Advancements in holographic photoelasticity

The success of double-exposure holography as an interferometric technique for experimental stress analysis has lead to several recent publications dealing with the theoretical expression which describes the resulting photoelastic patterns. This paper describes the extension of current theory to include the effect of an intensity difference between the light used during the first exposure and that used during the second exposure. It is shown that as the ratio of these two intensities is changed both the position and the intensity of the photoelastic-fringe pattern is altered. An interpretation of the photoelastic pattern as a simple combination of isochromatic and isopachic-fringe patterns is shown to be possible only under certain conditions. Using a pulsed ruby laser, single- and double-exposure photoelastic holograms of stress waves were obtained, and reconstructions of these holograms are presented. The clarity of the reconstructed images is comparable with photographs taken with a standard polariscope. In addition, the capability of magnifying any particular portion of the image by known holographic reconstruction techniques is illustrated.