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Dynamic crack propagation in weak snowpack layers: insights from high-resolution, high-speed photography

Abstract. Dynamic crack propagation in snow is of key importance
for avalanche release. Nevertheless, it has received very little
experimental attention. With the introduction of the propagation saw test
(PST) in the mid-2000s, a number of studies have used particle tracking
analysis of high-speed video recordings of PST experiments to study crack
propagation processes in snow. However, due to methodological limitations,
these studies have provided limited insight into dynamical processes such as the
evolution of crack speed within a PST or the touchdown distance, i.e. the
length from the crack tip to the trailing point where the slab comes to rest
on the crushed weak layer. To study such dynamical effects, we recorded PST
experiments using a portable high-speed camera with a horizontal resolution
of 1280 pixels at rates of up to 20 000 frames s−1. 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 enabled
us to calculate time derivatives to obtain velocity and acceleration fields.
We demonstrate the versatility and accuracy of the DIC method by showing
measurements from three PST experiments, resulting in slab fracture, crack
arrest and full propagation. We also present a methodology to determine
relevant characteristics of crack propagation, namely the crack speed
(20–30 m s−1), its temporal evolution along the column and touchdown
distance (2.7 m) within a PST, and the specific fracture energy of the weak
layer (0.3–1.7 J m−2). 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
provides insight into the three different propagation results found with the
PST and reveals intricate dynamics that are otherwise not accessible.
