Figure 59.1 Strain rate as a function of stress for basal slip in ice crystals and for creep of isotropic polycrystalline ice at -10°C.
When ice crystals are loaded such that there is no resolved shear stress on the basal plane, creep rates are so small that measurements are uncertain. A slight misorientation of the c-axis with respect to the direction of the applied axial stress induces basal slip. According to Duval et al. (1983), under the same prescribed stress, a crystal sheared parallel to its basal plane exhibits a strain rate that can be up to three orders of magnitude greater than that measured when an ice crystal is compressed along or perpendicular to the c-axis. The rapid glide of short-edge dislocations on non-basal planes has been observed by X-ray topography (Higashi et al., 1985; Ahmad & Whitworth, 1988). The fact that basal slip is dominant, in spite of faster movement of such non-basal dislocations, is attributed by these authors to the large difference in respective dislocation density. According to Hondoh et al. (1990), the long basal screw dislocations should be dissociated on the basal plane thereby impeding the glide of these dislocations on non-basal planes.
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