Crack nucleation mechanism in saline ice

A mechanism for crack nucleation in saline ice is presented, by considering a planar array of hexagonal grains containing the brine pockets as a model of poly crystalline saline ice. It is shown through a thermodynamic analysis that important local stresses arise associated with the internal pressure which builds up inside a brine pocket due to a drop in the temperature. As the temperature drops, the water inside the brine freezes, and because of the variation in the water density on freezing, this results in a buildup of pressure inside the pocket. For typical field conditions, assuming elastic behavior for the matrix, pressures as high as 7 MPa are estimated. Next, using a finite element method, the stress concentration at a grain triple junction is determined under the influence of the stress field associated with a nearby brine pocket. The resulting stress state is used to determine the condition for crack nucleation. The analysis is restricted to only elastic deformation regimes with isotropic grains, albeit with elastic constants corresponding to extreme values in a single freshwater ice crystal. The mechanism discussed here provides an explanation for the widely observed brine channels in sea ice. In addition, the effect of the externally applied stress is also considered, and the resulting stress singularities at the grain triple junctions analyzed by an asymptotic method as well as by a numerical solution. Both the strength and an approximate energy criteria suggest crack nucleation from the brine pocket surface towards the grain triple junction. The results are shown to be consistent with the experimental observations.