VISCOUS RESPONSE OF POLAR ICE WITH EVOLVING FABRIC

Re-orientation of individual crystal glide planes as isotropic surface ice is deformed during its passage to depth in an ice sheet creates a fabric and associated anisotropy. A simple macroscopic description is that these material glide planes are rotated towards planes normal to an axis of compression, and away from planes normal to an axis of ext ension, inducing an instantaneous orthotropic viscous response with re flexional symmetries in the planes orthogonal to the current principal stretch axes. An associated orthotropic viscous law expresses the str ess in terms of the strain-rate, deformation, and three structure tens ors based on the principal stretch axes. General frame indifferent for ms are analysed to determine the fabric induced during differential st retchings along fixed principal axes. Then, freezing the fabric by rem oval of the stress, and hence strain-rate, the instantaneous simple sh ear responses in different planes are determined and compared. The cor responding instantaneous viscosities are expressed in terms of the res ponse coefficients of the constitutive model. A simple model is adopte d to illustrate the evolution of the viscosities during axial stretchi ngs and shearing.