scholarly journals Modeling of crack propagation in weak snowpack layers using the discrete element method

2015 ◽  
Vol 9 (5) ◽  
pp. 1915-1932 ◽  
Author(s):  
J. Gaume ◽  
A. van Herwijnen ◽  
G. Chambon ◽  
K. W. Birkeland ◽  
J. Schweizer
Keyword(s):  
Mechanical Properties ◽  
Crack Propagation ◽  
Discrete Element ◽  
Tensile Fracture ◽  
Dynamic Crack Propagation ◽  
Dynamic Crack ◽  
Snow Layer ◽  
Weak Layer ◽  
Failure Initiation ◽  
Fracture Arrest

Abstract. Dry-snow slab avalanches are generally caused by a sequence of fracture processes including (1) failure initiation in a weak snow layer underlying a cohesive slab, (2) crack propagation within the weak layer and (3) tensile fracture through the slab which leads to its detachment. During the past decades, theoretical and experimental work has gradually led to a better understanding of the fracture process in snow involving the collapse of the structure in the weak layer during fracture. This now allows us to better model failure initiation and the onset of crack propagation, i.e., to estimate the critical length required for crack propagation. On the other hand, our understanding of dynamic crack propagation and fracture arrest propensity is still very limited. To shed more light on this issue, we performed numerical propagation saw test (PST) experiments applying the discrete element (DE) method and compared the numerical results with field measurements based on particle tracking. The goal is to investigate the influence of weak layer failure and the mechanical properties of the slab on crack propagation and fracture arrest propensity. Crack propagation speeds and distances before fracture arrest were derived from the DE simulations for different snowpack configurations and mechanical properties. Then, in order to compare the numerical and experimental results, the slab mechanical properties (Young's modulus and strength) which are not measured in the field were derived from density. The simulations nicely reproduced the process of crack propagation observed in field PSTs. Finally, the mechanical processes at play were analyzed in depth which led to suggestions for minimum column length in field PSTs.

scholarly journals Modeling of crack propagation in weak snowpack layers using the discrete element method

2015 ◽  
Vol 9 (1) ◽  
pp. 609-653 ◽  
Author(s):  
J. Gaume ◽  
A. van Herwijnen ◽  
G. Chambon ◽  
J. Schweizer ◽  
K. W. Birkeland
Keyword(s):  
Mechanical Properties ◽  
Crack Propagation ◽  
Field Measurements ◽  
Discrete Element ◽  
Dynamic Crack Propagation ◽  
Dynamic Crack ◽  
Weak Layer ◽  
Failure Initiation ◽  
Depth Analysis ◽  
Fracture Arrest

Abstract. Dry-snow slab avalanches are generally caused by a sequence of fracture processes including (1) failure initiation in a weak snow layer underlying a cohesive slab, (2) crack propagation within the weak layer and (3) tensile fracture through the slab which leads to its detachment. During the past decades, theoretical and experimental work has gradually led to a better understanding of the fracture process in snow involving the collapse of the structure in the weak layer during fracture. This now allows us to better model failure initiation and the onset of crack propagation, i.e. to estimate the critical length required for crack propagation. On the other hand, our understanding of dynamic crack propagation and fracture arrest propensity is still very limited. For instance, it is not uncommon to perform field measurements with widespread crack propagation on one day, while a few days later, with very little changes to the snowpack, crack propagation does not occur anymore. Thus far, there is no clear theoretical framework to interpret such observations, and it is not clear how and which snowpack properties affect dynamic crack propagation. To shed more light on this issue, we performed numerical propagation saw test (PST) experiments applying the discrete element (DE) method and compared the numerical results with field measurements based on particle tracking. The goal is to investigate the influence of weak layer failure and the mechanical properties of the slab on crack propagation and fracture arrest propensity. Crack propagation speeds and distances before fracture arrest were derived from the DE simulations for different snowpack configurations and mechanical properties. Then, the relation between mechanical parameters of the snowpack was taken into account so as to compare numerical and experimental results, which were in good agreement, suggesting that the simulations can reproduce crack propagation in PSTs. Finally, an in-depth analysis of the mechanical processes at play was carried out which led to suggestions for minimum column length in field PSTs.

