Photo-Elasticity in Mechanical Engineering

Abstract

TWO papers which make a very useful addition to the sparse literature on photo-elasticity were presented to the Institution of Mechanical Engineers on December 12, 1947. Mr. W. A. P. Fisher‘s paper dealt with the basic physical properties of model materials which are relied upon in the ‘frozen stress' technique of three-dimensional stress analysis. This technique, which was originated in Germany and the United States, is based on the fact that certain photo-elastic model materials have a very low Young‘s modulus at elevated temperatures and consequently suffer considerable deformations under small loads. If such small loads remain in position during subsequent cooling, it is found that the materials set in the strained position and retain, after the removal of the loads, virtually the whole of the strain and the photo-elastic effect associated with the loading at the elevated temperature ; hence the term ‘frozen stress'. This phenomenon is utilized in three-dimensional analysis by cutting thin slices from a three-dimensional model, which can then be examined in a photo-elastic polariscope in accordance with the ordinary two-dimensional technique. The ‘frozen stress' patterns are not found to be appreciably disturbed by careful cutting.