As the load is applied, the curve is proportional and this period of linearity is termed the elastic region. Once the curve deviates from a straight line and begins to yield, the material has reached the proportional limit. Once the material has yielded, it exhibits plastic behaviour or plasticity. Brittle materials do not exhibit much yield and are, therefore, less curved than ductile materials.
Ductile material curves have marked areas of yield and curvature illustrates the degree of ductility. At the top of the curve is the ultimate tensile strength of the material. Once the curve has peaked, stress continues to decline while strain continues to increase. This condition continues until failure.
Elasticity :Ability of a material to return to its original shape when the load causing the deformation is removed.
• Reversible or recoverable deformation (ε → 0 when σ → 0)
• Usually associated with linear region of s-e curve.
Brittle materials fail in tension at relatively low values of strain. These materials include ceramics, some metal alloys and thermosetting polymers. There is virtually no necking so the nominal stress=true stress
Plasticity :Tendency of a material to remain deformed, after reduction of the deforming stress, to a value equal to or less than its yield strength.
• Demarcates transition from elastic to plastic behaviour.