Limit of proportionality

The limit of proportionality and the onset of permanent, inelastic deformation are related ideas, but they should not simply be treated as the same named point. “No longer proportional” tells you about the force–extension relationship; “inelastically deformed” tells you that the object will not completely return to its original shape after the force is removed.

The point where the straight-line relationship ends is the limit of proportionality.

A spring may behave linearly at first but cease to do so beyond its limit of proportionality. Other materials, including elastic bands, can show a curved relationship over much of their range. A curve is still a meaningful scientific relationship: it should not be replaced by a straight line merely because it generally rises.