Elasto-Viscoplastic Micromechanical Modelling of the Transient Creep of Ice
Abstract
There are significant issues in Geophysics related to the elasto-viscoplastic behavior of polycrystalline materials such as ice and minerals. One can evoke for example the post-seismic deformations in fault regions due to the viscoelastic behavior of the lower crust and/or the upper mantle, the uplift of continents associated to post-glacial rebound, the attenuation of seismic waves associated to viscous dissipation along the travel path of the wave, and more generally all situations in which the mechanical response of the material involves both elasticity and viscoplasticity. For ice, transient effects associated to the elasto- viscoplastic behavior are encountered when ice flow changes direction rapidly, such as for glaciers flowing above irregular bedrocks or for sea-ice coming up against an off-shore rig. Besides those geophysical issues, applying recent micromechanical approaches to the transient creep response of ice, as in this first study, is highly valuable for understanding the elementary deformation mechanisms, such as glide and climb of dislocations. A salient feature of the rheology of polycrystalline ice is the decrease of the strain-rate by more than two orders of magnitude during transient creep, to reach a secondary creep regime at a strain which is systematically of ~ 1%. We use a recent mean-field micromechanical model, which aims at bridging scales between the rheology of single grain and the one of polycrystals by evaluating the intergranular interactions as deformation proceeds. The model takes into account the long-term memory effects which manifests itself by the fact that local stress and strain-rate in grains depend on the whole mechanical history of the polycrystal. It is shown that the strong hardening amplitude during the transient creep is entirely explained by the stress redistribution within in the different grains of the specimen, whereas the experimentally observed hardening kinetic is much too slow to be explained by the same process. This latter is attributed to the hardening of hard glide slip systems (prismatic slip) in the transient regime. Moreover, the model very well reproduces the permanent creep-rate of several highly anisotropic specimens of the GRIP ice core (Greenland) exhibiting pronounced crystallographic textures, when accounting for a single grain rheology that well matches the experimental one. Our results are also consistent with recent findings on dislocation dynamics in ice.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFMMR13A1701C
- Keywords:
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- 0738 Ice (1863);
- 3902 Creep and deformation;
- 3904 Defects;
- 3909 Elasticity and anelasticity;
- 8160 Rheology: general (1236;
- 8032)