13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- 3. Experimental Specimens made of Plexiglas, of thickness d = 3.0, 4.5, 9.5 and 12.5 mm and width w = 42.4, 47.5, 51.5 and 63.5 mm, with an edge notch of length a = 15.5 mm were subjected to a progressively increasing tensile loading in an Instron testing machine. The specimens were illuminated by a convergent, divergent or parallel monochromatic light beam produced by a Ne-He laser. The caustic curves obtained from the light rays reflected from the front or rear faces of the specimen, or those transmitted through the specimen, were recorded on a viewing screen placed at a distance z0 from the specimen. Caustics were obtained at different load levels for various values of the magnification factor of the optical arrangement, m, and the distance z0. In this way, a host of caustics were obtained from different values, r, of the initial curve from the crack tip. Experimental values of stress intensity factor, Kexp, were obtained. These values were compared with theoretical values of stress intensity factor Kth given by [7] (3) Note that the experimental values of stress intensity factor are obtained under the assumption that the initial curve of the caustic lies in the region near the crack tip where plane stress conditions dominate. Thus, if the values of Kexp and Kth coincide, this means that the initial curve of the caustic lies in the region where the state of stress is plane stress. In case the values of Kexp and Kth do not coincide, this implies that the initial curve lies in the region where the state of stress is three-dimensional Fig 2 present the variation of Kexp/ Kth versus r/d for a value of the specimen thickness d = 4.5, and different values of specimen width. Points in figure correspond to different values of the applied load, P, the magnification factor of the optical arrangement, m, the distance between the specimen and the viewing screen where the caustic is formed, z0, and the specimen thickness, d. Note form figure that the ratio Kexp/ Kth increases with r/d and reaches a plateau value equal to one as the radius of the initial curve takes a limiting value rc. At that value of r = rc the state of stress in the neighborhood of the crack tip becomes plane stress. For distances r smaller than rc the state of stress is three-dimensional, while for values of r larger than rc plane stress conditions dominate. It was obtained that the critical value of r for which the state of stress becomes plane stress depends not only on d, but also on the geometrical characteristics of the cracked plate, especially the ratio of the crack length to specimen thickness. 4. Limit of Applicability of the Method of Caustics The condition that the initial curve of the caustic should lie at distances from the tip approximately greater than half the specimen thickness introduces limitations in the parameters (distance between the specimen and the viewing screen where the caustics is formed, the magnification factor of the optical arrangement, the specimen dimensions and thickness, and applied loads) entering in the determination of stress intensity factors. These factors should be properly selected so that the initial
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