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Hai this is very interesting topic for the dental students and also for the PG of orthodontics .So just have a glance over it and always your suggestions are heartly welcome.please free to suggest and make necessary suggestions.
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GOOD MORNIN
G
Physical and Mechanical Properties and its application in orthodontics
Prepared by Dr.Hardik Lalakiya
Guided by Dr.Ajay Kubavat Dr.Chintan
Agrawal Dr.Ketan Mashru Dr.Bhavik Patel Dr.Manish Desai Dr.Vishal Patel
Department of Orthodontics
and Dentofacial Orthopaedics
OUTLINE
Introduction Crystal structure
and its arrangement
Principal metal structures and its arrangement
Classification Stress and its
types Strain True Stress strain
curve
Poisson’s ratio Mechanical
properties based on elastic deformation
Toughness Impact strength Proportional limit Elastic limit Yield strength
Permanent Plastic deformation
Strain hardening Strength and its
types Fatigue Static fatigue Brittleness Ductility Malleability Physical Properties
Abrasion and abrasion resistance.
Hardness Viscosity Creep and flow Color and color
perception Bezold brucke effect
Mechanical properties are defined by the laws of mechanics that is the physical science that deals with the energy and forces and their effects on bodies the discussion centers primarily on the static bodies –those at rest-rather than on dynamic bodies.
Many factors must be taken into account when considering which properties are relevant to the successful performance of the material used in dentistry
The Plantonic Solids
CUBE DODECAHEDRON ICOSAHEDRON
OCTAHEDRON TETRAHEDRON
http://home.teleport.com/~tpgettys/platonic.shtml
Atomic arrangements in crystalline solids can be described with respect to a network of lines in three dimensions.
The intersections of the lines are called “lattice sites” (or lattice points). Each lattice site has the same environment in the same direction.
A particular arrangement of atoms in a crystal structure can be described by specifying the atom positions in a repeating “unit cell”.
14 Bravais lattices
Principal metal crystal structures
There are three principle crystal structures for metals: –(a) Body-centered cubic (BCC) –(b) Face-centered cubic (FCC) –(c) Hexagonal close-packed (HCP)
Principal structures
Body centered cubic (BCC)
(BCC)
Face centered cubic (FCC)
(FCC)
Hexagonal closed packed (HCP)
(HCP)
Classification
Definition: When a force acts on a body tending to produce deformation . A resistance is developed to this external force application. The INTERNAL reaction is equal in intensity and opposite in direction to the applied external force and is called stress. Stress = Force/Area
STRESS
•Commonly expressed as Pascal 1Pa = 1N/m2. It is common to report stress in units of Megapascals (MPa) where 1 MPa = 106 Pa. •TYPES OF STRESS :- Tensile Compressive Shear
In english system of measurement ,the stress is usually expressed in pounds per square inch.
3 Types of stress
Tensile Compressive Stress Shear stress
Tensile Stress
A tensile Stress is caused by a load that tends to stretch or elongate a body .
for eg stress developed on the gingival side of 3 unit bridge bridge
Compressive stress If a body is placed under a load that
tends to compress or shorten it,the internal resistance to such a load is called compressive stress.
Shear stress A stress that tends to resist a twisting
motion or sliding of one portion of a body over another is shearing stress
For eg If a force is applied along the surface of tooth enamel by a sharp edged instrument parallel to the interface between the enamel and an orthodontic bracket may debond by shear stress failure of the resin luting cement
Complex stress Complex stress those
produced by applied forces that cause flexural or torsional deformation are called flexural stress
More than two
They are also called as bending stress.
STRAIN
o A force is applied to a body it undergoes deformation.
o Strain is described as the change in length (Δ L = L – LO) per unit length of the body when it is subjected to a stress.
Strain ( ) = Change in length = L – Lo = Δ L
Original length Lo Lo
Strain has no units of measurement.
· It is a Dimensionless quantity.
· Reported as an absolute value or as a percentage.
Facts
The Average max sustainable biting force is 756N (170 pounds) or (77kgs)
The Guiness Book Of World records
(1994) lists the highest biting force as 4337N (975 pounds).
Each type of stress is capable of producing a corresponding deformation in a body.
Tensile stress produces tensile strain.
Compressive stress produces compressive strain.
Shear stress produces shear strain.
Stress strain curve
Represents energy storage capacity of the wire so determines amount of work expected from a particular spring in moving a tooth.
True stress strain curve
A stress strain curve based on stresses calculated from a Non Constant Cross sectional area is called a true stress strain Curve.
