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
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.