FLOW IN POLYCRYSTALLINE ICE

Part 2 - Background information

By Chris Wilson and Brett Marmo

2.16 Tertiary Creep

Duval et al. (1983) have demonstrated that the acceleration of creep in polycrystalline ice is at least partially due to the formation of microcracks within polycrystalline ice. The microcracks are about equal to the grain size and their density increase with strain. The initiation and movement across microcracks results in additional stresses on uncracked crystals which produces localised internal stress variations. Duval et al. (1983) also showed that a sample deformed at 1.86 MPa had a steady increase in microcrack initiation that resulted in a concurrent increase in strain rate, while a sample that was deformed under 1 MPa increased in strain rate without any observed cracking which indicates that microcracking is not the only process that produces tertiary creep.

Dynamic recrystallisation also contributes to tertiary creep. Recrystallisation induces the development of a preferential c-axis orientation in ice that deforms close to its melting point which results in strain softening and an increase in strain rate. Dynamic recrystallisation occurs as a discontinuous process. At a certain critical strain a wave of recrystallisation will occur and pass through the deforming ice. This is well documented in metals where it occurs at ~20%, whereas within ice it typically occurs at ~1% (Duval et al. 1983).

For a given kind of deformation the ratio of the strain rate for steady-state tertiary creep to the minimum strain rate is found to be a constant independent of both stress and temperature. In uniaxial compression , and for simple shear the ratio is (Budd and Jacka 1989). This shows that the flow processes are similar in both secondary and tertiary creep, with the difference between them arising from the more favourable fabric established in the tertiary stage. In the case of shear the preferred fabric has all grains with their c-axes, but in compression the c-axes are distributed over a cone, and this is how shear deformation can give a greater enhancement than compression. If ice deformed into the tertiary stage is subsequently tested in a different orientation it is much more resistant to deformation. Eventually recrystallisation re-orients the fabric to one appropriate to the new stress regime. It is very important to recognize that ice flowing in an ice sheet has highly anisotropic properties that are developed during its recent history.