FLOW IN POLYCRYSTALLINE ICE

Part 2 - Background information

By Chris Wilson and Brett Marmo

2.14 Primary Creep

When a load is initially applied to a polycrystalline aggregate, both grains in easy glide and hard glide orientations deform elastically (Fig. 2.14.1, AB). With increasing stress the easy glide grains begin to glide and plastically deform. As the easy glide grains relax, stress is redistributed onto the hard glide grains which are progressively rotated and begin to deform in an elastic-plastic manner. If the stress is removed at this point, some but not all of the strain is recoverable, as the easy glide grains have deformed permanently. If the stress is continued then, hard glide grains will eventually begin to fail by plastic deformation and contribute to the bulk deformation at which point secondary glide is achieved.

Figure 2.14.1: Schematic creep curve for polycrystalline ice under constant load.

The region of decelerating "primary creep" extends from B to the inflection point C, after which the creep accelerates and eventually reaches a constant rate DE. Early work on ice by Glen (1955) and Barnes et al. (1971) identified a region of steady-state or "secondary creep" around the point C. Later experiments show only a broad minimum in , but the minimum strain rate represents a very important quantity in the analysis of the creep data. Deformation beyond the minimum is called "tertiary creep". The final steady state marked DE in Fig. 2.14.1 is hard to attain in laboratory experiments, but this is the region of most significance in geology and glaciology.