Item: FRACTURE ENERGIES RELEVANT FOR DRY-SNOW SLAB AVALANCHE RELEASE
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Title: FRACTURE ENERGIES RELEVANT FOR DRY-SNOW SLAB AVALANCHE RELEASE
Proceedings: International Snow Science Workshop Proceedings 2023, Bend, Oregon
Authors:
- Bastian Bergfeld [ WSL Institute for Snow and Avalanche Research SLF, Davos, Switzerland ]
- Alec van Herwijnen [ WSL Institute for Snow and Avalanche Research SLF, Davos, Switzerland ]
- Gregoire Bobilier [ WSL Institute for Snow and Avalanche Research SLF, Davos, Switzerland ]
- Jürg Schweizer [ WSL Institute for Snow and Avalanche Research SLF, Davos, Switzerland ]
Date: 2023-10-08
Abstract: In our current understanding of dry-snow slab avalanche release, the crack propagation phase is composed of two parts: first, the onset of crack propagation, i.e. the moment when an initial crack reaches a critical size at which the energy release rate of the system equals the specific fracture energy of the weak layer; second, the dynamic crack propagation phase, when the crack propagates across the slope. To assess the material resistance against crack propagation, one has to know the specific fracture energy. So far, this parameter has only been determined at the onset of crack propagation, and dynamic fracture energy has not yet been measured in snow. This is a relevant property, as during dynamic crack propagation, several processes may modify the fracture energy, and thus potentially stop crack propagation (e.g. crack branching, strain rate effects). With data from long flat field PSTs (up to 9 meters), we suggest a method to assess dynamic fracture energy. Using displacement fields during crack propagation derived from high-speed camera recordings of PSTs, we measured dynamic fracture energies (0.05 to 0.43 J m-2) that were similar to fracture energy measured at the onset of crack propagation (0.1 to 1.5 J m-2). Furthermore, we found that most of the total available energy dissipated during the compaction of the weak layer (30 times more), occurring after initial crack formation. The excess of available energy supplied by the settlement of the slab, was therefore not (viscous-) plastically absorbed in the slab, but in the weak layer itself subsequently to the formation of the primary fracture at the moving crack tip by the compaction of the weak layer. This interpretation of the cracking process is a peculiarity of closing cracks and both energies (dynamic fracture energy and compaction energy) are necessary for sustained crack propagation in flat terrain. Dynamic fracture energy required for the primary fracture that forms the moving crack tip and compaction energy for the subsequent compaction of the weak layer. This work illustrates how complex closing cracks are and that the propensity of the snowpack for self-sustained crack propagation cannot be assessed with typically sized PSTs.
Object ID: ISSW2023_P1.47.pdf
Language of Article: English
Presenter(s): Bastian Bergfeld
Keywords: avalanche release, fracture mechanics, weak layer properties, propagation saw test
Page Number(s): 401 - 407
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