Stress Triaxiality at Crack Tips Studied by Caustics

The optical method of caustics constitutes a powerful tool in the hands of the experimentalist for the solution of fracture mechanics problems. According to the method, the area in the vicinity of the crack tip is illuminated by a light beam and the reflected or transmitted light rays form an envelope in space. When this envelope is cut by a screen a highly illuminated curve, the so-called caustic, is formed. By measuring characteristic dimensions of the caustic the stress intensity factor is determined. The method has been based on the assumption that plane stress conditions dominate in the vicinity of the crack tip. However, experimental evidence has shown that the state of stress is not pure plane stress. It changes from plane strain near the crack tip to plane stress at a critical distance away from the tip through a three-dimensional region. In order the caustic to be generated from the light rays reflected or transmitted through the specimen from the plane stress region certain conditions among the dimensions of the optical arrangement, the specimen properties and specimen thickness need to be satisfied. These conditions are investigated in this work so that the method of caustics can safely be used for determination of stress intensity factors in fracture mechanics problems.