scholarly journals Influence of weak layer heterogeneity and slab properties on slab tensile failure propensity and avalanche release area

2015 ◽  
Vol 9 (2) ◽  
pp. 795-804 ◽  
Author(s):  
J. Gaume ◽  
G. Chambon ◽  
N. Eckert ◽  
M. Naaim ◽  
J. Schweizer
Keyword(s):  
Crack Propagation ◽  
Tensile Failure ◽  
Tensile Fracture ◽  
Snow Layer ◽  
Avalanche Size ◽  
Weak Layer ◽  
Failure Initiation ◽  
Wide Scale ◽  
Release Area ◽  
Fracture Arrest

Abstract. Dry-snow slab avalanches are generally caused by a sequence of fracture processes, including failure initiation in a weak snow layer underlying a cohesive slab followed by crack propagation within the weak layer (WL) and tensile fracture through the slab. During past decades, theoretical and experimental work has gradually increased our knowledge of the fracture process in snow. However, our limited understanding of crack propagation and fracture arrest propensity prevents the evaluation of avalanche release sizes and thus impedes hazard assessment. To address this issue, slab tensile failure propensity is examined using a mechanically based statistical model of the slab–WL system based on the finite element method. This model accounts for WL heterogeneity, stress redistribution by slab elasticity and possible tensile failure of the slab. Two types of avalanche release are distinguished in the simulations: (1) full-slope release if the heterogeneity is not sufficient to stop crack propagation and trigger a tensile failure within the slab; (2) partial-slope release if fracture arrest and slab tensile failure occur due to the WL heterogeneity. The probability of these two release types is presented as a function of the characteristics of WL heterogeneity and the slab. One of the main outcomes is that, for realistic values of the parameters, the tensile failure propensity is mainly influenced by slab properties. Hard and thick snow slabs are more prone to wide-scale crack propagation and thus lead to larger avalanches (full-slope release). In this case, the avalanche size is mainly influenced by topographical and morphological features such as rocks, trees, slope curvature and the spatial variability of the snow depth as often claimed in the literature.

scholarly journals Influence of weak layer heterogeneity and slab properties on slab tensile failure propensity and avalanche release area

2014 ◽  
Vol 8 (6) ◽  
pp. 6033-6057 ◽  
Author(s):  
J. Gaume ◽  
G. Chambon ◽  
N. Eckert ◽  
M. Naaim ◽  
J. Schweizer
Keyword(s):  
Crack Propagation ◽  
Tensile Failure ◽  
Tensile Fracture ◽  
Snow Layer ◽  
Avalanche Size ◽  
Weak Layer ◽  
Failure Initiation ◽  
Wide Scale ◽  
Release Area ◽  
Fracture Arrest

Abstract. Dry-snow slab avalanches are generally caused by a sequence of fracture processes including failure initiation in a weak snow layer underlying a cohesive slab followed by crack propagation within the weak layer (WL) and tensile fracture through the slab. During past decades, theoretical and experimental work has gradually improved our knowledge of the fracture process in snow. However, our limited understanding of crack propagation and fracture arrest propensity prevents the evaluation of avalanche release sizes and thus impedes hazard assessment. To address this issue, slab tensile failure propensity is examined using a mechanically-based statistical model of the slab–WL system based on the finite element method. This model accounts for WL heterogeneity, stress redistribution by elasticity of the slab and the slab possible tensile failure. Two types of avalanche release are distinguished in the simulations: (1) full-slope release if the heterogeneity is not sufficient to stop crack propagation and to trigger a tensile failure within the slab, (2) partial-slope release if fracture arrest and slab tensile failure occurs due to the WL heterogeneity. The probability of these two release types is presented as a function of the characteristics of WL heterogeneity and of the slab. One of the main outcomes is that, for realistic values of the parameters, the tensile failure propensity is mainly influenced by slab properties. Hard and thick snow slabs are more prone to wide-scale crack propagation and thus lead to larger avalanches (full-slope release). In this case, the avalanche size is mainly influenced by topographical and morphological features such as rocks, trees, slope curvature and the spatial variability of the snow depth as it is often claimed in the literature.

