Plastic Behavior of Predeformed Ice Crystals

• Published in: Journal of Glaciology
• Main Authors: Joncich, D. M., Holder, J., Granato, A. V.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Cambridge University Press (CUP) 1978
• Subjects: Earth-Surface Processes; Journal of Glaciology
• Online Access: http://dx.doi.org/10.1017/s0022143000033876; https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022143000033876

Abstract Conventional studies of plastic deformation have been complicated by the simultaneous variation of dislocation velocity and dislocation density during the tests. In the present study this difficulty was avoided by carrying out deformation tests at low stress levels on samples which had been predeformed to relatively higher stresses prior to the measurements. Creep, mechanical after effect, constant strain-rate, and stress relaxation tests were carried out as a function of measurement stress or strain-rate, temperature, and predeformation level. The results were analysed in terms of a linear stress-dependent dislocation velocity in order to determine whether that simple behavior is able to account for the macroscopic deformation behavior of ice crystals. This report is a brief summary of the results; a complete discussion is to be published elsewhere. The principal results of the study are as follows: 1. The observed behavior was particularly simple for predeformed samples. The creep strain showed a nearly linear increase with time, without the large positive curvature characteristic of conventional tests. No stress maxima were observed in the constant strain-rate tests as have been found in previous studies of non-predeformed samples. The (complete) stress relaxation curve was virtually identical in shape to the inverted constant strain-rate curve. 2. The steady-state creep and constant strain-rate behavior could be described to good approximation in terms of the motion of a constant density of dislocations moving with the same linear stress-dependent velocities as have been observed directly by others. The strain-rates were linear in stress and the estimated dislocation densities (4 to 16 × 10 cm -2 ) varied with the magnitude of the predeformation level in a manner consistent with previous observa-tions. The strain-rates or stress levels are exponential in 1/ T with an activation energy of 0.6 eV, which is approximately equal to the activation energy reported for the motion of dislocations and for the ...