Systematic slide-hold-slide experiments were performed at -10 °C on both first year sea ice and freshwater ice. The sliding velocity ranged from 10~(-6) to 10~(-4) m s~(-1) and the holding time from t_h = 1 to 10~4 s under an apparent normal stress of 60 kPa. The experiments established that the shear stress required to re-initiate sliding increases with holding time, following a threshold period that increases with decreasing sliding speed. The effect is termed static strengthening and is found to scale as either β log t_h or t_h~m, where β = 0.30 ± 0.03 and m = 0.5 ± 0.1 for both materials. The effect is a large one: upon holding for t_h, = 10~4 s the coefficient of static friction for both materials increases by about a factor of three, from μ_s = 0.5 to μ_s = 1.4. The behavior is explained in terms of the geometry and deformation of asperities that protrude from opposing surfaces and interact at points of contact, and a model is presented that incorporates creep, hardness and fracture.
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