The brittle compressive failure of orthotropic ice under triaxial loading

Experiments on the brittle compressive failure of S2 fresh-water columnar ice loaded triaxially at −10°C at a strain rate of 6×10−3/s have revealed three regimes of Coulombic-like behavior: 1 of lower across-column confinement where the along-column stress has no significant effect on the strength; 2 of higher across-column confinement where the along-column stress increases the strength; and 3 of predominantly along-column loading where the strength increases in proportion to the smaller of the two orthogonal across-column confining stresses. Within each regime the strength of the fresh-water ice is indistinguishable from that of porous salt-water ice (Gratz and Schulson, J. geophys. Res., 1997, 102, 5091) of otherwise similar microstructure, implying similar failure mechanisms. Both kinds of ice fail through macroscopic shear faulting which results from the linking up of cracks formed during the deformation. The principal difference between the two materials is that the deformation damage to the salt-water ice is restricted more to the vicinity of the faults, owing to the crack-stopping influence of pores incorporated during growth. Terminal failure is explained in terms of a new mechanism involving the bending of “splay cracks” under frictional sliding. “Splay cracks” are feather-like deformation features that emanate from one side of parent, sliding cracks. The idea is that slender microcolumns between the “splay cracks” are more likely to bend and break than are microcolumns between adjacent wing cracks because they do not have two fixed ends; instead, the end stemming from the sliding interface is free. A moment is then applied by frictional sliding. A first-order calculation shows that the stress required to break the columns and thus to initiate the fault is of the same order of magnitude as the terminal failure stress.