Microstructural change in ice: II. Creep behavior under triaxial stress conditions

Abstract

This work investigates the deformation of ice under deviatoric stresses and confining pressures expected during ice–structure interaction. Granular ice was tested under a range of confining pressures (5–60 MPa) and deviatoric stresses (up to 25 MPa), with sample temperatures between –8° and –10°C. Samples were deformed to increasing end-levels of axial strain, and were thin-sectioned and photographed immediately following testing.At all confinement levels, the original texture of the sample is completely transformed within the first 10–15% strain, to a fine-grained matrix with a few larger, isolated grains. At low confinements, grain-size reduction is associated with extensive microcracking. At high confinements, few cracks are observed. Observations suggest that microcracking, melting and recrystallization are active at all levels of confinement, though the relative importance of each depends on the confinement, stress and accumulated strain.Deviatoric stress is a strong factor in controlling the creep, reflected in both the time required for the sample to reach accelerated creep and the tertiary creep rate itself. Two exceptions to this pattern were noted. First, some samples experienced strain localization and eventual rupture. These specimens tended to have higher creep ratesevenin the initial stages of strain. Second, prior damage resulted in rapid softening compared with the behavior of undamaged specimens. However, when strain rates are compared among all samples at a given level of cumulative axial strain, the creep behavior obeys a power law over the whole range of strain levels tested. Effective viscosity decreased from 107.8to l06.4MPa−ns within the first 10% strain, during which the most substantial microstructural changes occurred, and then stayed relatively stable. The stress exponent,n, remained within the range 4.0–4.6.The dominant deformation mechanism appears to depend strongly on confining pressure (cracking at low pressure and dynamic recrystallization at high pressure). Creep rates at high confinement appear to increase relative to those at intermediate confinement, but the influence of temperature must be addressed further.

Plastic faulting: Brittle‐like failure under high confinement

“…[15] The temperature increase in the shear zone is bounded by the melting temperature, for as the temperature approaches this point the shear strength decreases, reducing the production of additional heat. Temperatures along P faults near the melting point are consistent with microstructural observations of recrystallized grains within P faults in ice [Meglis et al, 1999;Schulson and Gratz, 1999]. Evidence for recrystallization along P faults in rock is inconclusive, partially due to the extremely narrow nature of P faults and, perhaps, partially due to the complexity of the melting process in polymineralic material.…”

Rheological Properties of Water Ice—Applications to Satellites of the Outer Planets

“…Durham et al (2001) see signs of extended transient strain softening at lower stresses, perhaps due to the lower driving force for recrystallization. However, recrystallization is also very active in warm samples that exhibit tertiary creep (Meglis et al 1999), and the onset of recrystallization with increased strain rate at warmer temperatures in polar ice caps produces dramatic fabric changes from anisotropic to nearly isotropic ( Figure 7). Recrystallization under deviatoric stress adds another layer of complexity (Kamb 1961, Paterson 1973.…”