Microstructural evolution of polycrystalline ice during confined creep testing

Abstract

The mechanical properties of polycrystalline ice Ih have been observed to change under an applied hydrostatic pressure comparable to that present near the bottom of kilometer-thick ice sheets. To help determine the cause of these changes, we conducted confined creep testing of laboratory-prepared polycrystalline ice at pressures up to 20MPa (equivalent to ∼2000m of overburden) and subsequent microstructural analysis of specimens deformed by creep using optical microscopy and scanning electron microscopy, including extensive electron backscatter diffraction mapping of crystal orientations. Microstructural observations of the creep-deformed specimens revealed smaller median grain sizes, less regular, and more interlocked grain shapes in specimens deformed at higher pressure compared with those deformed at atmospheric pressure. Variable pressure testing reveals little change in strain rate for pressures less than 15MPa, leading to alternative hypotheses regarding the influence of confining pressure on the dislocation dynamics and associated creep behavior of polycrystalline ice. Our central hypothesis is that widely dissociated basal dislocations in ice begin to constrict after the confining pressure reaches a critical value. This critical pressure depends strongly on the (currently unknown) lattice dilatation induced in ice by the presence of stacking faults.