Frictional sliding across Coulombic faults in first-year sea ice: A comparison with freshwater ice

Frictional sliding plays a fundamental role in the deformation of the Arctic Ocean ice cover and in ice-structure interactions. To examine its character, sliding experiments were performed along freshly created Coulombic faults in first-year S2 sea ice harvested from the Beaufort Sea during the winters of 2003, 2007, and 2009. For comparison, and to complement experiments performed earlier, we also examined freshwater ice of similar microstructure. The principal variables were sliding velocity (from 8 x 10(-7) m s(-1) to 4 x 10(-3) m s(-1)), temperature (-40 degrees C, -10 degrees C, and -3 degrees C) and normal stress (from 0.02 to 2.7 MPa). Over the ranges explored, the coefficient of friction varies by a factor of four, from similar to 0.4 to similar to 1.6. The coefficient for both materials reaches a maximum value at an intermediate velocity, which is an order of magnitude higher for sea ice (similar to 10(-4) m s(-1)) than for freshwater ice (similar to 10(-5) m s(-1)). At lower velocities, sliding is characterized by velocity strengthening where, for a given velocity, the coefficient of friction of sea ice is lower than that of freshwater ice. At higher velocities, sliding is characterized by velocity weakening, where, for a given velocity, the coefficients of friction for sea ice is closer to that of freshwater ice. Velocity strengthening is explained in terms of creep deformation within contact zones along the fault. Velocity weakening is tentatively attributed to a combination of fracture and localized melting. An implication of stable versus unstable sliding is discussed for sea ice mechanics.