Abstract
AbstractThis work quantifies the increased temperature sensitivity of the constitutive behavior of ice with proximity to the melting point in terms of dislocation mechanics. An analysis of quasistatic and dynamic cyclic loading data for several ice types leads to the conclusion that high temperature (e.g. T ≥ −8°C) behavior is the result of a thermally induced increase in the number of mobile dislocations rather than an increase in the activation energy of dislocation glide or the introduction of a new deformation mechanism. The relationship between dislocation density and temperature is quantified and the model is shown to adequately represent the published minimum creep rate vs stress data for isotropic granular freshwater ice for −48 ≤ T ≤ −0.01°C.
Highlights
Creep experiments on granular freshwater ice as a function of temperature have long indicated that there is a breakpoint at ≈−8°C that defines a shift to a regime of greater temperature sensitivity than observed at lower temperatures (e.g. Barnes and others, 1971; Hooke and others, 1980)
The relevant experimental data are limited, the analysis indicates that a model that (1) allows the dislocation density to increase with proximity to the melting point and (2) has a constant activation energy adequately captures the quasistatic loading response of several ice types
What can be inferred from the limited number of high temperature creep experiments available for granular freshwater ice is that the behavior is a consequence of (1) the high density of thermally induced dislocations and (2) the mechanism of stress-induced dislocation multiplication continues to act even when the overall dislocation density is high
Summary
Creep experiments on granular freshwater ice as a function of temperature have long indicated that there is a breakpoint at ≈−8°C that defines a shift to a regime of greater temperature sensitivity than observed at lower temperatures (e.g. Barnes and others, 1971; Hooke and others, 1980). The alternative explanation for the observed high temperature behavior (e.g. increasing activation energy near the melting point) can be tested by applying a frequency shift to the data points and observing whether the shifted points lie along a master curve, making some allowance for experimental scatter, or diverge This approach was successfully applied in Cole (1995) to show that the anelastic straining due to dislocations in saline ice exhibited a constant activation energy for −50 ≤ T ≤ −10°C. As for the alternative of attributing high temperature effects to an increase in activation energy, this analysis indicates that when a frequency shift was applied to the experimental loss compliance data, they did not convincingly converge to a master curve, which argues against that explanation for the high temperature behavior of ice. more extensive experimental results are certainly called for to solidify the observed trends, the analysis presented below is based on the reasonable indications that high temperature effects are caused by temperature dependent increases in the mobile dislocation density. As noted for the Glen (1955) results, the fact that the data for T ≥ −22°C at the highest stress levels in Figure 12 fall above the model values is attributed to the onset of power law breakdown
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