FLOW IN POLYCRYSTALLINE ICE

Part 2 - Background information

By Chris Wilson and Brett Marmo

2.17 Deformation mechanism maps

Deformation maps describing mechanisms through which ice deforms have been calculated by Goodman et al. (1981), Frost & Ashby (1982), Duval et al. (1983) and Ashby & Duval (1985). The maps show the region, within stress-temperature space, where one of the flow mechanisms is dominant. Maps are generally plotted with the microscale variables, including grain size, held as constants. The upper boundary of the maps is defined by the region of brittle failure for ice. Figure 16 shows deformation maps for the grain sizes 0.1mm, 1mm and 10mm. Diffusional flow and glide controlled flow form two broad classes of mechanisms through which ice deforms.

Lines of strain rate are also mapped onto the diagrams. The strain rate for each component is calculated from the respective steady state flow laws. As diffusional flow and glide mechanisms occur concurrently the maps show the total strain rate:

The maps have three major regions: a low stress region dominated by diffusional flow; a moderate stress region where glide controlled creep dominates; and a high temperature - high stress area dominated by pressure melting. At high temperatures both recrystallisation and grain boundary melting contribute to the deformation.

Figure 2.17.1: Deformation mechanism maps for isotropic polycrystalline ice by Goodman et al. (1981). The fields show the deformation process that dominates, given the temperature and stress conditions, and do not indicate that these are the only processes active. The strain rate contours represent the cumulative strain for all processes contributing to deformation. Each contour represents one order of magnitude in . Note the variation in each field for different mean grain sizes.