FLOW IN POLYCRYSTALLINE ICE

Part 2 - Background information

By Chris Wilson and Brett Marmo

2.13 Plastic deformation

Glide and diffusional processes also play significant roles in the plastic deformation of polycrystalline ice. Compression experiments show that polycrystalline ice deforms instantly when a stress is applied suddenly (Barnes et al. 1971; Kamb 1972). Initially the strain rate slows with increasing strain; this is known as primary creep (Fig. 2.9.2). This reduction in strain rate reflects work hardening similar to that observed in a single crystal of ice that is being deformed in a hard glide orientation (see 2.9 Strain rate for glide on basal systems). Eventually the strain rate becomes constant; this is secondary creep, the minimum strain rate. Secondary creep is of principal interest as it gives rise to the steady state flow observed in glaciers. When the compressive stress exceeded a critical stress the strain rate increased significantly. This is known as tertiary creep and is due to recrystallisation of ice. Other complicating factors in the deformation of polycrystalline ice are grain boundary melting, pressure melting (Wilson et al. 1996) and the development of crystallographic fabrics which may enhance the flow of the ice.

In order that a polycrystalline solid can deform homogeneously into any arbitrary shape, with no volume change and maintain strain compatibility it must have at least five independent slip systems (Taylor 1938). If uniform strain is not pre-supposed then the polycrystals only require four independent systems (Hutchison 1976). The basal plane of ice crystals provides only two systems. The other two independent systems must be non-basal, but glide is 2 or 3 orders of magnitude more difficult to activate than for basal systems. The non-basal system therefore plays a major role in macroscopic behaviour making deformation of polycrystalline ice slower than that of a single ice crystal (Fig. 2.11.1). The additional non-basal system are most probably the prismatic systems {} <> and pyramidal {} <> slip systems (Fig. 1.1.2). The other possibility is that movement of dislocations on the two independent systems in the basal plane gives rise to dislocation climb normal to the basal plane; and provides a further two systems (Ashby & Duval 1985).