Static Friction, Healing, and Constitutive Laws for Time- and Slip-dependent Friction

I summarize laboratory friction experiments on quartz powder sheared at room temperature and humidity. Slide-hold-slide experiments indicate that static friction and related parameters vary systematically with both hold time and loading velocity. The data indicate that healing is closely related to gouge porosity and that static friction and healing are not simple friction parameters, but rather are system responses that vary with velocity and elastic coupling. The observed dependence of frictional healing on loading rate is predicted by rate and state friction laws and constitutive modeling indicates that both slipand time-dependent state evolution laws can fit data from individual tests. However, constitutive parameters vary with velocity and hold-time. Thus, the parameters determined from one set of conditions cannot, in general, be extrapolated to other conditions. This indicates either that the underlying physical processes vary with velocity and/or that the current rate and state friction laws are inadequate. I compare laboratory data with model predictions and document specific deficiencies in the current laws.