A true-stress strain curve may be quite different from an engineering stress-strain curve at high loads because significant changes in the area of specimen may occur.
STRESS STRAIN CURVE
Mechanical Properties Based On Elastic deformation
Elastic Modulus Shear Modulus Flexibility Resilience Poisson’s ratio.
Elastic modulus(young’s modulus or Elasticity)
The term elastic modules describes the relative STIFFNESS or RIGIDITY of a material which is measured by the elastic region of stress – strain diagram.
It is denoted by letter E
o Determined from stress stain curve by calculating ratio of stress to strain or slope of linear portion of curve.
Stress 6
Elastic Modulus = = Strain
Stress strain curve
Modulus of elasticity is independent of the ductility of a material and it is not a measure of its strength.
It is an inherent property of a material and cannot be altered appreciably by heat treatment, work hardening or any other kind of conditioning. This property is called STRUCTURAL INSENSITIVITY.
The Elastic modulus of a tensile test specimen can be calculated as follows where
E is elastic modulus P is the applied force or load A is the cross sectional area of material under
stress ^l is the increase in length Lo is the original length
Flexibility
The maximum flexibility is defined as the strain that occurs when the material is stressed to its proportional limit.
For example in an orthodontic appliance, a spring is often bent a considerable distance with a small stress resulting in such a case structure is said to be flexible.
Resilience
Popularly the term Resilience is associated with “springiness”.
Definition: It is defined as the amount of energy absorbed by a structure when it is stressed to its proportional limit.
Area bounded by the elastic region is measure of Resilience.
Poisson’s ratio
Any material when subjected to a tensile or compressive stress, there is simultaneous axial and lateral strain.
Within elastic range the ratio of lateral to axial strain is known as poisson’s ratio.
Dental materials have poisson’s ratio in the range of 0.3 to 0.5.
TOUGHNESS
It is defined as energy required to fracture a material.
It is measured as a total area under stress strain curve.
Toughness of the material is dependent on the ductility and malleability of the material than upon the flexibility or elastic modulus.
Conventional Tensile Stress Strain Curve
IMPACT STRENGTH
IMPACT: It is the reaction of a stationary object to a
collision with a moving object. Depending on the resilience of the object , energy is stored in the body without causing deformation or with deformation.
Impact resistance decreases with increase in stiffness.
Resilient material have high impact strength. Increase in volume leads to increase in impact resistance.
Impact Strength (continue).. It is the energy
required to fracture a material under force.
A charpey type tester is used. It has a heavy pendulum which swings down to fracture the specimen.
Another instrument called Izod impact tester can also be used.
Strength properties
Strength is the stress necessary to cause either fracture(ultimate strength) or a specified amount of plastic deformation(yields strength).
The strength of a material can be described by Proportional limit Elastic strain Yield strength Ultimate tensile strength,
shear ,compressive and flexural strength.
Proportional limit (PL)
It is defined as the greatest stress that a material will sustain without a deviation from the linear proportionality of stress to strain.
Hooke’s Law :- States that stress – strain ratio is constant upto the proportional limit, the constant in this linear stress-strain relationship is Modulus of Elasticity.
Below PL no permanent deformation occurs in a structure.
Region of stress stain Curve. Below PL – ELASTIC REGION Above PL – PLASTIC REGION
Elastic limit (EL)
Definition: It is defined as maximum stress that a material can withstand before it undergoes permanent deformation.
For all practical purposes PL and EL represent same stress. But they differ in fundamental concept :-
PL deals with proportionality of strain to
stress in structure.
EL describe elastic behavior of the material.
EL & PL limits are usually assumed to be identical although their experimental values may differ slightly.
Yield Strength(yield stress or proof stress)
It is defined as the stress at which a material exhibits a specified limiting deviation from proportionality of stress to strain.
Amount of permanent strain is arbitrarily selected for material being examined and may be indicated as 0.1%, 0.2% or 0.5% (0.001, 0.002, 0.005) permanent strain
Amount of permanent strain may be referred to as PERCENT OFFSET. Many specifications use 0.2% as convention.
Permanent (Plastic) deformation
If the material is deformed by a stress at a point above the proportional limit before fracture,the removal of applied force will reduce the stress to zero,but the strain does not decrease to zero because the plastic deformation has occurred .
Thus the object does not return to its original dimension when the force is removed.It remains bent,streched,compressed or otherwise plastically deformed.
