13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- materials three caustics are formed by the light rays reflected from the front and rear surfaces and those transmitted through the specimen. For opaque materials, only one caustic is formed by the reflected light rays from the front surface of the specimen. The dimensions of the caustic are related to the state of stress near the crack tip. For the case of a mode-I through-the-thickness crack the stress intensity factor Kexp is given by [1] (1) where z0 is the distance between the specimen and the viewing screen where the caustic is formed, c is the stress optical constant of the specimen under conditions of plane stress, t is the specimen thickness, m is the magnification factor of the optical arrangement defined as the ratio of a length on the reference screen where the caustic is formed divided by the corresponding length on the specimen and D is the transverse diameter of the caustic at the crack tip. The above equation is valid when the state of stress in the vicinity of the crack tip is plane stress, so that the value of stress-optical constant under conditions of plane stress is used. For optically isotropic materials, the caustic is created by the light rays reflected from the circumference of a circle, the so-called initial curve, which surrounds the crack tip. The radius of the initial curve is given by r = 0.316 m D (2) Figure 1. Optical arrangement for divergent (a), convergent (b) and parallel (c) light
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