Kink angle in Knoop- and Vickers-damaged glass for spontaneous and subcritical crack growth under mixed-mode loading

摘要

Crack growth in ceramics under multiaxial loading is often computed using the coplanar energy release rate criterion. This criterion is based on the assumption of a crack propagating in a self-similar manner. The failure condition reads in terms of stress intensity factors K_I~2 + K_(II)~2 = K_(Ic)~2 (1) where K_I, K_(II) are the stress intensity factors under mode-I and mode-II loading and K_(Ic) is fracture toughness. Since a self-similar crack shape is assumed, this criterion cannot describe failure under conditions of a changing crack plane. A simple method to check the applicability of the energy release rate criterion is to observe the crack path during crack propagation. If abrupt crack deflection occurs at the moment of spontaneous crack extension, Equation 1 must fail, since the crack does not grow self-similarly. In the following considerations this check is carried out first for spontaneous crack growth conditions. Since glasses exhibit a pronounced subcritical crack growth effect, it is of special interest to check whether subcritical crack propagation under mixed-mode loading conditions is similar to spontaneous crack extension.