The Mechanisms of Deformation of Pure and Debris-Laden Ice

Department Of Army Cold Regions Research & Engineering Lab, Vicksburg MS

Investigators

Abstract

Abstract OPP-0117371 Cole This is a collaborative proposal by the Cold Regions Engineering and Environmental Laboratory and Dartmouth College. Despite considerable progress in understanding the mechanisms of glacier flow, uncertainties still remain in understanding the relationship between stress and the resulting flow rate, and the effect of low concentrations of debris or particulate inclusions on the flow rate of ice. There are indications that power law behavior (n =3 to 4) is not universally observed at low stress levels. Laboratory experiments indicate that at low stresses, n1, and a linear stress dependence is often required to produce sensible results from calculations in ice sheet models. However, the underlying causes for the stress dependency of flow are not known and a physically based quantitative model of the process is not currently available. In fact, there is evidence that ice with small quantities of debris actually deforms faster than either pure ice or sediment-rich ice. Given the importance of the debris-laden layers to overall ice sheet movement, there is great interest in understanding the reason for this effect and developing a suitable constitutive relationship. Recent indications show that both the stress dependence of the flow rate and the influence of sediments can be addressed with a dislocation-based constitutive model. The application of dislocation-based models to ice has been hampered by an inability to quantify dislocation processes in specimens of a meaningful size. However, an approach has been developed that overcomes this obstacle, and a specimen's effective dislocation density (length of dislocation lines per unit volume of material)can now be determined as a function of its physical properties. Moreover, a combination of cyclic loading and creep experiments provides a way to track the dislocation density as a function of the specimen's stress/strain path and thermal history. Such experiments will be used to establish the dislocation density-stress/strain-temperature relationships for granular freshwater ice and debris-laden granular ice. Attention will be paid to the influence of debris concentration and thermal history on the grown-in (pre-deformation)dislocation density and on the dislocation density that evolves as a result of deformation. This approach has been successfully applied to ice having a variety of microstructures, but most extensively to sea ice, and a quantitative dislocation-based model has been developed. This study will verify critical aspects of the model for the case of laboratory-prepared granular freshwater ice, extend it to the case of ice with low sediment concentrations, and provide direct observations of the dislocation-particle interactions through the use of synchrotron x-ray topography on static and deforming specimens. Subsequent work will examine field cores for verification.