# Mechanical Properties of Dental Materials

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Mechanical Properties of Dental Materials

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 Stress: Internal resistance to applied external force.
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 Resilience A B Stress (Pa) Toughness C D

Resilience: the resistance of a material to permanent deformation
A: Proportional limit Elastic limit A Stress (Pa) Strain Resilience

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.

A B Stress (Pa) Strain Resilience

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 A B Stress (Pa) Strain Resilience

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 A B Stress (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

The importance of endurance limit? Stress (Pa) Strain Cycles Stress

Fluid behaviour and Viscosity
Viscosity: the resistance of a fluid to flow Viscosity= Shear stress/shear strain rate.

Viscous fluids The importance of thixotropic impression materials
Shear stress Newtonian Stress (Pa) Strain Shear rate Pseduplastic Dilatant 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??
Munsell colour system Hue Chroma Value

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