Conceptual and physical clarification of rate and state friction: Frictional sliding as a thermally activated rheology

We observed slow frictional slip occurring at a constant shear stress below the nominal friction level and compared it with the time-dependent strengt hening of the frictional interface, which was also tracked experimentally. It was found that slip velocity decreases as the interface strengthens due to aging, while it increases with the applied shear stress. These dependenc ies were both exponential and were of similar magnitudes, as implied by the framework law of rate- and state-dependent friction. In the spirit of the adhesion theory of friction the dependence of slip velocity on interface st rength is understood to be the result of the change of the shear stress act ing on frictional junctions due to the change of junction population, thoug h the observed dependence was somewhat stronger than a simple model based o n this idea predicts. By correcting the observed slip velocity for the effe ct of the change of the interface strength, we could obtain a unique relati onship between stress and slip velocity, which may be readily compared with a standard rheological formulation. Thus the obtained relationship between stress and slip velocity showed a reasonable agreement with the absolute r ate theory over a temperature range of 25-800 degreesC for the present expe rimental condition (fine albite powder, 20 MPa normal stress, no pore water ).