CRACK NUCLEATION MECHANISM IN SALINE ICE

A mechanism for crack nucleation in saline ice is presented, by consid ering a planar array of hexagonal grains containing the brine pockets as a model of polycrystalline saline ice. It is shown through a thermo dynamic analysis that important local stresses arise associated with t he internal pressure which builds up inside a brine pocket due to a dr op in the temperature. As the temperature drops, the water inside the brine freezes, and because of the variation in the water density on fr eezing, this results in a buildup of pressure inside the pocket. For t ypical field conditions, assuming elastic behavior for the matrix, pre ssures as high as 7 MPa are estimated. Next, using a finite element me thod, the stress concentration at a grain triple junction is determine d under the influence of the stress field associated with a nearby bri ne pocket. The resulting stress state is used to determine the conditi on for crack nucleation. The analysis is restricted to only elastic de formation regimes with isotropic grains, albeit with elastic constants corresponding to extreme values in a single freshwater ice crystal. T he mechanism discussed here provides an explanation for the widely obs erved brine channels in sea ice. In addition, the effect of the extern ally applied stress is also considered, and the resulting stress singu larities at the grain triple junctions analyzed by an asymptotic metho d as well as by a numerical solution. Both the strength and an approxi mate energy criteria suggest crack nucleation from the brine pocket su rface towards the grain triple junction. The results are shown to be c onsistent with the experimental observations.