Interplay of Cohesive, Griffith, and Geometric Scales in the Nucleation of Friction

Rupture of the contacts forming a frictional interface governs frictional failure in both natural and engineered systems. Often such failure is governed by brittle fracture, where a small cohesive length ξ regularizes the singular rupture tip. We experimentally study rupture nucleation under conditions where ξ is large, exceeding the interface width. Within ξ, initially 2D expanding fronts transition into slow 1D rupture fronts while still establishing ξ. Rapid rupture only ensues after ξ is established, at the Griffith length. This cohesive-scale-driven nucleation should be generic when ξ dominates geometrical scales, with ramifications for material failure, friction, and fault-zone dynamics.