Solution of Fracture Mechanics Problems by the Optical Method of Caustics- New Developments

triaxial state of stress, stress-optical constants Abstract. The optical method of caustics constitutes a powerful tool in the hands of the experimentalist for the solution of fracture mechanics problems. 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 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 thickness need to be satisfied. These conditions are fully investigated in this work. It is shown that special precautions should be taken when applying the method to determine stress intensity factor