13th International Conference on Fracture June 16–21, 2013, Beijing, China -9- Figure 7. Initial curves and respective caustics in a plate with crack subjected to tension made of birefringent materials with ξ = 0, 0.2, 0.4, 0.6, 0.8 and 1.0 7. Conclusions From the results of the present work the following conclusions may be drawn: a. Direct application of the method of caustics without taking special precautions for the determination of stress intensity factors may lead to erroneous results. b. The material, dimensions of the specimen, applied loads and geometrical dimensions of the optical arrangement should be properly selected to ensure that the initial curve lies in the plane stress region. c. For specimens made of Plexiglas the stress-optical constant for transmitted light rays is independent of the state of stress around the crack tip for a value of the index of refraction of the medium surrounding the specimen approximately equal to 1.35. Under such condition the plane stress stress-optical constant of the material can be used for any location of the initial curve of the caustic. On the contrary, for light rays reflected from the rear face of the specimen the stress-optical constant does not become independent of the state of stress around the crack tip for any value of the index of refraction of the medium surrounding the specimen. d. Optically anisotropic materials can effectively be used for the determination of the state of stress around the initial curve of the caustic and the correct determination of stress intensity factors. For such materials the two caustics formed can be used for the determination of the triaxiality coefficient and the subsequent calculation of the corresponding stress-optical constant.
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