The plastic behavior of predeformed ice crystals

Analysis of previous plastic deformation experiments in ice has been complicated by the fact that the dislocation density changes during the experiment. To overcome this difficulty, measurements were made at stress levels lower than a predeformation value in an attempt to maintain constant dislocation densities. Creep, constant strain rate, and stress relaxation tests were performed as a function of stress, strain rate, temperature, and predeformation value. The response can be described in good approximation by a linear second order differential equation resulting from a combination of recoverable and non-recoverable components of dislocation strain. The dislocation viscosity results from a Snoek-type viscous drag arising from proton rearrangement in the stress field of the dislocations, as suggested by Weertman. Since the viscous drag constant at -100 C is about seven orders of magnitude greater than typical room temperature phonon drag constants, the deformation rate is limited by the viscosity, rather than by obstacles as in most materials. No evidence was found for electrical charge on the dislocations, or for a dislocation contribution to the decrement at 500 Hz or 10 MHz. No feature of the present results was inconsistent with the simple model proposed here, and ice is now perhaps one of the best understood plastic materials.U of I OnlyThesi