Mechanisms of crack nucleation in ice
- Dartmouth College, Hanover, NH (United States)
This paper will review the recent literature on the mechanisms by which crack are nucleated in polycrystalline ice. In the absence of pre-existing cracks, crack nucleation is the first step in mechanical failure. A variety of mechanisms have been discussed in the literature, mostly involving the propagation of microcracks or precursors from initial sizes below the level of convenient detection to easily observable cracks. There are several candidate mechanisms for this concentration of stress: dislocation glide (leading to pile-ups of dislocations on particular slip planes); grain boundary sliding (leading to stress concentrations at the triple junctions at the edge of grain boundary facets); thermal expansion of extraneous inclusions (such as produced by progressive freezing of brine pockets upon cooling); and elastic anisotropy of the ice crystals. Both dislocation glide and boundary sliding are kinetic processes in which the stress redistribution occurs at a finite, temperature-dependent rate. These processes also contribute temperature-dependent internal friction and anelasticity and their operation can, therefore, be independently measured. Elastic anisotropy should produce stress concentrations in polycrystalline ice in an a thermal manner and should, therefore, become more important at very low temperatures and high loading rates. It has been shown, however, that in the absence of stresses due to brine pockets, the inherent elastic anisotropy of ice is not sufficient to nucleate cracks in purely two-dimensional models, such as perfectly columnar grains in plane strain. Calculated nucleation energies are far beyond available thermal activation energies. Therefore, some additional stress concentrating effects are required, such as irregularities in the third dimension (e.g. jogs in column boundaries) or inclusions (e.g. brine pockets or dirt particles).
- OSTI ID:
- 175213
- Report Number(s):
- CONF-950686--
- Country of Publication:
- United States
- Language:
- English
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