Mechanical Properties

a: stiff, rigid b: flexible Modulus of elasticity “Young’s Modulus” (E): The constant of proportionality between stress and strain. It represents the slope of the elastic portion of the stress-strain curve. It measures the stiffness or rigidity E = = = kg/cm2 MPa stress kg/cm2 Stress Strain a b strain cm/cm a: stiff, rigid b: flexible

Clinical significance: Denture base should be constructed of a rigid material: To allow load distribution on the whole area. To be used in thinner sections without the risk of bending. To avoid bone resorption

Ductility and malleability: Ductility is the ability of the material to withstand plastic deformation under tensile stress without fracture. i.e. the ability of the material to be drawn into wires without fracture. Malleability is the ability of the material to withstand plastic deformation under compressive stress without fracture. i.e. the ability of the material to be hammered into thin sheets without fracture. Stress Strain

Brittleness: When the material shows no or very little plastic deformation on application of load brittle. Stress Strain Stress Strain

Ductile materials Brittle materials Fracture occurs far away from the P.L Fracture occurs at or near the P.L Necking takes place before fracture Crack propagation take place till fracture Examples: gold, nickel chromium alloys Burnish-ability of casting margins Examples: amalgam, porcelain, composite Stress Strain Stress Strain

Resilience: It is the energy required to deform the material to the P.L. It is represented by the area under the elastic portion.

R= ½ stress x strain N.m/m3, joule/m3 b Strain R= ½ stress x strain N.m/m3, joule/m3

Clinical significance: Orthodontic wires capable of storing energy, and released over period of time Acrylic resin teeth are more resilient than porcelain teeth So transmit less forces to underlying bone

Toughness: It is the energy required to stress the material to the point of fracture. It is represented by the area under the elastic and plastic portion.

Stress a b Strain N.m/m3, joule/m3

Clinical significance: It is very important with brittle materials Dicor is brittle but after incorporation of mica crystal during ceraming, toughness is increased

Stress Strain Stress Stress Strain Stress Strain Stress Strain Stress

Fracture toughness: The ability of material to resist fracture through its resistance to crack propagation. Or the amount of energy required for fracture. F.T. is related to crack propagation not crack initiation, surface condition is of little importance

Fracture toughness: Clinical significance: Glass fillers in dental composite. Addition of zirconia particles in porcelain.