  We measure the elastic and dissipative responses of a multicontact interface,
formed between the rough surfaces of two contacting macroscopic solids,
submitted to a biased oscillating shear force. We evidence that beyond a linear
viscoelastic regime, observed at low shear amplitude, the interface response
exhibits a dissipative component which corresponds to the onset of frictional
dissipation. The latter regime exists whereas the tangential force applied, far
from the nominal static threshold, does not provoke any sliding. This result,
akin to that of Mindlin for a single contact, leads us to extend his model of
`microslip' to the case of an interface composed of multiple microcontacts.
While describing satisfactorily the elastic response, the model fails to
account quantitatively for the observed energy dissipation, which, we believe,
results from the fact that the key assumption of local Coulomb friction in
Mindlin's model is not legitimate at the sub-micrometer scale of the microslip
zones within microcontacts between surface asperities.
