Mechanical Properties of Dental Materials -- Part 1 Dr. Raghuwar D Singh Associate Professor Prosthodontic Department King George’s Medical University UP, Lucknow

Dental Materials: Properties of Dental Materials – Physical, Mechanical Biocompatibility Micro structure of metal alloys Impression materials Gypsum product Dental Resins – Denture base and Restorative Resin Dental Amalgam Dental Cements Direct filling Gold Dental casting alloys Finishing and polishing materials Dental Ceramics

Mechanical Properties of DM Stress Strain Mechanical Properties based on elastic deformation: Stress- Strain diagram/ curve Modulus of elasticity Poisson’s ratio Flexibility Resilience

Strength Properties: Proportional limit Elastic limit Yield strength Diametral tensile strength Flexural strength Fatigue strength Impact strength

Other Mechanical properties: Toughness Fracture toughness Brittleness Ductility and Malleablility Hardness

STRESS Force per unit area; Stress= Force/Area 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.

Stress: Internal resistance to applied external force.

Types of stresses:

Types of stresses.... Axial Compressive Stress- Tensile Stress-

....Types of stresses Non Axial Shear – Tends to resist the sliding of one portion of a body over another. Torsion Bending

Flexural Stress: Bending forces

STRAIN Strain: change in length per unit original length when stress is applied; = Length / Original length Strain(ε)= Deformation/Original length

Elastic Strain Plastic Strain

Mechanical Propertied based on Elastic deformation Stress- Strain diagram/ curve Modulus of elasticity Poisson’s ratio Flexibility Resilience

Stress-Strain curve Strain Resilience A B Stress (Pa) Toughness C D

A B Stress (Pa) Strain Resilience

ELASTIC MODULUS A B Stress (Pa) Strain Resilience 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

...ELASTIC MODULUS

POISSON’S RATIO Ratio of lateral to axial strain within the elastic range. For an ideal isotropic material of constant volume the ratio is 0.5. Most material have values of 0.3.

FLEXIBILITY Ability of a material to return to its original form indicates its elasticity, but the strain taking place at elastic limit is known as flixibility. Flexibility is bending capacity. It can be defined as the strain that occurs when the material is stretched to its proportional limit.

RESILIENCE Defined as the amount of energy absorbed within a unit volume of a structure when it is stressed to its proportional limit. The property if often described as “springback potential.”

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

STRENGTH PROPERTIES Proportional limit Elastic limit Yield strength Diametral tensile strength Flexural strength Fatigue strength Impact strength

Strength properties: Strength is the stress that is necessary to cause fracture or a specified amount of plastic deformation.

PROPORTIONAL LIMIT It is defined as the greatest stress that a material will sustain without a deviation from the linear proportionality of stress to strain.

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

YIELD STRENGTH Defined as the stress at which a material exhibits a specified limiting deviation from proportionality of stress to strain. It is the amount of stress required to produce a predetermined amount of permanent strain usually 0.1% or 0.2% which is called the Percent Offset.

For brittle materials such as composites and ceramic – Yield strength can not be measure. YS indicates a degree of permanent deformation (usually 0.2%) Continue……