Plastic more than brittle failure may govern standard propagation saw tests

Nicola M Pugno, Hamed Haftbaradaran, Mahmood Heshmati

Understanding standard (1 m) propagation saw test (PST) as an invaluable tool for studying failure of snowpacks has remained elusive. Although fracture mechanics theories are commonly applied for interpreting PSTs, recently performed standard PSTs under controlled conditions of slope angle/loading suggest that such theories cannot adequately explain critical cut length measurements vs. slope angle even when empirical fracture criteria are utilized. Here, we demonstrate that standard PSTs performed for a wide range of slope angles with both upslope and downslope cuts can be explained by a nonlinear plastic analysis using a simple cap failure envelope for the weak layer rather than by brittle fracture. Building on the well-established concept of brittle-to-ductile transition in structural failure mechanics, our analysis suggests that plastic collapse within the weak layer may remain the dominant mechanism well into the dimensions of standard PSTs, depending on characteristic lengths influenced by a number of factors, including elastic properties of the slab/weak layer, fracture toughness and plastic strength of the weak layer as well as the slope angle. Accordingly, a plastic-to-brittle transition is expected with increasing the size scale, thus longer PSTs are suggested to investigate the fracture properties relevant to avalanches. Importantly, this framework generalizes beyond classical weak-layer assumptions and linear elasticity, extending to conditions such as wet snow, which favor ductile behavior and are becoming increasingly relevant under climate change.