Full-field predictions of ice dynamic recrystallisation under simple shear conditions

Abstract

Understanding the flow of ice on the microstructural scale is essential for improving our knowledge of large-scale ice dynamics, and thus our ability to predict future changes of ice sheets. Polar ice behaves anisotropically during flow, which can lead to strain localisation. In order to study how dynamic recrystallisation affects to strain localisation in deep levels of polar ice sheets, we present a series of numerical simulations of ice polycrystals deformed under simple-shear conditions. The models explicitly simulate the evolution of microstructures using a full-field approach, based on the coupling of a viscoplastic deformation code (VPFFT) with dynamic recrystallisation codes. The simulations provide new insights into the distribution of stress, strain rate and lattice orientation fields with progressive strain, up to a shear strain of three. Our simulations show how the recrystallisation processes have a strong influence on the resulting microstructure (grain size and shape), while the development of lattice preferred orientations (LPO) appears to be less affected. Activation of non-basal slip systems is enhanced by recrystallisation and induces a strain hardening behaviour up to the onset of strain localisation and strain weakening behaviour. Simulations demonstrate that the strong intrinsic anisotropy of ice crystals is transferred to the polycrystalline scale and results in the development of strain localisation bands than can be masked by grain boundary migration. Therefore, the finite-strain history is non-directly reflected by the final microstructure. Masked strain localisation can be recognised in ice cores, such as the EDML, from the presence of stepped boundaries, microshear and grains with zig-zag geometries.

Physical analysis of an Antarctic ice core—towards an integration of micro- and macrodynamics of polar ice

“…Recent microstructural modelling studies [18,19,112] combined deformation by a viscoplastic full-field approach (taking strong crystal anisotropy into account [113]) with recrystallization (dynamic and static, continuous and discontinuous [114]). These simulations suggested that the effect of recrystallization on the LPO should be minor.…”

“…Under these conditions, grain boundary migration dominates the microstructure evolution and masks the deformation habitus of grains [18,19]. This has been observed with respect to the appearance of triple junctions [17] as well as grain boundary irregularities [77].…”

“…Grain elongation direction histograms derived from vertical sections (figure 5 b left) show a broad, but distinct distribution (cone angle of 45°) with a slight tendency towards double/multiple maxima. This can be interpreted as grains being partly elongated perpendicular to the main compression direction in shear, thus lining up in the instantaneous stretching direction [16], and grains partly being further rotated towards the shear plane [18,19]. These multiple maxima in the SPO description should not be confused with multiple maxima in the LPO.…”

“…Direct microstructural evidence on the microstructural scale, such as impurity accumulation along grain boundaries, is difficult to prove [109,110]. This effectively changes the deformation–recrystallization balance under increasing strain rate [18,19,79]. The step in grain elongation magnitude (figure 4 e ) suggests an increased influence of deformation on grain topology moving the microstructure from a recrystallization dominated one towards a deformation microstructure (see a comparison of end members in figure 2 in [19]).…”

“…This effectively changes the deformation–recrystallization balance under increasing strain rate [18,19,79]. The step in grain elongation magnitude (figure 4 e ) suggests an increased influence of deformation on grain topology moving the microstructure from a recrystallization dominated one towards a deformation microstructure (see a comparison of end members in figure 2 in [19]). Furthermore, although the ice is mainly clear, we see some isolated cloudy bands that are strongly folded, which is an effect of the very strong shear deformation.…”