FLOW IN POLYCRYSTALLINE ICE

Part 2 - Background information

By Chris Wilson and Brett Marmo

2.12 Diffusional flow

If temperature is sufficiently high, even a small stress will induce a flux of matter through or around the boundary of an ice grain. Diffusional flow has been extensively modeled and there is general agreement on the form of the relation describing the strain rate:

----------------------------------------equation (8)

where:

----------------------------------------equation (9)

where and are the lattice and grain boundary diffusion coefficients respectively, is a dimensionless constant, W is the molecular volume, k is Boltzmann's constant, d is the grain size and d is the grain boundary width.

Diffusional flow in ice has not been well studied as diffusion through the ice lattice is very slow and the grain size large compared to metals and ceramics. Transient creep processes, such as grain boundary sliding and dislocation motion, mask diffusional processes at low strains. Glide related deformation occurs significantly faster that diffusion, but dislocations require a stress greater than 340 Pa to propagate through the ice structure (Goodman et al. 1981). Thus, diffusional flow dominates deformation at low stress but is obscured by the glide at moderate stresses.