The proportional limit is defined as:

For a material to satisfy Hooke’s law, the elastic stress must be directly proportional to the elastic strain. The initial region of the stress-strain plot must be a straight line. Because direct proportionality between two quantities is graphically represented by a straight line, the linear portion of the graph in the figure satisfies this law. Because the proportional limit (stress corresponding to point P) is the greatest elastic stress possible in accordance with this law, it represents the maximum stress above which stress is no longer proportional to strain. Key concept: Proportional limit—Magnitude of elastic stress above which plastic deformation occurs.