A higher-order method for determining quasi-brittle tensile fracture parameters governing the release of slab avalanches and a new tool for in situ indexing

Abstract

The tensile fracture of heterogeneous earth materials such as snow, ice, and rocks can be characterized by two fracture parameters - the fracture toughness and the fracture process zone length. The latter length scale characterizes the zone of microcracking surrounding a crack tip in a heterogeneous material. For alpine snow, these two fracture parameters influence the release dimensions and thus destructive potential of slab avalanches. In general, it is difficult to determine these parameters concurrently, and most experimental methods are based on first-order scaling laws that have considerable errors unless very large test specimens are used. Here we introduce a simple experimental method based on a higher-order quasi-brittle scaling law that has never been applied to snow nor any other geophysical material. We conducted hundreds of beam bending experiments using natural cohesive snow samples to produce the most comprehensive measurements to date of the tensile fracture toughness and effective process zone length of snow. We also adopt a new penetration resistance gauge to index the fracture toughness data, addressing a longstanding need for better proxy measurements to characterize snow structure. The peak penetration resistance met by a thin blade proved better than the bulk snow density for predicting fracture toughness, a finding that will improve field predictions and facilitate comparisons of results across studies. The tensile fracture process zone, previously a highly uncertain length scale related to avalanche fractures, is shown to be about 5-10 times the snow grain size, implying nonlinear fracture scaling for the majority of avalanches. Key Points Higher-order quasi-brittle scaling law applied for first time to snow Tensile fracture toughness and process zone length determined Penetration resistance proves better than density for predicting toughness ©2013. American Geophysical Union. All Rights Reserved.