Fabrics in polycrystalline ice deformed experimentally at −10°C

Abstract

Blocks of polycrystalline ice were compressed experimentally in an apparatus that restricted the deformation to plane strain at −10°C, under constant load and at strain rates varying between 10 −8 to 10 −9 s −1. By inscribing initially circular markers on a face perpendicular to the axis of plane strain, it is possible to measure the strain heterogeneity. The shortening and cumulative strain ( γ oc) measured locally are seen, in general, to be different from the nominal bulk shortening strain. Using these strain markers it has been possible to compare the c-axis preferred-orientation with strain distribution. Variation in ice c-axis fabrics correlate with changes in: (1) mode of deformation from pure shear to combined pure and simple shear, and (2) the magnitude of the finite strain. In pure shear there is a rapid transition from randomly oriented c-axes of the initial ice aggregate to a broad diffuse pattern lying between the 45° and 85° small circles (centred about the shortening axis), and thence to a 60° small circle distribution containing two maxima that can be symmetrically related to the bulk axes of strain. Where there is a component of simple shear, this two-maxima pattern is more asymmetric, having a strong c-axis maximum that can be attributed to the simple shear and a weaker portion of the fabric contributed by the pure shear deformation. These fabric transitions are analogous to those described in natural ice-sheets.