Strain hardening
Strengthening by increase of dislocation density (Strain Hardening = Work Hardening = Cold
Working)
Ductile metals become stronger when they are deformed plastically at temperatures well below the melting point.
The reason for strain hardening is the increase of dislocation density with plastic deformation.
Average distance between dislocations decreases and dislocations start blocking the motion of each other.
The percent cold work (% CW) is often used to express the degree of plastic deformation:
%CW is just another measure of the degree of plastic deformation, in addition to strain.
Strength
It is the maximal stress required to fracture a structure.
Strength is not a measure of individual atom to atom
attraction or repulsion , but rather it is a measure of the
interatomic forces collectively over the material which
is stressed.
STRENGTH IS BASICALLY OF FOUR TYPES: Tensile Compressive Shear Flexure
Tensile strength
Tensile Strength is determined by subjecting a rod , wire or a dumbbell shaped specimen to a tensile loading.
It is defined as the maximal stress the structure will withstand before rupture.
Diametral Tensile Strength
Brittle material an indirect tensile test called Diametral compression test or Brazillian test is used .
A compressive load is placed on the diameter of a short cylindrical material .
Compressive strength
Crushing strength is determined by subjecting a cylindrical specimen to a compressive load.
The strength is obtained from the cross sectional area and force applied.
Complex failure
SHEAR SRENGTH
Maximum stress a material can withstand before failure in a shear mode of loading. It is tested using punch or pushout method.
Shear strength = Force/ Π punch dia * thickness
FLEXURE STRENGTH
Transverse strength or modulus of rupture or flexure strength Obtained using a beam supported at each end and load applied in the middle.
Also called three point
bending test.
Used in long span bridges. Neutral Axis
Fatigue
A Structure subjected to repeated or cyclic stress below its proportional limit can produce abrupt failure of these structure.
Fatigue behavior is determined by subjecting a material to a cyclic stress of known value and determining the number of cycles that are required to produce failure.
Static fatigue
Some material support a static load for a long period of time and fail abruptly. This type of failure may occur in wet environment.
Eg ceramic materials.
Brittleness
A brittle material fractures at or near its proportional limit.
It is opposite of toughness.
Brittle material will not bend appreciably without breaking.
Though a brittle material may have a very high compressive strength. E.g. glass.
Ductility
Ability of a material to withstand permanent deformation under a tensile load without rupture.
It is the ability of the metal to be drawn into wires.
Ductility depends on tensile strength.
It decreases with increase in temperature.
MEASUREMENT OF DUCTILITY 1.Percentage elongation after fracture Gauge length = 51
mm( STANDARD GAUGE LENGTH FOR DENTAL MATERIALS)
2.Measuring reduction in cross sectional areas of fractured ends in comparison to the original area of the wire. This is also called as reduction in area method.
3. cold bend test
Malleability
It is the ability of a material to withstand rupture under compression.
It is seen in hammering or rolling of a material into sheets.
It is not dependent on the strength of the material
It increases with temperature.
Gold is most ductile and malleable and
silver stands the second.
Platinum is third most ductile and
copper ranks third in malleability.
Stress concentration factors
THESE INCLUDES Surface flaws Internal voids air bubbles. Inclusions of other materials Hertzian load Sharp angles Notches Thermal mismatch
Some clinical relations with orthodontic wire
Tension Test Results; UTS and E for stainless steel and titanium material.
Material Type UTS (MPa) E (GPa)
Stainless steel 1300 193
titanium 1615 179
Stress-Strain curve of stainless steel specimen the x-axis the strain in the specimenand the y-axis stress (MP/mm2). By wp 300 tensile testing machine
Physical Properties
Abrasion and abrasion resistance
Phenomenon of wearing/ removal process that occurs whenever surfaces slide against each other
The material which causes wearing is called
abrasive
The material which is worn is called substrate.
Hardness is one of the common index of a material to resist abrasion or wear but not the only index.
Other factor which cause and influence abrasion / abrasion resistance are
Biting force Frequency of chewing, Abrasiveness of diet, Intra oral liquid, temperature changes, Surface roughness, Impurities and irregularities (Pits and grooves)
hardness
Resistance to surface penetration / surface scratching /ability to resist indentation.
Indentation is produced on the surface of the material from a applied force of a sharp point or an abrasive particle.
Most hardness test are based on ability of a surface of a material to resist penetration by diamond point or a steel ball under a specified
Common tests are
Barcol Brinell (BH) Rockwell (RH) Shore Vickers (HV) Knoop (KH) Microhardness
test
Macrohardness test
Brinell hardness number (BHN)
Oldest, simplest , convenient & extensively used
Hardened steel ball pressed with standard load on polished surface of material .
