Final answer:
When a material is subjected to stress greater than its yield strength, it undergoes plastic deformation, meaning stress and strain are no longer proportional, and it will not return to its original shape.
Step-by-step explanation:
When a stress is induced in a material that is greater than the material's yield strength, the stress is no longer proportional to the strain, and the material does not return to its original shape. This behavior is called plastic deformation. Initially, when the material is under low stress levels, it behaves elastically, meaning that stress and strain are directly proportional to each other.
This relationship is described by Hooke's law, similar to how a spring responds to applied forces. The constant of proportionality in this linear range is known as the elastic modulus, and the limit of this linear behavior is referred to as the linearity limit or proportionality limit.
However, once the stress exceeds the yield strength of the material, the relationship between stress and strain becomes nonlinear. If the stress stays between the yield strength and the elasticity limit, the material will still undergo elastic deformation, but beyond the elasticity limit, it enters into a plastic deformation phase.
During plastic deformation, the material is permanently deformed; that is, even after the stress is removed, the material will not regain its original shape or size. Different materials will respond differently to this excessive stress, exhibiting either ductile or brittle characteristics.