Department
of Earth and Space Sciences University of Washington, Seattle, USA
Deformation
rates of single hexagonal crystals, deforming by glide on the basal plane, are
described as a function of stress state and crystal orientation.
These results are used to infer the deformation rate of crystal
aggregates assuming that the stress distribution within the crystal aggregate is
homogeneous. Analytical equations
for the deformation rate of anisotropic ice aggregates are derived for
vertically symmetric girdle fabric. This type of fabric is approximated by a
uniform distribution of c-axis orientations between a cone angle and a smaller
girdle angle relative to the symmetry axis. For simple shear stress acting on a
single maximum fabric there is a slight de-enhancement for cone angles between
60° and 90°. In uniaxial compression a maximum enhancement of ~1.7 occurs at a
cone angle of 57°. A pure shear stress state has similar features with the
additional complication that it causes a non-zero transverse strain rate, except
for perfect vertical alignment of crystals and isotropic fabric. In combined
states of stress the contribution of each stress component to the strain rate
depends on fabric. A single enhancement factor is not adequate to describe the
effects of anisotropy for complex stress states.
Published in
Journal of Glaciology, Vol. 47, No. 158, p. 507-516, 2001.