Transition in brittle failure mode in ice under low confinement

Compressively loaded ice deformed within the brittle regime fails by axial splitting under no confinement and by shear faulting under moderate confinement. This paper investigates the transition between these two failure modes using a novel very low-confinement loading device. It is found that, with increasing confinement, the orientation of the macroscopic failure plane rotates rapidly away from the direction of maximum compression until the confining stress reaches ∼1% of the maximum compressive stress, and then, under higher confinement, remains within the range of orientations generally attributed to Coulombic faulting. Quantitative modeling of the effect of confinement on wing crack growth and on the distribution of excess shear stress ahead of the wing crack tip results in predictions of the macroscopic fault orientation in good agreement with experimental observations.