5.5 Elastic Deformation, Plastic Deformation and Elastic Limit

5.5 Elastic Deformation, Plastic Deformation and Elastic Limit

In a tensile test machine, metal wire is extended at a specified deformation rate, and stresses generated in the wire during deformation are continuously measured by a suitable electronic device fitted in the mechanical testing machine. Force–elongation diagram or stress–strain curve is plotted automatically on X-Y chart recorder. A typical stress–strain curve for a ductile material is shown in Fig. 5.5.

In the initial stage of deformation, stress is increased linearly with the strain till we reach point $A$ on the stress–strain curve. This is called proportional limit $(\sigma_{p})$. It is defined as the greatest stress that a material can withstand without losing straight line proportionality between stress and strain. Hooke's law which states that the strain (deformation) is directly proportional to stress (force or load) is obeyed in the region $OA$. From $A$ to $B$, stress and strain are not proportional.