Permalink : http://hdl.handle.net/2115/45405

KEYWORDS : creep;dislocations;critical dynamics;ice sheets;Hall-Petch law;normal grain growth;rotation recrystallization

It was recently shown thai crystals (including ice) plastically deform in an intermittent manner in usual laboratory conditions. The present paper aims at discussing whether such self-organised critical dynamics still apply to polar ice sheet conditions. Field data suggest that grains should contain between zero and one dislocation moving at a time. However, this is nothing but an average estimate. Field data also show that strong back-stresses are present, collesponding to a significant density of potentially mobile dislocations. These findings, together with the very low loading level, are consistent with critical dislocation dynamics, in which collective motion events occur for a short time, followed by long periods of inactivity during which grain growth, rotation recrystallization and other recovery processes contribute to the reduction of the long range internal stress field. The stress and grain size dependencies of the strain rate are derived on this basis. The applicability of the Hall-Petch law is also discussed.