Mechanical Properties of Dental Materials

Mechanical Properties of

Dental Materials

Resilience

AB

Str

ess

(Pa)

Strain

Resilience

AB

Str

ess

(Pa)

Strain

Occlusal forces

• Average occlusal forces for fully dentate patients :

150 Newton in the anterior region to 500N in posterior region

• Maximum occlusal forces: different reports in the literature up to 3500N.

• The occlusal forces for edentulous patients 15% of dentate patients.

Bulk Properties

Stress

Force per unit area; a force exerted on one body

that presses on, pulls on, pushes against, or tends to invest,

compress another body; the deformation caused in a body by such a force;

an internal force that resists an externally applied load or

force. It is normally defined in terms of mechanical stress, which

is the force divided by the perpendicular cross sectional area over

which the force is applied. GPT 2005, J Prosthetic Dentistry

Stress: Internal resistance to applied external force.

Stress= Force/Area

Types of stresses

• Axial

Compressive

Tensile

Types of stresses

• Non Axial Shear

Torsion

Bending

Strain

• Strain: change in length per unit length when stress is applied; the change in length/original length

GPT 2005, J Prosthetic Dentistry

• Strain(ε)= Deformation/Original length

Stress-Strain curve

Strain

Res

ilien

ceA

B

Str

ess

(Pa) Toughness

C D

Resilience: the resistance of a material to permanent deformation

A: Proportional limit

Elastic limit

A

Str

ess

(Pa)

Strain R

esili

ence

A

Str

ess

(Pa)

Strain

A: Proportional limit

• The greatest stress that a material will sustain without a deviation from the proportionality of stress to strain, below which no permanent deformation happens.

Elastic limit

• The maximum stress that a material will withstand without permanent deformation.

AB

Str

ess

(Pa)

Strain

Res

ilien

ce

AB

Str

ess

(Pa)

Strain

B:Yield strength(YS)

• The stress at which a material exhibits a specified limiting deviation from proportionality of stress to strain

* YS indicates a degree of permanent deformation (usually 0.2%)

YS indicates a functional failure!!!

Elastic modulus

• Is a measure of elasticity of the material: how stiff the material is in the elastic range

• Elastic modulus= Stress/Strain

• The slope of the curve

AB

Str

ess

(Pa)

Strain R

esili

ence

AB

Str

ess

(Pa)

Strain

Poisson’s ratio

• Ratio of lateral to axial strain within the elastic range

Ductility and malleability

• Ductility: The ability of a material to be plastically deformed.

• Malleability: The ability of a material to be hammered or rolled into thin sheets without fractureing.

Plastic deformation

Strain

AB

Str

ess

(Pa) Toughness

C D

C: Ultimate strength

• Tensile or Compressive.

• The Ultimate strength: The maximum that a material can withstand before failure (tension or compressive).

it does give an indication of the needed thickness (cross section) of the restorations before failure.

D: fracture strength

• The stress at which the material fractures.

Toughness

• The resistance of a material to fracture

So what does yellow area under curve represent?

Fracture toughness

• The amount of energy required for fracture.

• Bond strength:

the bond strength between two dental materials.

Either tensile or shear

Fatigue bond strength?

Bending and torsion

• Endodontic files and reamers

Transverse strength

• Modulus of rupture or flexural strength

3- point bending test

Fatigue strength

• Fatigue: Progressive fracture under repeated loading

• The importance of endurance limit?

Str

ess

(Pa)

Strain Cycles

Str

ess

Fluid behaviour and Viscosity

• Viscosity: the resistance of a fluid to flow

• Viscosity= Shear stress/shear strain rate.

Viscous fluids

She

ar s

tres

s

Newto

nian

Str

ess

(Pa)

Strain Shear rate

Pseduplasti

c

Dilata

nt

She

ar s

tres

s

The importance of thixotropic impression materials

Creep and stress relaxation

• Creep is the increase in strain in a material under constant pressure.

Creep test is used for study of new amalgam materials

Surface mechanical properties

Indentation hardness

• Brinell hardness test. Ball,(steel or T carbide),

• Knoop hardness: Microindentation, pyramid shape.

• Vickers: 136 degrees diamond pyramid.

• Rockwell:metal cone.

• Shore A hardness for rubber

Stress analysis

• Lab based studies.

• Photoelasticity

• Finite Element Analysis.

Wear

• Loss of material due to contact between two surfaces

Surface phenomena

• Atoms or molecules at surface different to bulk

Stainless steel Vs steel

Oxide layer

Colloidal systems

• Two or more phases with one highly dispersed on the other.

Types:

* Sols and Gels

* Emulsions

Gels

• Entangled framework of solid colloidal praticles in which liquid is tapped in the intestices in which liquid is trapped

Emulsions

• A uniform dispersion of minute droplets of one liquid into another with the aid of emulsifier.

Surface tension and wetting

Θ Θ

High contact angle= less wetting

Low contact angle= better wetting

Adhesion

• The bonding of dissimilar materials by either:

^ Chemical bonding (True) OR

^ Mechanical bonding (retention).

Optical properties

• Basic colours:Red, Green and Blue.

Why only three??

HueChromaValue

Munsell colour system

Hue

• Basic colour

Chroma

• Colourfulness OR saturation

Value

• lightness

Metamerism

When two colour samples match when viewed under one light source but not another.

Any significance in dentistry?

Flouresence

• The emission of luminous energy by a material when a beam of light is shown on it.

What impact does this have in anterior restorations.

Thermal properties

• Heat of fusion: melting or freezing heat.

• Coefficient of thermal expansion: of paramount importance in clinical dentistry, why???

• Glass transition temperature??? For non metallic structures; glasses and polymers