Three-dimensional photoelasticity provides one of the commonest and most widely used experimental methods for determination of three-dimensional states of stress. This method, like two-dimensional photoelasticity, is based upon the phenomenon of temporary or artificial birefringence, according to which when a transparent material is loaded its optical properties change; measurement of these changes provides data adequate for quantitative determination of the corresponding state of stress. Although the method of two-dimensional photoelasticity is simple and straightforward and the optical patterns obtained are directly related to the stresses developed in the material, many difficulties arise in the evaluation of a triaxial stress field, when the method of three-dimensional photoelasticity is applied. This is mainly due to variation of stress distribution from point to point along the light path; this causes rotation of the principal-stress directions from layer to layer through a three-dimensional photoelastic model.

Three-Dimensional Photoelasticity

The method of birefringent coatings allows the evaluation of the strain distribution on the surface of an opaque body, by using the principles of two-dimensional photoelasticity developed in Chapter 7. This method may be considered as an application and extension to opaque materials of two-dimensional photoelasticity, which is applicable to only transparent models. The method of birefringent coatings consists of cementing a piece of transparent material (the birefringent coating) on the surface of the body under investigation and measuring the amount and the principal directions of the temporary birefringence developed in the coating when the main object is subjected to a system of loads.

E. Neil W. Lane

Papers

Discussion of “Extended Frozen Stress Method”