIAL KHN

Enamel 343

Dentin 68

Cementum 40

Denture Acrylic 21

Zinc Phosphate Cement 38

Porcelain 460

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 89: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

STRESS CONCENTRATION EFFECTS

bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure

bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks

89

bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 90: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform

90

bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 91: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

bull 2 important aspects of the flaws

1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size

in the interior region

Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress

91

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 92: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM

1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects

2 Interior flaws such as voidsbull Little can be done about the interior flaws but to

ensure highest quality of the structure or toincrease the size of the object

92

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 93: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle

bull Design of the prosthesis should vary gradually thanabruptly

bull Notches should be avoidedbull All internal line angles should be rounded

4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface

bull The elastic moduli of the 2 materials should be closelymatched

bull Materials must be closely matched in theircoefficients of expansion or contraction

93

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 94: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the

occlusal contact areas in the brittle material is large

94

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 95: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

CONCLUSION

bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication

bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity

95

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 96: Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials - Dr Nithin Mathew

REFERENCES

bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition

bull Craigrsquos Restorative Dental Materials ndash 13th Edition

bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition

bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition

96

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97
Page 97: Mechanical Properties of Dental Materials

97

  • Slide Number 1
  • Mechanical Properties of dental materials
  • contents
  • Slide Number 4
  • Slide Number 5
  • introduction
  • Slide Number 7
  • Slide Number 8
  • Force
  • Occlusal Forces
  • Stress amp strain
  • Slide Number 12
  • Slide Number 13
  • Tensile Stress
  • Slide Number 15
  • Compressive Stress
  • Shear Stress
  • Slide Number 18
  • Slide Number 19
  • Slide Number 20
  • Flexural Stress (bending)
  • Slide Number 22
  • Slide Number 23
  • Slide Number 24
  • Slide Number 25
  • Slide Number 26
  • strain
  • Slide Number 28
  • Elastic and plastic deformation
  • Stress ndash strain curve
  • Mechanical properties based on elastic deformation
  • Youngrsquos Modulus
  • Slide Number 33
  • Slide Number 34
  • Slide Number 35
  • Slide Number 36
  • Hookersquos Law
  • Stress ndash strain curve Enamel amp Dentin
  • Poissonrsquos ratio
  • Slide Number 40
  • Dynamic youngrsquos modulus
  • Slide Number 42
  • Flexibility
  • Slide Number 44
  • resilience
  • Slide Number 46
  • Slide Number 47
  • Slide Number 48
  • Strength properties
  • Slide Number 50
  • Proportional limit
  • Slide Number 52
  • Slide Number 53
  • Slide Number 54
  • Elastic limit
  • Yield strength
  • Slide Number 57
  • Slide Number 58
  • Slide Number 59
  • Ultimate strength
  • Mechanical properties based on plastic deformation
  • Cold working (Strain HardeningWork Hardening)
  • Flexural strength
  • Flexural strength
  • Slide Number 65
  • Impact strength
  • Slide Number 67
  • Slide Number 68
  • Toughness
  • Fracture toughness
  • Slide Number 71
  • brittleness
  • ductility
  • Slide Number 74
  • MALLEABILITY
  • hardness
  • Slide Number 77
  • Slide Number 78
  • Brinell test
  • Brinell test
  • Slide Number 81
  • Rockwell hardness test
  • Slide Number 83
  • Vickerrsquos hardness test
  • Slide Number 85
  • Knooprsquos hardness test
  • Slide Number 87
  • Slide Number 88
  • Stress concentration effects
  • Slide Number 90
  • Slide Number 91
  • Causes for areas of high stress concentration and methods to minimize them
  • Slide Number 93
  • Slide Number 94
  • conclusion
  • references
  • Slide Number 97

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Mechanical & Physical Properties of Dental Materials (from Dental Materials Lectures Notebook, 20 Years ago)

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