CRACK NUCLEATION IN COLUMNAR ICE DUE TO ELASTIC-ANISOTROPY AND GRAIN-BOUNDARY SLIDING

The phenomenon of crack nucleation from grain triple junctions is inve stigated in columnar freshwater ice at rates of loading where the disl ocation pile-up process is inhibited. Two mechanisms are explored. Fir st, crack nucleation due to elastic anisotropy-induced singular stress field at the triple junction is investigated. To this end, both the s ingularity exponent acid the energy release rate values associated wit h crack nucleation are provided for random orientations of the grain b oundaries and material axes of the grains. The computed energy release rate values fall much short of those required for nucleation. Next, b y assuming a linear viscous response for the boundary, the stress conc entrations due to grain boundary sliding are computed, and the resulti ng energy release rate values are shown to be sufficiently high to ove rcome the barrier for crack nucleation. Experimental evidence for grai n boundary sliding-induced boundary decohesions at -10 degrees C are p resented in the companion paper. At such temperatures, sliding is acti vated even though the strain rate of loading invokes an overall brittl e response from the polycrystal.