Theory of Mixed-Mode Caustics for Stationary Cracks in Optically Anisotropic Materials

The optical method of caustics is one of the most powerful techniques for evaluating stress intensity factors in static as well as dynamic fracture mechanics problems. In the method of transmitted caustics, optically anisotropic (birefringent) materials such as Araldite B and polycarbonate were often used. Double caustic curves appear for a tip of the crack in an anisotropic (birefringent) plate, due to the birefringence. In this paper, a caustic theory was developed for stationary cracks in birefrigent materials under in-plane mixed-mode conditions. Completely analytical expressions were derived for the double caustic curves as well as for the double initial curves. The combined effects of optical anisotropy and mixed-mode state on the caustic curves and the initial curves were investigated. New procedures were also developed for the evaluation of mixed-mode stress intensity factors using the characteristic dimensions of the double caustic patterns.