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RESEARCH PRODUCT

Strain rate dependence for evolution of steady state grain sizes: Insights from high-strain experiments on ice

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Abstract Understanding of the microstructural evolution and equilibrium grain size development during steady state tertiary flow is essential in order to improve our knowledge of ice and rock deformation. This contribution presents results from in situ transmitted light deformation experiments of natural glacier ice, with the development of the microstructure in a tertiary flow regime. We conducted one relative slower ( 1 × 10 − 6 1/s) and two relative faster-strain rate ( 2 × 10 − 6 1/s) pure shear experiments at −10 °C, up to a shortening of ∼57%. Microstructure development was followed by time-lapse observations, and two new microstructure-based indicators, the ‘seeding rate’ and the ‘microstructure activity’, were introduced to evaluate whether a steady-state was reached. These indicate that a steady state was only reached in the two faster strain rate experiments. In contrast, during a slow deformation there is insufficient seeding of new grains to enable continuous recovery, and there is a bimodal grain size distribution. These results are explained by stress concentrations within grains. Particularly where basal planes are in unfavourable orientations for basal slip (hard glide orientation) coinciding with the development of a bimodal grain size distribution. In the case of bimodal grain size distributions the use of a stabilised mean grain size as a stress piezometer as a criterion for steady state should be handled with caution.