The Glide Direction in Ice

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Abstract
The failure to detect experimentally a glide direction in the ice crystal is satisfactorily explained by assuming that the crystal glides simultaneously in three symmetry-equivalent directions with a response to the shear stress component in each direction that is the same as that observed for the crystal as a whole or for polycrystalline aggregates—the typical non-linear, power-type flow law. A hexagonal crystal responding to stress by this type of “non-linear crystal viscosity” behaves very differently from a tetragonal one. For a tetragonal crystal, the glide directions are well defined in the response of the crystal if the power-flow-law exponent n exceedsn~ 1·5, whereas for a hexagonal crystal a well-defined glide direction can be observed only ifn>c. 5. The response of a hexagonal crystal is entirely independent ofa-axis orientation ifn= 3 exactly. For 3 <n<c. 5 the true glide direction should be weakly apparent, whereas for 1 <n< 3 the crystal should show a response weakly suggestive of preferred glide in a direction perpendicular to the true glide direction. In the observed range ofnvalues for ice, 2 <n< 4, the expected response to simultaneous glide differs so slightly from the hitherto-postulateda-axis-independent, “non-crystallographic” glide as to be practically undetectable experimentally. This circumstance makes it possible to identify <>as the glide direction, from structural considerations alone, and to accommodate the plastic properties of the ice crystal into the modern concepts of crystal plasticity. It may be expected that hexagonal close packed and face-centred cubic metals at high temperatures, in steady state creep, will show translation gliding without well-defined glide directions.