Follow-up at the small scale during snow deformation. Microstructure evolution and local heterogeneities at various strain-rates.

Snow deforms naturally in a large range of strain rates covering ductile to brittle regimes. In all situations, snow deformation is characterized by complex mechanisms taking place at the microstructure scale, with interplay between metamorphism, sintering and grain rearrangements. Current modeling efforts require better understanding and formulation of the microstructure-scale complexity.
We performed compression experiments on snow samples in order to follow the microstructure evolution at various imposed displacement velocities. The resulting strain-rates varied between 10^-2 s^-1 and 10^-7 s^-1. Samples (15 mm height, 15 mm diameter) were made out of rounded grains with an initial density of about 250 kg m^-3. Samples evolution was followed by means of micro-computed X-Ray tomography (microCT) with full 3D scans performed during the slower tests and simple radiographies at a high frequency for faster tests.
In this presentation we will focus on the various metrics used to analyse the microstructure evolution on one side, in particular specific surface area (SSA) and the minimum cut area. On the other side, we will present recent developments based on digital image and volume correlations (DIC and DVC) performed on the radiographies and the 3D microCT images with the open access SPAM software (https://hal.univ-grenoble-alpes.fr/hal-03020460), in order to follow the local strain field.
We will provide analyses of the interplay between metamorphism and strain in the microstructure evolution and of the impact of mechanisms at bonds in the various strain-rate regimes explored (ductile to brittle). Snow deformation localisations revealed by DIC and DVC observations will be presented. They can be inherited from initial sample heterogeneities or take the shape of compaction bands, depending on the strain rate.
All these data and analyses will be further interpreted regarding the understanding of the small scale mechanisms of metamorphism and deformation of snow and their modeling frame.