Avalanche study published

I saw this in the journal Science today and thought that some might find it interesting. The following is the abstract. For the more technical readers, they can register online to read the full text or go to the library and find the article.

Science 11 July 2008:
Vol. 321. no. 5886, pp. 240 - 243
DOI: 10.1126/science.1153948

Anticrack Nucleation as Triggering Mechanism for Snow Slab Avalanches
J. Heierli,1,2* P. Gumbsch,2,3 M. Zaiser1

Snow slab avalanches are believed to begin by the gravity-driven shear failure of weak layers in stratified snow. The critical crack length for shear crack propagation along such layers should increase without bound as the slope decreases. However, recent experiments show that the critical length of artificially introduced cracks remains constant or, if anything, slightly decreases with decreasing slope. This surprising observation can be understood in terms of volumetric collapse of the weak layer during failure, resulting in the formation and propagation of mixed-mode anticracks, which are driven simultaneously by slope-parallel and slope-normal components of gravity. Such fractures may propagate even if crack-face friction impedes downhill sliding of the snowpack, indicating a scenario in which two separate conditions have to be met for slab avalanche release.

1 Centre of Materials Science and Engineering, University of Edinburgh, Sanderson Building, The King's Buildings, Edinburgh, EH9 3JL, UK.
2 Institut für Zuverlässigkeit von Bauteilen und Systemen, Universität Karlsruhe, Kaiserstr. 12, 76131 Karlsruhe, Germany.
3 Fraunhofer Institut für Werkstoffmechanik, Wöhlerstr. 11, 79108 Freiburg, Germany.

Alan

The pdf-inclined might do well to have a quick gander at the bottom of this TGR thread
Link removed - check out David's arXiv link below. Kudos to the authors for arXiving it, even if that's basically normal now.

I've not yet finished fully chewing on it myself, so I'm unprepared to rate it on the garbage-to-epiphany-o'meter at present.

Thanks Alan, I had a difficult time getting past the jargon until I looked a bit closer at the Fletcher-Pollard model that they are applying here. Figure 3 of this paper really cleared things up for me. A crack is an opening between the shear planes, an anticrack is just the opposite.
arxiv.org/vc/physics/papers/0511/0511142v1.pdf

I just haven't had time to go through the whole article. I may also not have the background to fully appreciate the models, but wanted to get this out so people could look at it.

Dave, being a physicist, I was hoping you would translate this down to the essentials for us who work in the lesser sciences!

In principle I can access the full article since I subscribe, but have not registered as a user for the online service. Again, once I have time I will give it a shot, but may refrain since I am not so sure that I would want to disseminate copyright info freely over a public forum. The abstract was free to all who gets to the site.

Science is a very reputable journal, but that doesn't mean that the topic and the research is beyond reproach, or that is has any practical value. On the other hand, the audience does not have to be the layman.

Science is also a very widely subscribed journal, often found on magazine racks. It should be in many public libraries, even though technical articles are typically of the highest quality and highly reviewed.

Hopefully I will get a chance to read this.

I can try. Not sure I'll get it right or make it any clearer.

Snow slab avalanches are believed to begin by the gravity-driven shear failure of weak layers in stratified snow. The critical crack length for shear crack propagation along such layers should increase without bound as the slope decreases.

In this model, slab release is initiated by formation of a crack, an opening up of the weak layer. The crack then grows in length. At some critical length, the crack then allows the slab to release and slide. This is what initially had me confused. I thought that crack meant the fracture or the crown. By crack, they mean that the weak layer gets wider or opens up. See figure 3 in that paper I linked.


However, recent experiments show that the critical length of artificially introduced cracks remains constant or, if anything, slightly decreases with decreasing slope.

This crack model cannot account for the dependence of critical crack length on slope angle, as given by experiment

This surprising observation can be understood in terms of volumetric collapse of the weak layer during failure, resulting in the formation and propagation of mixed-mode anticracks, which are driven simultaneously by slope-parallel and slope-normal components of gravity.

Instead of a crack, or an opening up of the weak layer, perhaps it is a collapse of the weak layer, or an anti-crack and propagation thereof that causes the slab to release. This new anti-crack model apparently either gets around the anomalous dependence of critical crack length on slope angle, or solves the anomaly altogether. I'm not sure from the abstract.

Such fractures may propagate even if crack-face friction impedes downhill sliding of the snowpack, indicating a scenario in which two separate conditions have to be met for slab avalanche release.

We've all set off "whumpfs", even on flat terrain. To get slab release, you need two things, a healthy whumpf (long-range anti-crack propagation) and subsequent low friction between the slab and the underlying surface