Load is divided by the area of projected surface of indentation .
Thus for a given load smaller the indentation, larger is the number and the harder is the material
Rockwell hardness number (RHN)
Conical diamond point is used.
Depth of penetration is measured directly by the dial gauge on the Instrument
RHN and BHN are used for measuring hardness of metal and alloys and they are not suitable for brittle materials.
Vickers hardness test HV test employs square
based pyramid of 136 Degrees
Method of computation is the load divided by the projected area of Indentation.
The length of the diagonals are measured and averaged.
Can be used for brittle materials.
also called 136 degree diamond pyramid test.
Knoop hardness number (KHN)
Uses diamond tip tool.
Rhombohedral pyramid diamond tip is used of dimension 130 degree and 172.30 degree
The length of the largest diagonal is measured .
The projected area is divided in to the load to give KHN
Can be used for extremely hard and soft materials.
KHN and HV are called as micro hardness test.
BHN and RHN are macro hardness test.
Shore and Barcol test are sometimes employed to measure hardness of rubber and plastic type of dental materials.
These have spring loaded metal indenter point.
Viscosity Resistance of a liquid to flow Study of flow
character of a material is the basis
for Rheology Importance of knowing flow: impressions, Gypsum products, cements, waxes. Resistance to flow is controlled by internal frictional forces. Thus viscosity is the measure of consistency of a medium and its inability to flow.
Change in Viscosity
Whenever a force is applied to a material it will
deform. The force / area is called stress. The calculation of deformation is the
strain. Strain = change in length / initial length. Unit of viscosity is MPa / second or
CETIPOISE
Viscosity of most liquids decreases with increase in temperature i.e. its flow increases
To explain viscous nature of some materials , shear stress / shear strain rate curve is plotted .
Based on Rheologic behavior fluids are
classified in to four types
Newtonian fluid
Pseudoplastic
Dilatant fluid
Plastics
Newtonian fluid
Ideal fluid which demonstrates a shear strain proportional to the shear stress
The plot on the graph is a straight line
Newtonian fluids has a constant viscosity and is independent of the shear strain rate.
Pseudoplastic fluid
When the viscosity of a material decreases with increasing strain rate until it reaches the constant value such a material is called Pseudoplastic materials or fluid.
Dilatant fluid These are the liquids
that
becomes more rigid as the
rate of deformation
increases.
These liquids show
opposite tendency as
described for
pseudoplastic
Plastic
Some classes of material
behave like a rigid body until
some minimum value of
shear stress is reached (off
set value)
These fluids which exhibits
rigid behavior initially and
then attend constant
viscosity are referred to as
plastic.
Ketchup is a familiar
example .
Thixotrophic material
Viscosity of liquid also depends on previous deformation of liquid
A liquid of this type that becomes less viscous and more fluid under more repeated application of pressure is called as Thixotrophic materials
Examples: Dental polishing paste, plaster of paris,
impression materials, resins and cements
Importance of Viscosity Properties
Teaches us the best way to manipulate the materials
Guides as on the best use of the materials
Measure of working time
Thixotropic materials stays on tray but on applying pressure in the mouth the material flows
Creep and flow
If the metal is held at the temperature near its melting point and subjected to constant applied stress, the resulting strain will increases over time.
Creep is defined as the time dependant plastic strain of a materials under static / constant load.
Sag is same as creep but the load is the mass of the same material .
Creep and flow (continue…)
A filling material called “Amalgam” has low melting range. So when in mouth it is close to the melting point and is subjected to constant biting forces. It gets get deformed. Here the biting forces keep changing and continuous Dyanamic creep.
For waxes term flow rather than creep is used as it is amorphous. The flow of wax is its potential to deform under small static load / or its own mass.
Creep and flow (continue…)
Flow is measured using compressive forces mostly.
Testing flow: A cylinder prescribed dimension is subjected to a given compressive stress for a specified time and temperature.
The creep or flow is measured as percentage decrease in length.
Significance of creep / sag.
Thermophysical properties
Heat transfer through solid substances most commonly occur by means of conduction.
The conduction of heat through metals occurs through the interaction with atoms.
Thermal conductivity (k) is the thermophysical measure of how well heat is transferred through a material by conductive flow.
The measurement of thermal conductivity is performed under steady state conditions.
