Mixed-mode I/III fracture of anticracks in porous weak layers of snow

Sustained fracture propagation driven by out-of-plane tearing (mode III ) is rarely observed, as cracks tend to rotate or kink toward energetically favorable mode I opening paths. Interfaces, however, can alter this behavior by confining crack growth along a weak plane. In porous weak layers subjected to compressive loading, fracture can occur through collapse-type mechanisms, commonly referred to as anticracks. Besides their confinement, the compressive fracture state is favorable for propagation under mode III loading. A natural example of such a process occurs in dry snow slab avalanches, where the stratified snowpack on inclined terrain enables crack propagation under combined mixed-mode loading involving compressive mode I , shear mode II , and out-of-plane mode III . Despite its potential relevance to dry snow slab avalanche release, the influence of mode III on anticrack-type fracture has not yet been investigated. We present displacement-controlled field experiments that enable controlled mode III loading in weak snow layers under mixed-mode conditions. Using an analytical weak-interface model, we quantify mode-specific energy release rates and identify an interaction law in G I / G III space. The results show that mode III fracture requires substantially less energy than mode I and that moderate mode III contributions already reduce the critical energy for fracture, providing a basis for incorporating out-of-plane loading into weak layer fracture modeling.