FRACTURE OF MULTIYEAR SEA-ICE

The fracture and flow of multiyear sea ice was investigated under tria xial compression and uniaxial tension in the temperature range -40 deg rees to -3.5 degrees C, for strain rates from 10<SUP>-7</SUP> to 10<SU P>-2</SUP> s<SUP>-1</SUP>, and for confining pressures up to 30 MPa us ing 40 mm diameter specimens. Specimens both in the horizontal plane o f the multiyear flee and perpendicular to this plane were tested. The results of short-rod fracture toughness tests on multiyear and first-y ear sea ice at temperatures -20 degrees C are also reported. The multi year sea ice came from an unridged portion of a single flee 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 ra te, temperature, and confining pressure. In compression, four main typ es of deformation were observed. (1) Under uniaxial compression, compl etely brittle fracture at high strain rates (of 10<SUP>-3</SUP> to 10< SUP>-2</SUP> s<SUP>-1</SUP>) was characterized by multiple<SUP></SUP> axial splitting. (2) Application of even a small confining pressure in hibited splitting, and fracture took place by the formation of a narro w shear fault inclined at 45 +/- 3 degrees. (3) At higher confining pr essures, plastic deformation accompanied substantial cracking activity . (4) However, at still higher confining pressures: cracking was compl etely inhibited and deformation was entirely plastic. At -20 degrees C , shear fracture occurred according to a maximum shear stress criterio n and hence was pressure independent, with crack nucleation dominating the fracture behavior. At -40 degrees C, however, the shear fracture stress was found to be strongly pressure dependent up to 14 MPa and co uld be described in terms of a Coulombic failure envelope. The unusual 45 degrees orientation of ice shear fractures, together with the unus ual pressure dependencies of ice peak strengths, may be explained by t he fact that low-stress slip and cleavage occurs in the basal planes o f ice crystals.