  When two solids start rubbing together, frictional sliding initiates in the
wake of slip fronts propagating along their surfaces in contact. This
macroscopic rupture dynamics can be successfully mapped on the elastodynamics
of a moving shear crack. However, this analogy breaks down during the
nucleation process, which develops at the scale of surface asperities where
microcontacts form. Recent atomistic simulations revealed how a characteristic
junction size selects if the failure of microcontact junctions either arises by
brittle fracture or by ductile yielding. This work aims at bridging these two
complementary descriptions of the onset of frictional slip existing at
different scales. We first present how the microcontacts failure observed in
atomistic simulations can be conveniently "coarse-grained" using an equivalent
cohesive law. Taking advantage of a scalable parallel implementation of the
cohesive element method, we study how the different failure mechanisms of the
microcontact asperities interplay with the nucleation and propagation of
macroscopic slip fronts along the interface. Notably, large simulations reveal
how the failure mechanism prevailing in the rupture of the microcontacts
(brittle versus ductile) significantly impacts the nucleation of frictional
sliding and, thereby, the interface frictional strength. This work paves the
way for a unified description of frictional interfaces connecting the recent
advances independently made at the micro- and macroscopic scales.
