A THEORETICAL INVESTIGATION INTO THE NUCLEATION OF STABLE CRACK PRECURSORS IN POLYCRYSTALLINE ICE

The possible formation of small stable cracks (crack precursors) at grain triple points in polycrystalline S2 ice subjected to stresses below that required to nucleate grain-size cracks is examined theoretically at -10 degrees C. The investigation is based on the theory that stress concentrations can arise from the crystal elastic anisotropy and the pile-up of grain boundary dislocations at the triple point. Using an energy approach, numerical simulations of a model show that (i) small stable cracks can initiate from triple points under small stresses, (ii) unstable Griffith cracks can also nucleate, (iii) the smallest nucleation stresses are weakly dependent on hydrostatic compression and crystal orientation, and obey approximately the Hall-Fetch relation with respect to the mean grain size, (iv) the crack to grain boundary length ratios are statistically distributed rather than constant, and (v) crack nucleation is strongly influenced by the orientations of the grain boundaries with respect to the applied stress and by the grain boundary dislocation configurations (positive or negative).