Crack nucleation in columnar ice due to elastic anisotropu and grain boundary sliding

The phenomenon of crack nucleation from grain triple junctions is investigated in columnar freshwater ice at rates of loading where the dislocation pile-up process is inhibited. Two mechanisms are explored. First, crack nucleation due to elastic anisotropy-induced singular stress field at the triple junction is investigated. To this end, both the singularity exponent and the energy release values associated with crack nucleation are provided for random orientations of the grain boundaries and material axes of the grains. The computed energy release rate values fall much short of those required for nucleation. Next, by assuming a linear viscous response for the boundary, the stress concentrations due to grain boundary sliding are computed, and the resulting energy release rate values are shown to be sufficiently high to overcome the barrier for crack nucleation. Experimental evidence for grain boundary sliding-induced boundary decohesions at — 10°C are presented in the companion paper. At such temperatures, sliding is activated even though the strain rate of loading invokes an overall brittle response from the polycrystal.