A Room Temperature Photo-elastic Technique for Three-dimensional Problems

For the application of photo-elasticity to the stress analysis of three-dimensional problems, the existing methods rely on the “stress-freezing” property of thermosetting resins. This again requires the model material to have certain characteristics that are not always present. The lack of these leads to experimental difficulties that may be overcome only by skilful technique.

In the “sandwich” method described in the paper, the difficulties associated with stress-freezing— as, for example, large distortion of the model and rind effect—are avoided by loading at room temperature. In addition a series of fringe patterns is obtained for the specific plane selected for investigation, as distinct from one pattern only for a frozen slice. These patterns are repeatable for the same loading. The difficulties that arise in the production of a sandwich in “Perspex”, with “Catalin 800” as the birefringent layer, are discussed in detail. It is emphasized that, before it is made up into the sandwich, the Catalin 800 must be subjected to additional curing to bring its elastic properties into line with those of Perspex; this also improves its behaviour at room temperature.

Semicircular notches on round bars subjected to axial tension have been selected to demonstrate the correctness of the solutions obtained, and the results are in good agreement with those predicted by Neuber (1945)^‡. The method has been applied also to the case of a thick cylinder under internal pressure, to investigate the increased stresses arising at a diametral port, and also to the estimation of the stress distribution in nut and bolt assemblies.

Despite certain limitations met with in each particular application, it is evident that the “composite model technique” described in the paper provides a very useful tool for stress analysis in three-dimensional problems.