  Digital Image Correlation is used to study the micromechanics of a
multi-contact interface formed between a rough elastomer and a smooth glass
surface. The in-plane elastomer deformation is monitored during the incipient
sliding regime, i.e. the transition between static and sliding contact. As the
shear load is increased, an annular slip region, in coexistence with a central
stick region, is found to progressively invade the contact. From the
interfacial displacement field, the tangential stress field can be further
computed using a numerical inversion procedure. These local mechanical
measurements are found to be correctly captured by Cattaneo and Mindlin (CM)'s
model. However, close comparison reveals significant discrepancies in both the
displacements and stress fields that reflect the oversimplifying hypothesis
underlying CM's scenario. In particular, our optical measurements allow us to
exhibit an elasto-plastic like friction constitutive equation that differs from
the rigid-plastic behavior assumed in CM's model. This local constitutive law,
which involves a roughness-related length scale, is consistent with the model
of Bureau \textit{et al.} [Proc. R. Soc. London A \textbf{459}, 2787 (2003)]
derived for homogeneously loaded macroscopic multi-contact interfaces, thus
extending its validity to mesoscopic scales.measurements allow for the first
quantitative test of Cattaneo and Mindlin (CM) classical model of the incipient
sliding of a smooth interface. Small deviations are observed and interpreted as
a result of the finite compliance of the rough interface, a behavior which
contrasts with Amontons' law of friction assumed to be valid locally in CM's
model. We illustrate how these measurements actually provide a method for
probing the rheology of the rough interface, which we find to be of the
elasto-plastic type.
