When the distortion gets big

Graphic - No title - beyondthelimit

The linear relationship between restoring force (the more general terminology is stress) and distortion (alias strain) is typical of the behaviour of solid materials when the strains are small.

For larger strains the relationship is in general no longer linear. Springs generating forces that are non-linear functions of their length can give rise to exotic (not at all simple harmonic) motion, of a kind we shall explore in the final stage of this course.

For larger strains still the material ruptures. The how, when and why of large-scale deformation and rupture are amongst the core concerns of materials science.