the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
06 Jan 2021
06 Jan 2021
Dynamic crack propagation in weak snowpack layers: Insights from high-resolution, high-speed photography
- 1WSL Institute for Snow and Avalanche Research SLF, Davos, Switzerland
- 2Météo-France, CNRS, CNRM, Centre d‘Etudes de la Neige, Grenoble, France
- 3Institute for Mechanical Systems, ETH Zurich, Zurich, Switzerland
- 1WSL Institute for Snow and Avalanche Research SLF, Davos, Switzerland
- 2Météo-France, CNRS, CNRM, Centre d‘Etudes de la Neige, Grenoble, France
- 3Institute for Mechanical Systems, ETH Zurich, Zurich, Switzerland
Abstract. To assess snow avalanche release probability, information on failure initiation and crack propagation in weak snowpack layers underlying cohesive slab layers are required. With the introduction of the Propagation Saw Test (PST) in the mid-2000s, various studies used particle tracking analysis of high-speed video recordings of PST experiments to gain insight into crack propagation processes, including slab bending, weak layer collapse, crack propagation speed and the frictional behavior after weak layer fracture. However, the resolution of the videos and the methodology used did not allow insight into dynamic processes such as the evolution of crack speed within a PST or the touchdown distance, which is the length from the crack tip to the trailing point where the slab sits on the crushed weak layer at rest again. Therefore, to study the dynamics of crack propagation we recorded PST experiments using a powerful portable high-speed camera with a horizontal resolution of 1280 pixels at rates up to 20,000 frames per second. By applying a high-density speckling pattern on the entire PST column, we then used digital image correlation (DIC) to derive high-resolution displacement and strain fields in the slab, weak layer, and substrate. The high frame rates allowed time derivatives to obtain velocity and acceleration fields. On the one hand, we demonstrate the versatile capabilities and accuracy of the DIC method by showing three PST experiments resulting in slab fracture, crack arrest and full propagation. On the other hand, we present a methodology to determine relevant characteristics of crack propagation: the crack speed and touchdown distance within a PST, and the specific fracture energy of the weak layer. To estimate the effective elastic modulus of the slab and weak layer as well as the weak layer specific fracture energy we used a recently proposed mechanical model. A comparison to already established methods showed good agreement. Furthermore, our methodology also provides insight into the three different propagation results found with the PST and reveals intricate dynamics that are otherwise not accessible.
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Bastian Bergfeld et al.
Status: closed
-
RC1: 'Reviewer comments on Bergfeld et al. (2021)', Philipp Rosendahl, 29 Jan 2021
The manuscript presents a methodology for full-field measurements of snowpack displacements using digital image correlation. The work opens numerous possibilities for the extraction of snowpack properties. Among these, the authors discuss ways to obtain an effective homogenized elastic modulus of the slab, the weak layer fracture toughness and the speed of cracks running in the weak layer. The study focuses on the comparison of different methodologies using three representative examples.
Please consult the supplemented PDF file for a detailed review of the article.
- AC1: 'Reply on RC1', Bastian Bergfeld, 24 Mar 2021
-
RC2: 'Comment on "Dynamic crack propagation in weak snowpack layers: Insights from high-resolution, high-speed photography" by Bergfeld et al.', Edward Bair, 11 Feb 2021
In "Dynamic crack propagation in weak snowpack layers: Insights from high-resolution, high-speed photography" by Bergfeld et al., fracture in Propagation Saw Tests (PSTs) is examined using a new high-speed/resolution camera combined with digital image correlation for particle tracking. I enjoyed reviewing this article as it contains high-speed measurements and analysis of fracture at the PST scale that improves resolution of acceleration and other derivative measures that were too noisy using older equipment. The strain measurements in the weak layer and the collapse wavelength measurement are notable. The methods employed are sound and varied, e.g. three different techniques were employed for crack speed estimates.
Most of my critiques are minor and are included as an annotated manuscript. My major critique is that no attempt is made to link these measurements to slope scale avalanches or practical use, which should be overarching goals. Since its inception, the PST has been used to study fracture in snow, however we know that, as with any small-scale stability test that involves isolated blocks of snow, it is contrived and not fully representative of the avalanche process. Recent work (e.g. Gaume et al., 2019) suggests that the PST can effectively represent collapse waves in low angle terrain, but that the exaggerated bending is not representative of slope scale failure. For example, slab fracture in the PST begins at the top of the snowpack, while the simulated crowns in Gaume et al. (2019) open from the bottom. Crack speeds measured in avalanches (Hamre et al., 2014) are several times faster than 21-30 m/sec values measured in the PSTs here. Thus, I suggest further discussion on the motivation and utility of these high speed PST measurements towards understanding the avalanche process. Why are we still doing PSTs and carefully studying them? Slope angles of the PSTs are not provided, but should be. Judging from the vegetation in the background, it looks like they were conducted on nearly flat slopes. Thus, some discussion of the slower collapse wave measured on flat ground versus the faster shear fractures on steep slopes is advised. As discussed by others (Rosendahl and Weißgraeber, 2020; van Herwijnen et al., 2016), the specific fracture energies reported here are comparable to the tensile fracture of solid ice, meaning that either: 1) there is something wrong with the elastic modulus and fracture energy measurements; or 2) there is a lot of energy being dissipated during the collapse process. I suggest at least mentioning these issues with the reported values.
The data statement does not comply with The Cryosphere's stated policy. I also found numerous grammatical errors, particularly with respect to use of verb tense and subject agreement, e.g. from the abstract "The high frame rates allowed time derivatives to obtain velocity and acceleration fields." I did not highlight or correct all of these errors and suggest the use of an English language service.
