Thursday, 28 January 2021

Speaker: Prof. Johan Gaume

Snow and Avalanche Simulation Laboratory, École polytechnique fédérale de Lausanne (EPFL), Switzerland

From sub-Rayleigh to supershear fracture in snow slab avalanche release

Abstract

Snow slab avalanche release can be separated in four distinct phases : (i) failure initiation in a weak snow layer buried below a cohesive snow slab, (ii) the onset and, (iii) dynamic phase of crack propagation within the weak layer across the slope and (iv) the slab release. The highly porous character of the weak layer implies volumetric collapse during failure which leads to the closure of crack faces followed by the onset of frictional contact. To better understand the mechanisms of dynamic crack propagation, we performed numerical simulations, snow fracture experiments, and analyzed the release of full scale avalanches. Simulations of crack propagation are performed using the Material Point Method (MPM), finite strain elastoplasticity and constitutive models based on critical state soil mechanics. Experiments consist of the so-called Propagation Saw Test (PST). Concerning full scale measurements, an algorithm is applied to detect changes in image pixel intensity induced by slab displacements. We report the existence of a transition from sub-Rayleigh anticrack to supershear crack propagation following the Burridge-Andrews mechanism. In detail, after reaching the critical crack length, self-propagation starts in a sub-Rayleigh regime and is driven by slab bending induced by weak layer collapse. If the slope angle is larger than a critical value, and if a so-called super critical crack length is reached, supershear fracture occurs. The corresponding critical angle may be lower than the weak layer friction angle due to the loss of frictional resistance during volumetric collapse. The sub-Rayleigh regime is driven by mixed mode anticrack propagation while the supershear regime corresponds to a pure mode II propagation with intersonic fracture velocities. This intersonic regime thus leads to pure tensile slab fractures initiating from the bottom of the slab as opposed to surface initiations induced by slab bending in the sub-Rayleigh regime. Key ingredients for the existence of this transition are discussed such as the role played by friction angle, collapse height and slab secondary fractures.


Biography

Johan Gaume received his PhD from the Grenoble University in 2013. He was then a postdoc at ETH Zürich and at the WSL Institute for Snow and Avalanche Research SLF in Davos, Switzerland. In 2016, he joined EPFL as are search and teaching assistant. He was a visiting scholar in the Department of Mathematics of UCLA (2017) and in the Computer and Information Science Department of UPenn (2018). In 2019, he became Assistant Professor at EPFL and head of SLAB, the Snow and Avalanche Simulation Laboratory. His research focuses on the initiation and propagation of gravitational mass movements with a particular focus on snow and avalanche mechanics which includes the development of multiscale numerical methods. He developed, in collaboration with Prof. Joseph Teran at UCLA and Prof. Chenfanfu Jiang at UPenn, new constitutive snow models to simulate avalanche release and flow at the slope scale in a unified manner using the Material Point Method. He received the Excellence Scholarship from the Swiss Government and was awarded the Ambizione grant and Eccellenza Professorial fellowship from the Swiss National Science Foundation.


Notes

by Marco Moscatelli and Antoni Joubert.

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