Static strengthening of frictional surfaces of ice

Systematic slide-hold-slide experiments were performed at -10 degrees 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 beta log t(h) or t(h)(m), where beta = 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 mu(s) = 0.5 to mu(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. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.