Some materials become optically anisotropic when a mechanical stress is applied. This phenomenon is variously referred to as photoelasticity, stress birefringence, or mechanical birefringence. It is useful for the visualization of mechanical stress in otherwise transparent materials. The colors are seen when the stressed materials are placed between crossed polarizers.
Photoelastic materials are used for thin-film strain gauges for the study of mechanical stresses in mechanical parts. If a replica of a cast part is made out of photoelastic material and then subjected to stresses like those anticipated for the finished part, the maximum stress points in the shape can be identified. This could lead to the strengthening of the design at those points and eliminate costly breakdowns.
Optics, Sec 8.9