Fracture of multiyear sea ice

The fracture and flow of multiyear sea ice was investigated under triaxial compression and uniaxial tension in the temperature range −40° to −3.5°C, for strain rates from 10−7 to 10−2s−1 and for confining pressures up to 30 MPa using 40 mm diameter specimens. Specimens both in the horizontal plane of the multiyear floe and perpendicular to this plane were tested. The results of short-rod fracture toughness tests on multiyear and first-year sea ice at temperatures −20°C are also reported. The multiyear sea ice came from an unridged portion of a single floe about 7 m thick, which was found to be massive and not blocky with large voids. The ice had low salinity and high porosity. The inelastic deformation of multiyear sea ice was found to be strongly dependent upon strain rate, temperature, and confining pressure. In compression, four main types of deformation were observed. (1) Under uniaxial compression, completely brittle fracture at high strain rates (of 10−3 to 10−2 s−1) was characterized by multiple axial splitting. (2) Application of even a small confining pressure inhibited splitting, and fracture took place by the formation of a narrow shear fault inclined at 45±3°. (3) At higher confining pressures, plastic deformation accompanied substantial cracking activity. (4) However, at still higher confining pressures, cracking was completely inhibited and deformation was entirely plastic. At −20°C, shear fracture occurred according to a maximum shear stress criterion and hence was pressure independent, with crack nucleation dominating the fracture behavior. At −40°C, however, the shear fracture stress was found to be strongly pressure dependent up to 14 MPa and could be described in terms of a Coulombic failure envelope. The unusual 45° orientation of ice shear fractures, together with the unusual pressure dependencies of ice peak strengths, may be explained by the fact that low-stress slip and cleavage occurs in the basal planes of ice crystals.