  The onset of frictional sliding between contacting bodies under shear load is
nucleated by the quasi-static growth of localized slip patches. After reaching
a certain critical size, these patches become unstable and continue growing
dynamically, eventually causing the sliding of the entire interface. Two
different theories have been used to compute the nucleation length of such
patches depending on the dominant process driving their growth. If it is only
the yielding of contact asperities (large-scale yielding), a stress criterion
is applied, based on linear stability analysis, whereas if fracture dominates,
an energy criterion is applied, based on fracture mechanics and classical
nucleation theory. Both approaches contain important underlying assumptions
that are well-suited to describe either one situation or the other. However,
what happens in-between is not captured by any of them. In this work, we use
numerical simulations to study what is the dominant underlying process driving
nucleation for different conditions of heterogeneity and what are the
implications for nucleation dynamics and the onset of frictional sliding. We
show that large frictional heterogeneities enable a transition from a
yielding-driven nucleation phase to a fracture-driven one. This transition
occurs only above a certain level of heterogeneity and can either be
quasi-static (stable) or dynamic (unstable), depending on the correlation
length of frictional strength along the interface and the difference in
strength between the strongest and the weakest point (the amplitude). Unstable
transitions generate localized dynamic slip events, whose magnitude increases
with higher correlation length and decreases with larger amplitude. Our work
sheds new light on the role of heterogeneity and fracture in the nucleation of
frictional slip, bridging the gap between the two main governing theories for
nucleation.