Thermoconductivity Properties The Thermal conductivity or coefficient of
thermal conductivity is the quantity of heat in calories per second that passes through a specimen 1 cm thick having a cross sectional area of 1cm2 ,when the temperature difference between the surfaces Thermoconductivity Properties perpendicular to the heat flow of the specimen is 10 K.
Materials that have a high thermal conductivity are called conductors, whereas materials of low thermal conductivity are called insulators.
Thermoconductivity Properties(Cont..)
The international system (SI) unit or measure for thermal conductivity is watt / meter / second /o Kelvin
Increase in thermal conductivity , greater is the ability to transfer thermal energy.
Metal restoration – increase conductivity compared to other materials.
Thermal Diffusivity
The value of thermal diffusivity of a material controls the time rate of temperature change as heat passes through a material.
It is a measure of the rate at which a body with a nonuniform temperature reaches a state of thermal equilibrium.
For a given volume of material, the heat required to raise the temperature , to a given amount depends on its heat capacity or specific heat and the density.
Thermal Diffusivity (cont)..
The formula that related thermal diffusivity to thermal conductivity is
h = k / cpρ
h = Thermal diffusivity k = Thermal conductivity cp = Heat capacity ρ = temperature dependent density
Thermal Diffusivity (cont)..
Square root of thermal diffusivity is indirectly proportional to thermal insulation ability.
SI unit is square meter per second commonly used.
Coefficient of thermal expansion
Coefficient of thermal expansion, is defined as the change in length / unit of the original length of a material when its temperature is raised 1degree K.
SI unit μm /m0 K or ppm / k0 A tooth restoration may contract or expand more
than the tooth during the change in temp which may cause micro leakage or debond of restoration of teeth.
To reduce this, selection of material whose expansion or contraction coefficient should be matched approximately within 4%.
PFM
Color and color perception (cont)..
Sensation induced from color of various wavelength reaching the eye.
Eye is sensitive to wavelength of
400nm(violet) to 700nm(dark red).
For an object to be visible, it must reflect and transmit incident light at certain wavelength.
Color is measured using munsell system.
Color and color perception (cont)..
Color and color perception (cont)..
Thus, Light from object
Incident on eyes
Focused in retina →rods and cones
Converted into nerve impulses
Transmitted to brain
Color and color perception (cont)..
Three dimension of color are: 1. Hue 2. Value 3. Chroma
Color and color perception (cont)..
Hue: Dominant color of an object
E.g. red, blue, green (dominant wavelength).
The normal human teeth have hue range of 6.3
yellow red to 9.3 yellow red.
Color and color perception (cont)..
Value
Relative lightness or darkness of color.
The human teeth have a value in the range of 0-7.
Color and color perception (cont)..
CHROMA
Degree of saturation of particular hue. Higher the chroma, more intense and
mature the color. Chroma cannot exist itself and it is
always associated with hue and value. Normal human teeth has chroma of 4
to 7.
Color and color perception (cont)..
Color Solid:
Central rod = value
Spikes = hue
Volume = chroma
Color and color perception (cont)..
CIE SYSTEM: Commission
International Eclairage. Based on Adam
system Colour in L*a*b L = value a = measure along r-g axis b= measure along y-b
axis
Color and color perception (cont)..
Shade Guide : In the dental laboratory, color matching
is usually performed by the shade guide.
The most commonly used guide is VITA shade guide.
The range is from A1 to D4 .From left to right the darkness increase.
Color and color perception (cont)..
Metamerism: Object that appear to be color matched
under one type of light may appear different under another light source.
Day light, incandescent lamps, fluorescent lamps are most common source of light in dental operatory.
Two or more sources of light should be used to prevent metamerism causing wrong selection of
Metamerism
Color and color perception (cont)..
Near ultraviolet radiation: Natural tooth structure absorbs light at
wave lengths too short to be visible at human eye.
These wave lengths between between 300nm- 400nm are referred as near ultraviolet radiation.
Sources are natural sunlight, photoflash lamps, UV light
Color and color perception (cont)..
Fluorescence: Energy that the tooth absorbs is
converted into light with longer wavelength in which case the tooth actually becomes a light source.
The phenomenon is called Fluorescence. Ceramics, composites – fluorescent
agents are added.
Fluorescence
Color and color perception (cont)..
BEZOLD BRUCKE EFFECT:
At low light levels, rods of human eye are dominant and color perception is lost. As the brightness becomes more intense , color appears to change.
BEZOLD BRUCKE EFFECT
BEZOLD BRUCKE EFFECT