MICROMECHANICS OF THE VELOCITY AND NORMAL STRESS DEPENDENCE OF ROCK FRICTION

Among the second-order effects on friction the most important are thos e of variable normal stress and of slip velocity. Velocity weakening, which is usually considered the source of the stick-slip instability i n rock friction, has been observed in velocity stepping experiments wi th Westerly granite. The friction change, Delta mu, was -0.01 to -0.00 8 for a tenfold velocity increase. Using normal closure measurements, we observed dilation upon each increase in sliding rate. We also obser ved, for the first time, time-dependent closure between surfaces durin g static loading. The dilation that occurred during the velocity stepp ing experiment was found to be that expected from the static time-depe ndent closure phenomenon, This change in closure was used to predict f riction change with an elastic contact model. The calculated friction change which results from a change in contact area and asperity interl ocking, is in good agreement with the observed velocity dependence of steady-state friction. Variable normal stress during sliding has two e ffects, first in creating new partial slip contacts and locking some e xisting fully sliding contacts and second in increasing interlocking, for instance when normal load is suddenly increased. As a result, a tr ansient change in friction occurs upon a sudden change in normal load.