Shear response of a frictional interface to a normal load modulation

We study the shear response of a sliding multicontact interface submitted to a harmonically modulated normal load, without loss of contact. We measure, at low velocities (V< 100 <mu>m s(-1)), the average value (F) over bar of the friction force and the amplitude of its first and second harmonic components. The excitation frequency (f= 120 Hz) is chosen much larger than the natural one, associated with the dynamical aging of the interface. We show the following: (i) In agreement with the engineering thumb rule, even a modest modulation induces a substantial decrease of (F) over bar. (ii) The Rice-Ruina state and rate model, though appropriate to describe the slow frictional dynamics, must be extended when dealing with our ''high'' frequency regime. That is, the rheology which controls the shear strength must explicitly account not only for the plastic response of the adhesive junctions between load-bearing asperities, but also for the elastic contribution of the asperities bodies. This "elastoplastic" friction model leads to predictions in excellent quantitative agreement with all our experimental data.