Investigation of the ductile-to-brittle transition in snow with compression tests and tomography monitoring

Once fallen on the ground, snowflakes evolve quickly and form the snowpack. Under its own weight, the snowpack slowly deforms and settles. On a steep slope, some layers may rapidly deform and fail, which yield to the release of an avalanche. At the macroscopic scale, snow mechanical behavior is highly strain-rate dependent: ductile at low strain rates and brittle at high strain rates. The ductile-to-brittle transition has recently been shown to occur in two stages, with an intermediate regime of intermittent brittle failures, assumed to result from a competition between different time scales. At the micro-scale, this mechanical behavior is controlled by the microstructure and the visco-plastic and sintering properties of the ice skeleton. In this work, we investigate snow brittle-to-ductile transition by conducting displacement-controlled compression tests monitored with X-ray micro-computed tomogaphy.

We specifically designed a loading apparatus to perform displacement-controlled compression tests in cold environment and the constrained space of the tomographic cabin, so that microstructure evolution could be followed by regular scans. Samples (14 mm in diameter, 14 mm in height) were prepared from natural snow, sieved directly into samples holders, in batch of 10 samples and sintered for 72h at -20°C then stored at -50°C to prevent further microstructure evolution. The sample were taken out and placed in the compression device 30min before the first scan. We explored strain rates from 10^-6 s^-1 to 10^-2 s^-1 by vertically compressing 30 samples, up to a peak stress of 250 kPa and at a constant temperature of -18.5 °C. At high strain rates, only the initial and final 3D microstructures were scanned and simple radiographs were acquired during loading at a rate of 5 frames per second. At low strain rates, the 3D microstructure was regularly scanned during the loading. The obtained time series comprises one of the most-resolved (8.5 µm, 1h) and complete dataset on snow microstructure evolution near the ductile-to-brittle transition to date.

Results indicate a clear dependency of snow mechanical response on the strain rate. At strain rates larger than about 10^-3 s^-1, snow samples display heterogeneous deformations with the formation of compaction bands, while the stress-strain curve shows a serrated behavior. To relate this macroscopic behavior to micro-structural evolution, quantitative investigation of local density and specific surface area changes, as well as of bond network evolution, will be presented. These results should help identifying the micro-scale mechanisms at play during deformation of snow through both ductile and brittle range.