Interference of the two partially reflected light beams from the front and back surfaces of a cracked thin plexiglas plate yielded a fringe pattern, which depicted the thickness variations of the plate due to loading. The interferogram at the vicinity of the crack tip consisted of a dense fringe pattern, which represented the constrained zone. The dense pattern was separated from the remaining sparce pattern by a caustic. The caustic was created by the reflected light rays emerging from the plate, which were twice refracted through the thickness and reflected on the back face of the plate and which were retarded according to Maxwell and Neumann’s law. A general theory was developed where the dense fringe pattern was directly related to the complex intensity factor K* = KI − iKII, combining the opening mode together with the edge-sliding mode of fracture. In this way, any combination of the two modes may be studied by this reflected shadow method and the contribution of each mode to fracture can be evaluated. A series of experiments in cracked plexiglas plates under longitudinal tension were executed. The internal symmetric crack in each plate was inclined to the axis of application of the load by a different angle β, so that various combinations between KI and KII were obtained and checked with theory.

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