Creep of ice in compression: An investigation of intracrystalline deformation processes.

摘要

The purpose of this study is to gain a better understanding of the physics of ice deformation, more specifically of the rheological processes in the deformation of ice crystals. First, a procedure for the growth and reproduction is devised in response to a need for a simple, versatile and systematic method for producing ice. Crystals and polycrystals were machined in rectangular prisms and deformed under a constant load at temperatures ranging from {dollar}-2spcirc{dollar}C to {dollar}-10spcirc{dollar}C. The effect of three types of boundary conditions is compared on the crystals. Friction at the ice-platens interface introduces a biaxial state of stress represented by a plastic bending moment. Relieving the constraints and minimising friction results in a bending moment of opposite direction and an uneven distribution of lattice slip. It is attributed to the propensity of ice to deform predominantly by slip along the basal (0001) plane. When the specimen's ends were prevented from widening, shear zones of variable width developed in the centre of the crystal. A technique for etching and replicating the specimens is used to monitor distribution of dislocations and crystal lattice rotation, the latter shown by the variation in geometry of sublimation pits. The progressive spatial development of a tilt wall into a high-angle crystal boundary is also observed, consistent with a geometry expected from basal slip. A new design for transducer set-up is proposed which takes into consideration the high plastic anisotropy inherent to ice crystals. It is also suggested that the creep behaviour of this material follows a polynomial form rather than power-law, as reported in previous investigations. The polycrystalline ice display a preferred orientation of the (0001) crystal axes parallel to the load direction, but also providing some evidence for the influence of a pre-deformation fabric. This preferred orientation is also observed in a core, representing the complete stratigraphic thickness of the Ward Hunt ice shelf, which was sectioned at various sites along its length. A detailed analysis of these sections failed to identify textural variations. The study of polycrystalline ice, both deformed in a laboratory and in a natural setting indicates that, when confinement is high enough to prevent basal glide, recrystallisation becomes an important strain accommodating mechanism.