The Hooke's law is valid for

Hooke's Law states that stress is directly proportional to strain, which is represented by a linear relationship on a stress-strain curve. This proportionality is strictly valid only up to the proportionality limit. While the material remains elastic up to the elastic limit—meaning it can return to its original shape—the relationship between stress and strain often becomes non-linear between the proportionality limit and the elastic limit. In this non-linear elastic zone, Hooke's Law (expressed as stress = E × strain) no longer holds true because the constant of proportionality is lost. Beyond the elastic limit, the material enters the plastic region, where deformation is permanent and irreversible. Therefore, Hooke's Law is specifically valid only for the proportional region of the stress-strain curve.

Sources

1. [1] Geography of India ,Majid Husain, (McGrawHill 9th ed.) > Chapter 2: Physiography > Earthquakes in India > p. 69