Grain-boundary sliding and crack nucleation in ice

Using a special geometry of columnar ice samples. we studied the process of grain-boundary sliding (GBS) in ice and its role in crack nucleation. It is shown that GBS is a viscous process leading to the formation of discontinuous features, called 'decohesions', along the boundary plane. In the early stages of deformation, corresponding to primary creep, the sliding rate is high and is linearly related to the resolved shear stress acting on the boundary plane. As GBS proceeds, localized openings of the boundary, that is decohesions, appear, separated by regions of stress concentration near bumps or ledges. Further sliding during later stages of deformation (steady state for creep tests) requires accommodation within the grains. At low strain rates, accommodation occurs by intracrystalline dislocation creep, without microcracking. At higher strain rates, part of the stress concentration is relaxed by localized mode I cracking. This was evident from the initiation of cracks from the decohesions, which points our the role of GBS in crack nucleation. GBS and the formation of decohesions are enhanced at higher temperatures, owing to the viscous character of sliding.