Acoustic emission signatures prior to snow failure

Capelli, Achille; Reiweger, Ingrid; Schweizer, Jürg

doi:10.1017/jog.2018.43

Cited by 16 publications

(37 citation statements)

References 61 publications

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“…It appears that load relaxation in combination with sintering changes the failure behavior from continuous to abrupt. These model results are in accordance to results of loading experiments of snow, where at high loading rates precursors to failure were observed, whereas for slow loading rates, for which sintering and load relaxation are supposed to be most relevant, no precursors were noted [9].…”

Section: B Load Relaxationsupporting

confidence: 89%

“…For all strain rates, snow fails at strain and stress values that are much higher than the linear elastic limit [7]. This rate dependence of the failure behavior is also reflected in the acoustic emission (AE) response of snow (e.g., [8,9]). The AE response in turn is used for measuring precursors of material breakdown and to predict the time of failure.…”

Section: Introductionmentioning

confidence: 90%

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Fiber-bundle model with time-dependent healing mechanisms to simulate progressive failure of snow

et al. 2018

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Snow is a heterogeneous material with strain-and/or load-rate-dependent strength. In particular, a transition from ductile-to-brittle failure behavior with increasing load rate is observed. The rate-dependent behavior can partly be explained with the existence of a unique healing mechanism in snow that stems from its high homologous temperature (temperature close to melting point). As soon as broken elements in the ice matrix get in contact, they start sintering and the structure may regain strength. Moreover, the ice matrix is subjected to viscous deformation, inducing a relaxation of local load concentrations and, therefore, further counteracting the damage process. Ideal tools for studying the failure process of heterogeneous materials are the fiber-bundle models (FBMs), which allow investigating the effects of basic microstructural characteristics on the general macroscopic failure behavior. We present an FBM with two concurrent time-dependent healing mechanisms: sintering of broken fibers and relaxation of load inhomogeneities. Sintering compensates damage by creating additional intact, load-supporting fibers which lead to an increase of the bundle strength. However, the character of the failure is not changed by sintering alone. With combined sintering and load relaxation, load is distributed from old stronger fibers to new fibers that carry fewer load. So as we additionally incorporated load redistribution to the FBM, the failure occurred suddenly without decrease of the order parameter-describing the amount of damage in the bundle-and without divergence of the fiber failure rate. Moreover, the b value, i.e., the power-law exponent of frequency-magnitude statistics of fibers breaking in load redistribution steps, at failure converged to b ≈ 2, a value higher than that of a classical FBM without healing (b = 3 2 ). These results indicate that healing, as the combined effect of sintering and load relaxation, changes the type of the phase transition at failure. This change of the phase transition is important for quantifying or predicting the failure (e.g., by monitoring acoustic emissions) of snow or other materials for which healing plays an important role.

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Section: B Load Relaxationsupporting

confidence: 89%

Section: Introductionmentioning

confidence: 90%

Fiber-bundle model with time-dependent healing mechanisms to simulate progressive failure of snow

et al. 2018

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“…In our case, the failure envelope is directly linked to ℎ , since any failure envelope can be expressed as a function of ℎ . Weak layer failure behavior was not affected by the heterogeneity induced by different types of random ball deposition and by the sample size if the sample size was larger than 0.3 m  0.3 m. This size is typically found in field tests (PST, ECT;van Herwijnen et al, 2016;Reuter et al, 2015;Bair et al, 2014) and laboratory experiments (Capelli et al, 2018).…”

Section: Discussionmentioning

confidence: 95%

“…To select the model parameters and ℎ ,we used the elastic modulus computed as the derivative of the normal stressstrain curve and the strength values from the experiments (Table 1), as well as the relations for strength and modulus derived below. Digital image analysis of the experiments had revealed that the deformation was concentrated in the weak layer (Capelli et al, 2018). We therefore simulated the weak layer with a rigid actuator layer on the top.…”

