Permanent Deformation:

A permanent change occurs in a body when the elastic limit of the material of the body is crossed. It is called permanent deformation. For example, when a wire is stretched by a stretching force, then within the elastic limit the longitudinal strain is proportional to the stress, this means that it obeys Hooke’s law. On the other hand, beyond the elastic limit, strain is not proportional to the stress, as a result, a permanent deformation is attained when the deforming force is withdrawn.

Breaking Stress:

If the stretching force is further increased beyond the elastic limit, a stage reaches when the wire just breaks, the corresponding force per unit area, i.e., the corresponding stress is called breaking stress.