13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- values of the index of refraction n0 of the surrounding medium. Note that ct varies linearly with k. From Fig. 5 it is observed that ct remains almost constant for n0 = 1.35. This means that when the index of refraction of the medium surrounding the specimen is equal to n0 = 1.35 the stress-optical constant ct is independent of the state of stress near the crack tip. Under such circumstances Eq. (1) can be used for the correct determination of stress intensity factor KI for any values of the parameters entering in Eq. (1). Analogous results for the stress-optical constant cr are shown in Fig. 6. Note that in this case cr does not become constant for any value of n0. Figure 6. Variation of stress-optical constant cr versus triaxiality coefficient k for PMMA for various values of the index of refraction n0 of the surrounding medium. k = 0 and 1 correspond to conditions of plane stress and plane strain, respectively. 6. Use of Optically Anisotropic Materials In optically anisotropic materials the variation of the optical path of a light ray traversing the specimen along the two principal stress directions is given by: ( ) ( ) [ ]d s c r t t t 2 , 1 2 1 1,2 σ σ ξ σ σ − Δ = + ± (6) where the coefficient r t,ξ characterizes the optical anisotropy of the material for light rays reflected from the rear face (r) or traversing (t) the specimen. The plus and minus signs in equation correspond to the values σ1 and σ2 of the principal stresses. Under such conditions the parametric equations of the caustic are given by [8]:
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