An optical method was developed for the study of constrained zones around cracks in transparent specimens made of birefringent materials under conditions of plane stress. The absolute retardations of light rays impinging normally at the plate, partly reflected from the back face, and twice refracted at the front face create a caustic when emerged from the plate, which has the shape of a generalized epicycloid, when it is projected on a screen far from the plate. The caustic defines the constrained zone surrounding a crack tip. The characteristic properties of the caustic were studied in relation to the straining mode of the plate.
P. S. Theocaris, "Reflected Shadow Method for the Study of Constrained Zones in Cracked Plates," Appl. Opt. 10, 2240-2247 (1971)