Micro-mechanical modeling using DEM to study the effect of mechanical properties on crack propagation for snow slab avalanches

2021 ◽  
Author(s):  
Bobillier Gregoire ◽  
Bergfled Bastian ◽  
Gaume Johan ◽  
van Herwijnen Alec ◽  
Schweizer Jürg
Keyword(s):  
Mechanical Properties ◽  
Steady State ◽  
Crack Propagation ◽  
Sensitivity Study ◽  
Tensile Fracture ◽  
Crack Speed ◽  
Stress Concentrations ◽  
Snow Layer ◽  
Stress Regime ◽  
Weak Layer

<p>Dry-snow slab avalanche release is a multi-scale process starting with the formation of localized failure in a highly porous weak snow layer below a cohesive snow slab, which can be followed by rapid crack propagation within the weak layer. Finally, a tensile fracture through the slab leads to its detachment. About 15 years ago, the propagation saw test (PST) was developed. The PST is a fracture mechanical field test that provides information on crack propagation propensity in weak snowpack layers. It has become a valuable research tool to investigate the processes involved in crack propagation. While this has led to a better understanding of the onset of crack propagation, much less is known about the ensuing propagation dynamics. Here, we use the discrete element method to numerically simulate PSTs in 3D and analyze the fracture dynamics using a micro-mechanical approach. Our DEM model reproduced the observed PST behavior extracted from experimental analysis. We developed different indicators to define the crack tip that allowed deriving crack speed. Our results show that crack propagation in level terrain reaches a stationary speed if the snow column is long enough. Moreover, we define stress concentration sections. Their length evolution during crack propagation suggests the development of a steady-state stress regime. Slab and weak layer elastic modulus, as well as weak layer shear strength, are the key input parameters for modeling crack propagation; they affect stress concentrations, crack speed, and the critical length for the onset of crack propagation. The results of our sensitivity study highlight the effect of these mechanical parameters on the emergence of a steady-state propagation regime and consequences for dry-snow slab avalanche release. Our DEM approach opens the possibility for a comprehensive study on the influence of the snowpack mechanical properties on the fundamental processes for avalanche release.</p>

scholarly journals Temporal evolution of weak layer and slab properties in view of snow instability

2016 ◽  
Author(s):  
Jürg Schweizer ◽  
Benjamin Reuter ◽  
Alec van Herwijnen ◽  
Bettina Richter ◽  
Johan Gaume
Keyword(s):  
Mechanical Properties ◽  
Crack Propagation ◽  
Temporal Evolution ◽  
Sensitivity Study ◽  
Distinct Pattern ◽  
Swiss Alps ◽  
Snow Layer ◽  
Weak Layer ◽  
Failure Initiation ◽  
Snow Stratigraphy

Abstract. If a weak snow layer below a cohesive slab is present in the snow cover, unstable snow conditions can prevail for days or even weeks. We monitored the temporal evolution of a weak layer of faceted crystals as well as the overlaying slab layers at the location of an automatic weather station in the Steintälli field site above Davos (Eastern Swiss Alps). We focussed on the crack propagation propensity and performed propagation saw tests on seven sampling days during a two-month period from early January to early March 2015. Based on video images taken during the tests we determined the mechanical properties of the slab and the weak layer and compared them to the results derived from concurrently performed measurements of penetration resistance using the snow micro-penetrometer (SMP). The critical cut length, observed in PSTs, showed a distinct pattern of temporal evolution that differed from the trend of other mechanical properties suggesting that it is not possible to assess crack propagation propensity by simply monitoring some of the relevant mechanical properties. A simple sensitivity study showed the complex interplay between these properties. Traditional and newly-developed metrics of snow instability describing either the failure initiation or the crack propagation propensity, calculated from simulated snow stratigraphy (SNOWPACK) or derived from the SMP signal, did partially reproduce the observed temporal pattern. Whereas our unique dataset of quantitative measures of snow instability provides new insights into the complex slab-weak layer interaction, it also showed some deficiencies of the modelled metrics of instability – calling for an improved representation of the mechanical properties.

