The Initiation of Frictional Motion-The Nucleation Dynamics of Frictional Ruptures

Frictional interfaces lose stability via earthquake-like ruptures, which are close analogues of shear cracks that are well-described by fracture mechanics. Interface ruptures, however, need to be first formed-or nucleated. Rupture nucleation therefore determines the onset of friction, replacing the concept of a characteristic static friction coefficient . Utilizing rupture arrest at an imposed barrier, we experimentally determine nucleation locations, times and stresses at the origin of each subsequent rupture event. This enables us study the nucleation process via real-time measurements of real contact area and local strain. Nucleation events initiate as 2D patches that expand at nearly constant velocities, v(nuc), that are orders of magnitude lower than the dynamic rupture velocities described by conventional fracture mechanics. We find that: (a) Nucleation has location-dependent stress thresholds, (b) v(nuc) is roughly proportional to the local stress level, (c) the nucleation process continues until the patch size reaches L-tran & SIM; L-G, the Griffith length for the onset of dynamic fracture (d) scaling time by tau = L-tran/v(nuc), nucleation patches exhibit self-similar dynamics (e) dynamic ruptures' cohesive zones are not fully established until significantly beyond L-tran. Many details of nucleation are governed by the local contact area topography, which is roughly invariant under successive rupture events in mature interfaces. Topography-dependent details of the nucleation process include: precise nucleation site location, patch geometry, critical stress thresholds and the proportionality constant of v(nuc) with stress. We believe that these results shed considerable light on both how frictional motion is triggered and earthquake initiation.

Automated summary

Frictional interfaces become unstable due to earthquake-like ruptures, which are similar to shear cracks. The nucleation of rupture determines the onset of friction and it is influenced by local stress and contact area topography. Nucleation events occur as 2D patches that expand at low velocities and are affected by the local stress level.