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Tensile strength and fracture mechanics of cohesive dry snow related to slab avalanches Borstad, Christopher P.


Fracture mechanics has been applied for over 30 years to explain the release of slab avalanches, but most studies have focused on the initial shear fracture which governs the loss of slab stability rather than the ultimate tensile fracture which releases the avalanche. The application of continuum fracture mechanics to snow—a porous material near the melting temperature—requires a homogenization scheme which accounts for the characteristic length scales associated with the diffuse nature of cracking in snow. An experimental campaign was conducted to measure the strength, fracture mechanical properties, and length scales in the tensile fracture of cohesive dry snow related to slab avalanches. Over 1000 natural snow samples were fractured in beam bending tests in a cold laboratory. Significant rate and size effects were observed in the experiments, though the loading rates were sufficiently high to justify an effective elastic analysis of the data. Using beam theory, the tensile strength was calculated from hundreds of unnotched bending tests and compared with over 2000 synthesized tensile strength measurements from the literature. From the results of three different types of fracture experiments, the fracture toughness and effective fracture process zone length were calculated using equivalent elastic fracture mechanics, which approximately accounts for the nonlinearity engendered by the distributed nature of microcracking in snow. A thin-blade penetration resistance gauge was developed which characterizes structural variations in cohesive snow. The maximum force of penetration was the best index variable for correlating with tensile strength and fracture toughness. A nonlocal damage mechanics model, implemented in a finite element code, was calibrated using the results of ten series of experiments, providing a foundation for future predictive modeling applications related to slab avalanches. The tensile strength and fracture toughness of cohesive snow are now well constrained as functions of the snow density, penetration resistance, grain size, strain rate and sample size. The tensile fracture process zone was determined to be about 10-20 times the grain size, a length scale which necessitates the use of nonlinear fracture mechanics in the analysis of all but the very largest slab avalanches.

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