Elastic Limit (B):

If we increase the strain a little bit beyond the point A, the stress is no longer proportional to the strain. However, the wire still remains elastic. This means that, when the external force is removed, the body regains the original state. Therefore, point B denotes the maximum value of stress for which a body (wire) shows elastic property and this point is called elastic limit.

Yield Point (C)

When the wire is stressed beyond the elastic limit B, strain increases more rapidly and the body behaves like a plastic material; that is, if the load is removed, wire will contract but it will not regain its original length. A material follows dotted line CD on the curve when the deforming force is removed and the residual strain OD is known as a permanent set.

When the applied stress is further increased, we reach a point E. Beyond the point E, none of the extension is recoverable and the material exhibits completely plastic behaviour.

Breaking Point (F)

Beyond point E, strain increases much more rapidly and near point F the length of wire increases continuously even without increase of deforming force or even by reducing the force a little. In other words the wire breaks at point F. This is called breaking point. The stress corresponding to breaking point F is called breaking stress or tensile strength.

If large deformation takes place between the elastic limit and the breaking point, the material is called ductile. If it breaks soon after the elastic limit is crossed, it is called brittle.