Static strengthening of frictional surfaces of ice

Abstract

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 10−4ms−1 and the holding time from th=1 to 104s under an apparent normal stress of 60kPa. 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 βlogth or thm, where β=0.30±0.03 and m=0.5±0.1 for both materials. The effect is a large one: upon holding for th=104s the coefficient of static friction for both materials increases by about a factor of three, from μs=0.5 to μs=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.