An Experimental Investigation of the Effect of Grain Size on "Dislocation Creep" of Ice

The creep behavior of ice is believed to be dominated by dislocation creep at high stresses (>1 MPa) and high homologous temperatures (>0.9). Dislocation creep of ice is often described by the Glen law, epsilon?=B sigma n, with a canonical value of n= 3, and is independent of grain size. Laboratory studies of ice deformation at elevated pressures, however, suggest that dislocation creep of ice is characterized by a value of n approximate to 4. Here, we deformed ice samples with initial grain sizes of 0.23 and 0.63 mm, at 263 K and confining pressures of 10 or 20 MPa. The experiments yield a value of n= 3.6 using the peak stresses obtained at a strain epsilon approximate to 0.02, and reveal a marked dependence of the stress on initial grain size. The nominally constant flow stresses at larger strains, up to 0.2, yield a value of n= 3.9, and no influence of the initial grain size on the stress is observed. The lower value of n at peak stresses and the decrease in peak stress with decreasing initial grain size suggest a contribution to the strain rate from a grain-size-sensitive process, such as grain boundary sliding. Our data yield two flow laws, one for isotropic ice and one for highly anisotropic ice deformed to large strains, the latter of which may be used to model flow in high-strain natural environments.