Elastic snow properties for the optimization of weak layer fracture toughness estimates

Dry-snow slab avalanches release due to crack propagation in a weak layer inside the snowpack. Understanding the fracture characteristics of the weak layer is essential for describing the onset of crack propagation and hence for predicting avalanche release. Avalanches release on steep slopes, thus crack propagation is a mixed mode fracture problem. Yet, thus far little is known about the mixed-mode fracture toughness of weak layers, a material property describing the resistance to crack growth under different loading conditions, from mode I normal to the crack faces to mode II parallel to the crack face. Here, we present experiments that were conducted to derive a full range interaction between mode I and mode II fracture toughness of natural weak layers. Using a mechanical model, we derived fracture toughness values under different mixed-mode loading conditions. Crucial model variables are the elastic properties of the slab and the weak layer, which we retrieved from high-speed video recordings of the experiments and digital image correlation. These elastic properties allow for optimization of the estimates for weak layer fracture toughness values. Our results show that the specific fracture energy is larger in mode II than in mode II. This agrees with the behavior observed in other materials. In future we will investigate the fracture properties of numerous weak layer microstructures. Since the snow microstructure most likely controls the mechanical properties, a characterization of the microstructure is essential. The connection between weak layer fracture and the microstructure of weak snowpack layers can be used to ultimately improve slab avalanche forecasting.