Proportional limit is the point on the stress-strain diagram where the curve becomes nonlinear. The proportional limit stress is the value of stress corresponding to the elastic limit of the material. For strain levels below the elastic limit strain, Hooke's law may be used to relate stress to strain. The proportional limit is commonly assumed to coincide with the yield point unless otherwise stated in the problem statement.

This is a typical shear stress-strain diagram.

This is an ideal stress-strain diagram where the proportional limit and the yield point coincide and stress remains constant beyond the elastic limit. This type of material is known as elasto-plastic.