13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- 5. Determination of Stress Intensity Factors When the initial curve of the caustic lies at distances where three-dimensional effects dominate the proper value of the stress-optical constant, c, should be used. The value of the stress-optical constant changes from its plane strain value near the tip to its plane stress value at distances away from the tip approximately equal to half the specimen thickness. In order to characterize the three-dimensionality of the stress field near the crack tip an empirical triaxiality factor k is introduced, such that σz = k ν (σx + σy) (5) where σz is the normal stress perpendicular to the plane of the specimen, and σx and σy are the in-plane stresses. k takes the values of 0 and 1 for plane stress (σz = 0) and plane strain [σz = ν (σx + σy)], respectively. Figure 3. Variation of minimum value of z0 versus KI for parallel light. d= 0.5, 15 and 10 mm.
RkJQdWJsaXNoZXIy MjM0NDE=