COMPARISONS OF FABRIC DEVELOPMENT IN POLYCRYSTALLINE ICE AT BASAL AND ATMOSPHERIC PRESSURES

We conducted compressive creep tests on artificially prepared, 917 kg m^-3 polycrystalline ice specimens at both atmospheric and 20 MPa hydrostatic pressures (equivalent to 0 and ~2,000 m depths within polar ice sheets, respectively). Initial specimen grain orientations were random and mean grain diameter was 2.0 mm. We applied an axial stress of 0.6 MPa over 625 h at -5±0.1°C to induce axial strains of ~10% in each specimen.

Subsequent microstructural analyses on the deformed specimens allows examination the differences in crystal fabric evolution between the two cases. Because of the large size of ice grains at the end of testing (~2.5 mm diameter) and the short life of an ice specimen within the vacuum chamber of an SEM, we have developed a microstructural analysis technique which simultaneously collects grain shape and size data from micrographs and obtains crystallographic orientation data via EBSD conducted over a coarse grid. Combining these measurements allows analysis of the ice crystallographic fabric over large numbers of grains, yielding statistically useful numbers of grain size and full c- and a-axis grain orientation data.

The combined creep and microstructural data help us understand pressure-dependent effects on the mechanical and microstructural evolution of polycrystalline ice. We discuss possible mechanisms for the observed phenomena, and future directions for hydrostatic creep testing of polycrystalline ice.