Lecturer:  Shahar Gvirtzman - HUJI
 Abstract:

 The initiation of frictional sliding is mediated by rapid rupture fronts. Recent experiments have demonstrated that these fronts are shear cracks and that rupture propagation is fully described within the framework of fracture mechanics. Ruptures, however, need to be created within the frictional interface before they can propagate. This formation process, occurring below the critical (‘Griffith’) length for crack instability, is not described by our current understanding of fracture mechanics.  

By conducting controlled nucleation experiments, in which the interface is continuously imaged, we have been able to gain a detailed description of the rupture formation process. We find that the expansion of the nucleation patch is qualitatively different from the propagation of the fully formed rupture front. It occurs at extremely slow and constant velocities, and it is 2D in nature. Some of the features of this expansion, such as deterministic evolution and stress-dependent timescales, are general. However, some of the details of this process, such as the 2D shape of the patch, are governed by the local conditions at the nucleation region. 

As nucleation is not described by the usual frameworks that are used to explain rupture propagation, understanding the driving mechanism of it is of fundamental importance to questions ranging from earthquake nucleation and prediction to processes governing material failure. We propose a theoretical model for this mechanism and show that it captures the unique characteristics of the nucleation process as well as the transition to dynamic rupture propagation that is described by standard fracture mechanics.