Hot-pressure sintering of low-density snow analyzed by X-ray microtomography and in situ microcompression

We analyzed the impact of confined compression at different strain rates on the microstructure of low-density snow. Experimental conditions in a cold laboratory with fragile snow samples at porosities of almost 0.9 required the construction of a new microcompression device for in situ microtomography measurements. Under isothermal conditions, compression experiments were conducted at low, constant velocities of 0–0.4 μμm s−1 up to macroscopic stresses of about 13 kPa, followed by subsequent stress relaxation measurements. During compression, the temporal evolution of the stress for different strain rates follows a universal loading curve after rescaling. During relaxation, a different scaling is observed, which is quantitatively analyzed within a Maxwell model, indicating nonlinear viscoplastic behavior with ε̇∼σn and a stress exponent of around n≈2.2n≈2.2. The main differences between compression and relaxation are revealed by the microstructural analysis of the tomography data. The specific surface area shows a rate-dependent decrease during compression. The decrease is related to the evolution of the topology of the ice matrix, suggesting a decrease in the surface area at the cost of grain boundary area creation due to the formation of new contacts. The significant structural changes are absent during the relaxation stage, when only minor topological coarsening effects remain and stresses can relax in an almost invariant microstructure.