Section: Laboratory Experimentsmentioning

confidence: 99%

Micromechanical modeling of snow failure

Bobillier¹,

Bergfeld²,

Capelli³

et al. 2019

Preprint

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Abstract. Dry-snow slab avalanches start with the formation of a local failure in a highly porous weak layer underlying a cohesive snow slab. If followed by rapid crack propagation within the weak layer and finally a tensile fracture through the slab appears, a slab avalanche releases. While the basic concepts of avalanche release are relatively well understood, performing fracture experiments in the lab or in the field can be difficult due to the fragile nature of weak snow layers. Numerical simulations are a valuable tool for the study of micromechanical processes that lead to failure in snow. We used a three-dimensional discrete element method (3D-DEM) to simulate and analyze failure processes in snow. Cohesive and cohesionless ballistic deposition allowed us to reproduce porous weak layers and dense cohesive snow slabs, respectively. To analyze the micromechanical behavior at the scale of the snowpack (~ 1 m), the particle size was chosen as a compromise between a low computational cost and a detailed representation of important micromechanical processes. The 3D-DEM snow model allowed reproducing the macroscopic behavior observed during compression and mixed-modes loading of dry snow slab and weak snow layer. To be able to reproduce the range of snow behavior (elastic modulus, strength), relations between DEM particle/contact parameters and macroscopic behavior were established. Numerical load-controlled failure experiments were performed on small samples and compared to results from load-controlled laboratory tests. Overall, our results show that the discrete element method allows to realistically simulate snow failure processes. Furthermore, the presented snow model seems appropriate for comprehensively studying how the mechanical properties of slab and weak layer influence crack propagation preceding avalanche release.

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“…Yet, it is known that the deformation of the weak layer is crucial for deformations and the local load transfer in the snowpack (Reiweger and Schweizer, 2010). Chiaia et al (2008) and Gaume et al (2013) consider weak-layer deformability only in shear.…”

Section: Introductionmentioning

confidence: 99%

Modeling snow slab avalanches caused by weak-layer failure – Part 1: Slabs on compliant and collapsible weak layers

Rosendahl

Weißgraeber

2020

The Cryosphere

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Abstract. Dry-snow slab avalanche release is preceded by a fracture process within the snowpack. Recognizing weak-layer collapse as an integral part of the fracture process is crucial and explains phenomena such as whumpf sounds and remote triggering of avalanches from low-angle terrain. In this two-part work we propose a novel closed-form analytical model for a snowpack under skier loading and a mixed-mode failure criterion for the nucleation of weak-layer failure. In the first part of this two-part series we introduce a closed-form analytical model of a snowpack accounting for the deformable layer. Despite the importance of persistent weak layers for slab avalanche release, no simple analytical model considering weak-layer deformations is available. The proposed model provides deformations of the snow slab, weak-layer stresses and energy release rates of cracks within the weak layer. It generally applies to skier-loaded slopes as well as stability tests such as the propagation saw test. A validation with a numerical reference model shows very good agreement of the stress and energy release rate results in several parametric studies including analyses of the bridging effect and slope angle dependence. The proposed model is used to analyze 93 propagation saw tests. Computed weak-layer fracture toughness values are physically meaningful and in excellent agreement with finite element analyses. In the second part of the series (Rosendahl and Weißgraeber, 2020) we make use of the present mechanical model to establish a novel failure criterion crack nucleation in weak layers. The code used for the analyses in both parts is publicly available under https://github.com/2phi/weac (last access: 6 January 2020) (2phi, 2020).

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Acoustic emission signatures prior to snow failure

Cited by 16 publications

References 61 publications

Fiber-bundle model with time-dependent healing mechanisms to simulate progressive failure of snow

Fiber-bundle model with time-dependent healing mechanisms to simulate progressive failure of snow

Micromechanical modeling of snow failure

Modeling snow slab avalanches caused by weak-layer failure – Part 1: Slabs on compliant and collapsible weak layers

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