scholarly journals Micromechanical modeling of snow failure

2020 ◽  
Vol 14 (1) ◽  
pp. 39-49 ◽  
Author(s):  
Grégoire Bobillier ◽  
Bastian Bergfeld ◽  
Achille Capelli ◽  
Jürg Dual ◽  
Johan Gaume ◽  
...  
Keyword(s):  
Discrete Element Method ◽  
Crack Propagation ◽  
Discrete Element ◽  
Micromechanical Modeling ◽  
Tensile Fracture ◽  
Snow Layer ◽  
Macroscopic Behavior ◽  
Weak Layer ◽  
Snow Model ◽  
Element Method

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, a slab avalanche releases. While the basic concepts of avalanche release are relatively well understood, performing fracture experiments in the laboratory 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 (3-D 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 low computational costs and detailed representation of important micromechanical processes. The 3-D-DEM snow model allowed reproduction of the macroscopic behavior observed during compression and mixed-mode loading of dry-snow slab and the weak snow layer. To be able to reproduce the range of snow behavior (elastic modulus, strength), relations between DEM particle and 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 us to realistically simulate snow failure processes. Furthermore, the presented snow model seems appropriate for comprehensively studying how the mechanical properties of the slab and weak layer influence crack propagation preceding avalanche release.

scholarly journals Temporal evolution of crack propagation propensity in snow in relation to slab and weak layer properties

2016 ◽  
Vol 10 (6) ◽  
pp. 2637-2653 ◽  
Author(s):  
Jürg Schweizer ◽  
Benjamin Reuter ◽  
Alec van Herwijnen ◽  
Bettina Richter ◽  
Johan Gaume
Keyword(s):  
Mechanical Properties ◽  
Crack Propagation ◽  
Temporal Evolution ◽  
Effective Elastic Modulus ◽  
Swiss Alps ◽  
Snow Layer ◽  
Weak Layer ◽  
Failure Initiation ◽  
Specific Fracture ◽  
Measurement Period

Abstract. If a weak snow layer below a cohesive slab is present in the snow cover, unstable snow conditions can prevail for days or even weeks. We monitored the temporal evolution of a weak layer of faceted crystals as well as the overlaying slab layers at the location of an automatic weather station in the Steintälli field site above Davos (Eastern Swiss Alps). We focussed on the crack propagation propensity and performed propagation saw tests (PSTs) on 7 sampling days during a 2-month period from early January to early March 2015. Based on video images taken during the tests we determined the mechanical properties of the slab and the weak layer and compared them to the results derived from concurrently performed measurements of penetration resistance using the snow micro-penetrometer (SMP). The critical cut length, observed in PSTs, increased overall during the measurement period. The increase was not steady and the lowest values of critical cut length were observed around the middle of the measurement period. The relevant mechanical properties, the slab effective elastic modulus and the weak layer specific fracture, overall increased as well. However, the changes with time differed, suggesting that the critical cut length cannot be assessed by simply monitoring a single mechanical property such as slab load, slab modulus or weak layer specific fracture energy. Instead, crack propagation propensity is the result of a complex interplay between the mechanical properties of the slab and the weak layer. We then compared our field observations to newly developed metrics of snow instability related to either failure initiation or crack propagation propensity. The metrics were either derived from the SMP signal or calculated from simulated snow stratigraphy (SNOWPACK). They partially reproduced the observed temporal evolution of critical cut length and instability test scores. Whereas our unique dataset of quantitative measures of snow instability provides new insights into the complex slab-weak layer interaction, it also showed some deficiencies of the modelled metrics of instability – calling for an improved representation of the mechanical properties.

scholarly journals Role of LPSO Phase in Crack Propagation Behavior of an As-Cast Mg-Y-Zn Alloy Subjected to Dynamic Loadings

