The reflected caustics method for the evaluation of mode III stress intensity factor

The optical method of reflected caustics, which has been applied to the evaluation of stress intensity factors in cracked plates deformed under mode I and II, is extended in this paper to the evaluation of the same factor in cracked plates subjected to mode III deformation. It is shown that the method of reflected caustics is capable of detecting and evaluating this factor, whereas all the other experimental methods, i.e. photoelasticity, holographic interferometry and especially the method of transmitted caustics are incapable to yield this quantity. Based on the first-order approximation of the elastic solution around the crack tip and on Sneddon's formulas, the theory of formation of the reflected caustics is developed and the characteristic geometric properties of this envelope curve are defined. It is shown that this envelope is again a generalized epicycloid, whose characteristic dimensions are directly related to K_III. Experimental evidence with specimens made of optically isotropic materials (plexiglas), elastically loaded, has corroborated the theoretical results.