NB 2/11/2021
Gaume, J., van Herwijnen, A., Gast, T., Teran, J. and Jiang, C., 2019. Investigating the release and flow of snow avalanches at the slope-scale using a unified model based on the material point method. Cold Regions Science and Technology, 168: 102847.
Hamre, D., Simenhois, R. and Birkeland, K., 2014. Fracture speeds of triggered avalanches, International Snow Science Workshop, Banff, pp. 174-178.
Rosendahl, P.L. and Weißgraeber, P., 2020. Modeling snow slab avalanches caused by weak-layer failure – Part 1: Slabs on compliant and collapsible weak layers. The Cryosphere, 14(1): 115-130.
van Herwijnen, A., Gaume, J., Bair, E.H., Reuter, B., Birkeland, K.W. and Schweizer, J., 2016. Energy-based method for deriving fracture energy and elastic properties of snowpack layers. Journal of Glaciology.
- AC2: 'Reply on RC2', Bastian Bergfeld, 24 Mar 2021
Status: closed
-
RC1: 'Reviewer comments on Bergfeld et al. (2021)', Philipp Rosendahl, 29 Jan 2021
The manuscript presents a methodology for full-field measurements of snowpack displacements using digital image correlation. The work opens numerous possibilities for the extraction of snowpack properties. Among these, the authors discuss ways to obtain an effective homogenized elastic modulus of the slab, the weak layer fracture toughness and the speed of cracks running in the weak layer. The study focuses on the comparison of different methodologies using three representative examples.
Please consult the supplemented PDF file for a detailed review of the article.
- AC1: 'Reply on RC1', Bastian Bergfeld, 24 Mar 2021
-
RC2: 'Comment on "Dynamic crack propagation in weak snowpack layers: Insights from high-resolution, high-speed photography" by Bergfeld et al.', Edward Bair, 11 Feb 2021
In "Dynamic crack propagation in weak snowpack layers: Insights from high-resolution, high-speed photography" by Bergfeld et al., fracture in Propagation Saw Tests (PSTs) is examined using a new high-speed/resolution camera combined with digital image correlation for particle tracking. I enjoyed reviewing this article as it contains high-speed measurements and analysis of fracture at the PST scale that improves resolution of acceleration and other derivative measures that were too noisy using older equipment. The strain measurements in the weak layer and the collapse wavelength measurement are notable. The methods employed are sound and varied, e.g. three different techniques were employed for crack speed estimates.
Most of my critiques are minor and are included as an annotated manuscript. My major critique is that no attempt is made to link these measurements to slope scale avalanches or practical use, which should be overarching goals. Since its inception, the PST has been used to study fracture in snow, however we know that, as with any small-scale stability test that involves isolated blocks of snow, it is contrived and not fully representative of the avalanche process. Recent work (e.g. Gaume et al., 2019) suggests that the PST can effectively represent collapse waves in low angle terrain, but that the exaggerated bending is not representative of slope scale failure. For example, slab fracture in the PST begins at the top of the snowpack, while the simulated crowns in Gaume et al. (2019) open from the bottom. Crack speeds measured in avalanches (Hamre et al., 2014) are several times faster than 21-30 m/sec values measured in the PSTs here. Thus, I suggest further discussion on the motivation and utility of these high speed PST measurements towards understanding the avalanche process. Why are we still doing PSTs and carefully studying them? Slope angles of the PSTs are not provided, but should be. Judging from the vegetation in the background, it looks like they were conducted on nearly flat slopes. Thus, some discussion of the slower collapse wave measured on flat ground versus the faster shear fractures on steep slopes is advised. As discussed by others (Rosendahl and Weißgraeber, 2020; van Herwijnen et al., 2016), the specific fracture energies reported here are comparable to the tensile fracture of solid ice, meaning that either: 1) there is something wrong with the elastic modulus and fracture energy measurements; or 2) there is a lot of energy being dissipated during the collapse process. I suggest at least mentioning these issues with the reported values.
The data statement does not comply with The Cryosphere's stated policy. I also found numerous grammatical errors, particularly with respect to use of verb tense and subject agreement, e.g. from the abstract "The high frame rates allowed time derivatives to obtain velocity and acceleration fields." I did not highlight or correct all of these errors and suggest the use of an English language service.
NB 2/11/2021
Gaume, J., van Herwijnen, A., Gast, T., Teran, J. and Jiang, C., 2019. Investigating the release and flow of snow avalanches at the slope-scale using a unified model based on the material point method. Cold Regions Science and Technology, 168: 102847.
Hamre, D., Simenhois, R. and Birkeland, K., 2014. Fracture speeds of triggered avalanches, International Snow Science Workshop, Banff, pp. 174-178.
Rosendahl, P.L. and Weißgraeber, P., 2020. Modeling snow slab avalanches caused by weak-layer failure – Part 1: Slabs on compliant and collapsible weak layers. The Cryosphere, 14(1): 115-130.
van Herwijnen, A., Gaume, J., Bair, E.H., Reuter, B., Birkeland, K.W. and Schweizer, J., 2016. Energy-based method for deriving fracture energy and elastic properties of snowpack layers. Journal of Glaciology.
- AC2: 'Reply on RC2', Bastian Bergfeld, 24 Mar 2021
Bastian Bergfeld et al.
Video supplement
Crack Propagation in a Propagation Saw Test Bastian Bergfeld https://doi.org/10.5446/50536
Bastian Bergfeld et al.
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Cited
1 citations as recorded by crossref.
dynamic crack propagation phasein which a whole slope gets detached. The present work contains the first field methodology which provides the temporal and spatial resolution necessary to study this phase. We demonstrate the versatile capabilities and accuracy of our method by revealing intricate dynamics and present how to determine relevant characteristics of crack propagation such as e.g. crack speed.
dynamic crack propagation...