Ductile failure of columnar S2 saline ice under triaxial compressive loading

Triaxial compressive loading experiments were performed on columnar S2 saline ice, a plastically orthotropic material with planar isotropy normal to the columnar grains, to describe the effect of confinement on the strength and deformation of the material. Samples of columnar S2 saline ice were tested at −10°C under three types of triaxial loading—Proportional loading where the ratios of the normal stresses, applied along and across the columns, were varied in a systematic method; Conventional loading where the ice was confined under a constant hydrostatic pressure and then a uniaxial compressive load was applied perpendicular to the columns; And off-axis loading where proportional normal stresses were applied at an angle to the columns. In all three cases the nominal strain rate in the direction of greatest load was 4 × 10−5 s−1. At the given strain rate and temperature all samples deformed in a macroscopically ductile manner. The flow behavior was characterized by the yield and peak stresses, and the flow stress at 1% and 2% inelastic deformation. The yield and peak stresses rose with increasing across-column confinement, but were generally insensitive to along-column confinement. The flow stress showed a dependence on both the along- and across-column confinements at both 1% and 2% inelastic deformation. The ice behavior is discussed in terms of dislocation processes, cracking, grain boundary sliding and dynamic recrystallization. The yield and peak stresses agree with Hill's [1950] criterion at low to moderate confinements, but do not compare very well at higher confinements. It is then shown that alternative criterion by Horrigmoe and Zeng [1994] (modified) and Melton [1999] reasonably describe the data over a wider range of confinement. Correspondingly, the inelastic deformation generally obeys the associated flow rules.