ACOUSTIC-EMISSION IN SINGLE-CRYSTALS OF ICE

The dislocation dynamics during the creep deformation of single crysta ls of ice Ih was studied using acoustic emission (AE) measurements. Th e AE activity was recorded during uniaxial compression and torsion cre ep tests. The results were interpreted in terms of dislocation dynamic s with the help of an AE source model relating the amplitude of an aco ustic event to the number of dislocations involved in the event and to their velocity. This model was first validated by a comparison betwee n the global AE activity and the global strain rate. Then, it was poss ible to evaluate the density of moving dislocations during creep defor mation. Two regimes were revealed. Without significant polygonization, the density of mobile dislocations, deduced from AE, was proportional to the stress, but increased much faster after polygonization, in agr eement with theoretical arguments. Finally, the power law distribution s observed for AE amplitudes, the slow driving process, the very large number of interacting dislocations involved, argued for the dislocati on dynamics to be a new example of a class of nonlinear dynamics defin ed as a self-organized critical state (SOC). It would imply that, from a global point of view, the creep of ice single crystals is a margina lly stable state rather than a steady-stable state.