Brittle Failure of Ice

Ice exhibits either ductile or brittle behavior, depending upon the conditions under which it is loaded. Glaciers, for instance, are loaded by gravity, under deviatoric stresses of ~0.1 MPa or lower. At temperatures of interest, they flow through dislocation creep at strain rates of the order of 10−9 s−1 or lower (Patterson 1994). Sheets of sea ice—another terrestrially important ice feature—are loaded predominantly by wind, under global compressive stresses similar in magnitude to the shear stresses within glaciers (Richter-Menge and Elder 1998). These bodies deform through a combination of creep and fracture, the latter process manifested by oriented leads or open cracks and by compressive shear faults which often form as conjugate sets that traverse a large fraction of the Arctic Basin (Kwok 1999; Schulson 2002). The cold, icy crust of Europa, an extraterrestrial feature which may shield an ocean beneath within which a form of life may exist or may once have existed (Reynolds et al. 1987; Hoppa et al. 1999; Pappalardo et al. 1999; Greenberg et al. 2000; Kargel et al. 2000), is loaded by the motion of diurnal tides and by non-synchronous rotation (Greenberg and Weidenschilling 1984). The crust deforms in a brittle manner, as evident from the networks of cracks that lace through it (Greeley et al. 2000). Of the two kinds of ice deformation, creep is probably the better known by readers of the geological and geophysical literature, and is certainly the more fully explored and better understood. The interested reader may wish to consult a number of excellent reviews of the subject (e.g., see Duval et al. 1983; Weertman 1983; Durham and Stern 2001). Fracture, in comparison, has only recently been systematically examined. The motivation with respect to terrestrial mechanics (Schulson 2001) comes largely …