Proportional Limit: Also known as the elastic limit, is the point where the stress-strain curve becomes nonlinear. The stress and strain values at this point are known as the proportional-limit stress and strain, respectively. This is the point beyond which Hooke's law can no longer be used to relate stress and strain in axial or shear deformation. The figure below shows the proportional limit point associated with axial loading. A similar point also exists in shear stress-strain curves.
