Snow as a foam of ice: plasticity, fracture and the brittle-to-ductile transition

At strain rates lower than 10(4) s(1), snow deforms plastically and fractures in a ductile manner; at higher strain rates it is brittle. The brittle-to-ductile transition has an activation energy of 0.6 +/- 0.1 eV. Plasticity preceding fracture is characterized by an activation energy of 0.6 +/- 0.05 eV for temperatures below -6 degreesC, and about 2.7 +/- 0.4 eV above. The basic deformation mechanism of snow, an ice foam, is power-law creep of ice. As in silicon, the activation energy of the brittle-to-ductile transition is the lowest of the activation energies of all deformation processes available, but in ice these are the same, 0.6 eV, for dislocation glide, diffusion and sublimation.