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 fractur es in a ductile manner; at higher strain rates it is brittle. The brittle-t o-ductile transition has an activation energy of 0.6 +/- 0.1 eV. Plasticity preceding fracture is characterized by an activation energy of 0.6 +/- 0.0 5 eV for temperatures below -6 degreesC, and about 2.7 +/- 0.4 eV above. Th e basic deformation mechanism of snow, an ice foam, is power-law creep of i ce. As in silicon, the activation energy of the brittle-to-ductile transiti on is the lowest of the activation energies of all deformation processes av ailable, but in ice these are the same, 0.6 eV, for dislocation glide, diff usion and sublimation.