Performing mixed-mode fracture tests to assess crack propagation in weak snowpack layers

Dry-snow slab avalanches release due to widespread crack propagation in a weak layer buried below cohesive slab layers. To understand the onset of crack propagation, it is essential to measure fracture properties of weak layers. As crack propagation in snow commonly occurs on inclined terrain, the interaction of different fracture modes also needs to be accounted for. Mode I denotes loading normal to the crack faces and mode II loading parallel to the crack surface but normal to the crack front. So far, experimental results on this mode interaction are lacking. Here we present results using a novel field method to derive the mixed-mode fracture toughness of weak layers, a material property describing the resistance to crack growth. Crack propagation will begin once the energy release rate exceeds the specific fracture energy, which is a measure for the fracture toughness. In order to cover the entire interaction range between mode I and mode II, we performed tilted fracture mechanical field experiments to determine fracture characteristics of different types of weak layers. Fitting the obtained results with a power law allows to represent the correlation between fracture characteristics and the full range of mode interactions. Our first results suggest a quadratic interaction and the measured specific fracture energy is larger for mode II than for mode I which both is in agreement with observed behavior in other materials. The observed fracture energies have the same order of magnitude as previous, comparable experiments. These results provide the first measurements of the mixed-mode fracture toughness of different weak layers and can be used to establish a link between snow microstructure and mechanical properties to ultimately improve avalanche forecasting.