Photoelasticity

Photoelasticity is a method to determine the stress distribution in a material experimentally. 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 around discontinuities in materials. The method is an important tool for determining critical stress points in a material, and is used for determining stress concentration in irregular geometries.

The photoelastic phenomenon was first described by the Scottish physicist David Brewster . Photoelasticity was developed at the beginning of the twentieth century with the works of E. G. Coker and L. N. G. Filon of University of London. Their book Treatise on Photoelasticity, published in 1930 by Cambridge Press, became a standard text on the subject. Between 1930 and 1940, many other books appeared on the subject, including books in Russian, German and French. At the same time, much development occurred in the field – great improvements were achieved in technique, and the equipment was simplified. With refinements in the technology, the application of photoelasticity was extended to determining three-dimensional states of stress. Many practical problems were solved using photoelasticity, and it soon became widely used. A number of photoelastic laboratories were established at educational institutions and in industry.

With the advent of the digital polariscope – made possible by light-emitting diodes – continuous monitoring of structures under load became possible. This led to the development of dynamic photoelasticity, which has contributed greatly to the study of complex phenomena such as fracture of materials.

The method is based on the property of birefringence, as exhibited by certain transparent materials. Birefringence is a phenomenon in which a ray of light passing through a given material experiences two refractive indices. The property of birefringence (or double refraction) is observed in many optical crystals. Upon the application of stresses, photoelastic materials exhibit the property of birefringence, and the magnitude of the refractive indices at each point in the material is directly related to the state of stresses at that point. Information such as maximum shear stress and its orientation are available by analyzing the birefringence with an instrument called a polariscope.

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