FLOW IN POLYCRYSTALLINE ICE

Part 2 - Background information

By Chris Wilson and Brett Marmo

2.6 Crystal Structure

Ice like other crystalline solids deforms with both an elastic and a plastic component (Petrenko & Whitworth, 1999). If a small amount of stress is applied to ice for a short period of time, the ice will elastically deform. When the stress is removed the ice will return to its initial shape; the strain is completely recoverable. If the stress exceeds a certain amount the ice will be deformed irreversibly. This is plastic deformation and the critical stress above which the crystal deforms permanently is known as the elastic limit. Ice also will be permanently deformed if stress below the elastic limit is applied for a prolonged period of time. Such time dependent plastic deformation is known as creep.

Figure 2.6.1: The Ice Ih lattice. a) Perpendicular to the c-axis. The puckered layer of atoms composes the glide plane, or g-plane, and the vertical bonds accommodating hydrogen atoms is the shuffle plane, or s-plane. b) Viewed down the c-axis, a is the unit cell and Burgers vectors parallel this direction (After Hobbs, 1974).

Two mechanisms are known to contribute to creep in ice. The motion of dislocations allows the relative movement of molecular planes within the crystal lattice (Fig. 2.6.1) to permit deformation. This processes in known as glide. A second deformation mechanism involves the movement of single ions (Nabarro 1948) or dislocations (Herring 1950) through the volume of a crystal lattice or around the grain boundary (Coble 1963). Diffusional processes act very slowly in ice and are observed when the deformation regime involves very low stresses.