Abstract
The flow of glaciers and polar ice sheets is controlled by the highly anisotropic rheology of ice crystals that have hexagonal symmetry (ice lh). To improve our knowledge of ice sheet dynamics, it is necessary to understand how dynamic recrystallization (DRX) controls ice microstructures and rheology at different boundary conditions that range from pure shear flattening at the top to simple shear near the base of the sheets. We present a series of two-dimensional numerical simulations that couple ice deformation with DRX of various intensities, paying special attention to the effect of boundary conditions. The simulations show how similar orientations of c-axis maxima with respect to the finite deformation direction develop regardless of the amount of DRX and applied boundary conditions. In pure shear this direction is parallel to the maximum compressional stress, while it rotates towards the shear direction in simple shear. This leads to strain hardening and increased activity of non-basal slip systems in pure shear and to strain softening in simple shear. Therefore, it is expected that ice is effectively weaker in the lower parts of the ice sheets than in the upper parts. Strain-rate localization occurs in all simulations, especially in simple shear cases. Recrystallization suppresses localization, which necessitates the activation of hard, non-basal slip systems.This article is part of the themed issue 'Microdynamics of ice'.
Highlights
Ice is one of the most common minerals found at the Earth’s surface
The aim of this paper is to present the state of the art of the VPFFT/ELLE modelling of ice, paying particular attention to the effect of vorticity boundary conditions and DRX
Regardless of the amount of DRX and ice flow, a single c-axes maximum develops that is oriented approximately perpendicular to the maximum finite shortening direction and which in simple shear rotates towards the normal to the shear plane
Summary
Ice is one of the most common minerals found at the Earth’s surface. Most of it is concentrated in polar ice sheets, formed originally from precipitation of snow. Two main layers define the data structure of the models: (i) a contiguous set of polygons (figure 1a,b) that are used to define grains and that are themselves defined by straight boundary segments joined at boundary nodes (bnodes; figure 1d) and (ii) a high-resolution grid of unconnected nodes (unodes) that store physical properties within grains, such as the local lattice orientation, defined by three Euler angles, and the dislocation density (figure 1e) These unodes serve as Fourier points to simulate crystallites for the viscoplastic deformation calculations [25]. The stored energy ( H) is calculated from the difference in dislocation density across a grain boundary ( ρ), the shear modulus (G, assumed to be isotropic) and the Burgers vector (b): This routine picks single bnodes in a random order and uses four orthogonal small trial moves (parallel to the x- and y-axes, and of 1/100 of the average distance between bnodes) of that bnode to determine the direction (n) with the maximum driving stress ( f ) [33]. This procedure is repeated for each unode in random order
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