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 $\bar{F}$ 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 to the dynamical ageing of the interface. We show that: (i) In agreement with the engineering thumb rule, even a modest modulation induces a substantial decrease of $\bar{F}$. (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. Namely, 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 ``elasto-plastic'' friction model leads to predictions in excellent quantitative agreement with all our experimental data.