Photoelasticity

Photoelasticity is an experimental method to determine stress distribution in a material. The method is mostly used in cases where mathematical methods become quite cumbersome. Unlike, the analytical methods of stress determination, photoelasticity gives a fairly accurate picture of stress distribution even around abrupt discontinuities in a material. The method serves as an important tool for determining the critical stress points in a material and uis often used for detrmining stress concentration factors in irregular geometries.

Principle of Photoelasticity

The method is based on the property of birefringence, which is exhibited by certain transparent materials. Birefringence is property by virtue of which a ray of light passing through a birefringent material experiences two refractive indices. The property of bifringence or double refraction is exhibited by many optical crystals. But photoelastic materials exhibit the property of birefringence only on the application of stress and the magnitude of the refractive indices at each poitn in the material is directly related to the state of stress at that point. Thus, the first task is to develop a model made out of such materials. The model has a similar geometry to that of the structure where stress analysis is to be performed. When a ray a of light is passed through a photoelastic material, it gets resolved along the two principal stress directions and each of these components experience different refractive indices. The two waves are then brought together in a polariscope. The phenomena of optical interference takes place and we get a fringe pattern. By studying the fringe pattern one can determine the state of stress at various points in the material.