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 rates even in 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 107.8 to l06.4 MPa−n s 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.

Experimental Investigation of Compressive Failure of Truncated Conical Ice Specimens

“…Results of Meglis et al (1999) also supported their work, indicating that under higher confining pressures, dynamic recrystallization is an important factor resulting in the enhancement of creep rate. Work by Findley et al (1976) and Schapery (1997) also prepared a similar viscoelastic model.…”

“…They are: internal energy sources, surface energy of grain boundaries, intragranular lattice defect energy and elastic energy due to external loading. Meglis et al (1999), Passchier and Trouw (2005) and Wei and Dempsey (1991) have further investigated the influence of dynamic recrystallization with the influence of the creeping effect, pressure melting and kinking mechanisms.…”

“…A number of researchers have confirmed that the failure mechanisms of hpzs are not associated exclusively with fractures (for example Meglis et al, 1999, Xiao, 1997, Wells et al, 2010. Jordaan et al (2008) focused on the softening processes that occur within this damaged layer.…”

Small-scale laboratory experiments on the indentation failure of polycrystalline ice in compression: Main results and pressure distribution

“…Previous work by the present group of researchers has shown that the hpzs have a failure mechanism that does not include large fractures (see for example Meglis et al, 1999;Melanson et al, 1999;Xiao, 1997). Within hpzs, the state of stress is triaxial.…”

“…From these tests it was determined that the flow of energy into and out of the indenter is an important consideration during dynamic ice crushing (Jordaan and Timco, 1988). These early field observations also highlighted the important role of damage processes during an icestructure interaction, and guided laboratory tests (see for example Meglis et al, 1999;Melanson et al, 1999) aimed at understanding the relationship between confining pressure and damage.…”