Materials ◽  
2019 ◽  
Vol 12 (3) ◽  
pp. 498 ◽  
Author(s):  
Xuezhi Shi ◽  
Yunqian Long ◽  
Huiqiu Zhang ◽  
Liqiao Chen ◽  
Yingtang Zhou ◽  
...  
Keyword(s):  
Mechanical Properties ◽  
Crack Propagation ◽  
Crack Opening ◽  
Dynamic Crack Propagation ◽  
Dynamic Crack ◽  
Lpso Phase ◽  
Propagation Behavior ◽  
Dynamic Loadings ◽  
Crack Propagation Behavior

In this work, the role of long period stacking ordered (LPSO) phase in the crack propagation behavior of an as-cast Mg95.5Y3Zn1.5 alloy was investigated by dynamic four-point bent tests. The as-cast Mg95.5Y3Zn1.5 alloy is mainly composed of Mg matrix, 18R LPSO phase located at the grain boundaries and 14H LPSO phase located within the Mg matrix. The alloy exhibits excellent dynamic mechanical properties; the yield stress, maximum stress and strain to failure are 190.51 ± 3.52 MPa, 378.32 ± 4.26 MPa and 0.168 ± 0.006, respectively, at the strain rate of ~3000 s−1. The LPSO phase effectively hinders dynamic crack propagation in four typical ways, including crack tip blunting, crack opening inhibition, crack deflection and crack bridging, which are beneficial to the mechanical properties of the alloy under dynamic loadings.

scholarly journals Micromechanical modeling of snow failure

2019 ◽  
Author(s):  
Grégoire Bobillier ◽  
Bastian Bergfeld ◽  
Achille Capelli ◽  
Jürg Dual ◽  
Johan Gaume ◽  
...  
Keyword(s):  
Discrete Element Method ◽  
Crack Propagation ◽  
Computational Cost ◽  
Discrete Element ◽  
Micromechanical Modeling ◽  
Tensile Fracture ◽  
Macroscopic Behavior ◽  
Weak Layer ◽  
Snow Model ◽  
Element Method

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.

Sharp transition in modes of dynamic crack propagation in dry-snow slab avalanche release

2020 ◽  
Author(s):  
Bertil Trottet ◽  
Alec van Herwijnen ◽  
Stephanie Wang ◽  
Chenfanfu Jiang ◽  
Joseph Teran ◽  
...  
Keyword(s):  
Crack Propagation ◽  
Failure Modes ◽  
Slope Angle ◽  
Propagation Speed ◽  
Friction Angle ◽  
P Wave ◽  
Dynamic Crack Propagation ◽  
Dynamic Crack ◽  
Sharp Transition ◽  
Snow Layer

<p>Dry-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) onset, (iii) dynamics of crack propagation in the weak layer and eventually (iv) slab release. While a lot has been done to study the first two phases, less is known about dynamic crack propagation and slab release, especially at slope scale. </p><p>In this study, we used the Material Point Method and elastoplasticity to simulate the dynamics of 20 m long centered Propagation Saw Tests (PST). We improved the recent constitutive snow model of Gaume et al. (2018) by developing a new softening law based on the total plastic deformation (volumetric and deviatoric parts) to remove artifacts observed in failure modes.</p><p>Interestingly, several regimes of propagations are observed depending on slope angle Θ. For slope angles smaller than the friction angle (Θ < Φ), crack propagates faster in the downslope direction than upslope. The propagation speed increases with slope angle and appears closely related to the bending mechanism which sustains the propagation. For slope angles higher than the friction angle (Θ > Φ), a sharp transition is observed once the crack reaches a critical length l<sub>f</sub>. We interpret this transition as a change from slab bending to slab tension due to the increasing load in the downslope direction. An estimation of l<sub>f</sub> is proposed using a basic analytical shear model with residual friction similar to the one developped by McClung in 1979. In this case, the crack propagation speed seems to be mostly related to the P-wave speed in the slab. In this study, we explain the gap between propagation speeds based on 2 m PSTs and some observations of avalanche triggering. Finally, our results show the relevance of shear models which appear sufficient to describe slab avalanche release on steep terrain.</p>

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