The Transition from Dislocation Creep to Plasticity in Ice
Abstract
The flow of glaciers and ice sheets is typically described with the Glen law, a power law relationship between strain rate and stress, with a value of the power law exponent, i.e., the stress exponent, n, of 3. Numerous laboratory experiments, however, demonstrate that at sufficiently high stresses, values of n ≈ 4 are obtained. A key question is, might values of n ≈ 4 represent a transition with increasing stress from a creep mechanism characterized by n=3 to plasticity? Experimental evidence of such power law breakdown behavior in ice is difficult to obtain, because the associated high stresses would result in brittle fracture of samples deformed at atmospheric pressure. To investigate power law breakdown in ice, we deformed samples with uniform grain sizes of 0.25 to 1 mm at constant displacement rate at a temperature of 263 K, strain rates of 1x10-4 to 1.2x10-2 s-1, and gas confining pressures of 30 MPa. Samples were deformed at a confining pressure in excess of the differential stresses obtained in our experiments to preclude brittle fracture of the samples. Stress-strain curves show a peak stress at 2-3 % strain, followed by dramatic strain weakening due to the development of crystallographic preferred orientation and grain size reduction due to dynamic recrystallization. For peak stresses <17 MPa, the peak stress vs. strain rate data yield a value of the stress exponent n=3.8 +/- 0.2, indicative of dislocation creep. For peak stresses greater than 17 MPa, the stress exponent increases to values > 7, indicating the onset of power law breakdown behavior. Our experimental data demonstrate that the dislocation creep regime proper in ice is characterized by n ≈ 4 and persists over 2 orders of magnitude in strain rate at the conditions of our experiments. The value of n ≈ 4 therefore cannot be attributed to a transition from a creep mechanism characterized by n=3 to the plasticity regime.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMMR31B0087H
- Keywords:
-
- 3625 Petrography;
- microstructures;
- and textures;
- MINERALOGY AND PETROLOGYDE: 8004 Dynamics and mechanics of faulting;
- STRUCTURAL GEOLOGYDE: 8120 Dynamics of lithosphere and mantle: general;
- TECTONOPHYSICSDE: 8160 Rheology: general;
- TECTONOPHYSICS