Configurational stability of a crack propagating in a material with mode-dependent fracture energy - Part I: Mixed-mode I+III

In a previous paper (Leblond et al., 2011), we proposed a theoretical interpretation of the experimentally well-known instability of coplanar crack propagation in mode I+III. The interpretation relied on a stability analysis based on analytical expressions of the stress intensity factors for a crack slightly perturbed both within and out of its original plane, due to Gao and Rice (1986) and Movchan et al. (1998), coupled with a double propagation criterion combining Griffith (1920)’s energetic condition and Goldstein and Salganik (1974)’s principle of local symmetry. Under such assumptions instability modes were indeed evidenced for values of the mode mixity ratio - ratio of the mode III to mode I stress intensity factors applied remotely - larger than some threshold depending only on Poisson’s ratio. Unfortunately, the predicted thresholds were much larger than those generally observed for typical values of this material parameter. While the subcritical character of the nonlinear bifurcation from coplanar to fragmented fronts has been proposed as a possible explanation for this discrepancy (Chen et al., 2015), we propose here an alternative explanation based on the introduction of a constitutive relationship between the fracture energy and the mode mixity ratio, which is motivated by experimental observations. By re-examining the linear stability analysis of a planar propagating front, we show that such a relationship suffices, provided that it is strong enough, to lower significantly the threshold value of the mode mixity ratio for instability so as to bring it in a range more consistent with experiments. Interesting formulae are also derived for the distributions of the perturbed stress intensity factors and energy-release-rate, in the special case of perturbations of the crack surface and front obeying the principle of local symmetry and having reached a stationary state (corresponding to instability modes in near-threshold conditions).