Friction

Friction is important for many processes in nature and industry. It is a multiscale problem by the fact that the smallest asperities on the microscale can determine the sliding behavior and wear of metals as well as earthquake dynamics. Here we study in particular the motion of slow fronts between stick and slip regions.

Introduction

For many industrial applications, but also daily life as well as geological phenomena friction plays an important role. In particular the failure of frictional interfaces and the spatiotemporal structures that accompany it are central to a wide range of geophysical, physical and engineering systems. In contrast, the observation of ruptures in earthquakes, recent geophysical and laboratory observations indicated that interfacial failure can also be mediated by slow slip rupture phenomena. Several geophysical and laboratory observations have pointed to the possibility that stress releasing interfacial slip can be mediated by the propagation of rupture fronts whose velocity is much smaller than elastic wavespeeds. Although these discoveries have influenced the way we think about frictional motion, yet the nature and properties of slow rupture are not completely understood. Therefore, the understanding of the dynamic processes that govern interfacial failure and frictional sliding, e.g., an earthquake along a natural fault, remains a major scientific challenge. Several experiments clearly demonstrate the existence of a minimal propagation velocity below which no fronts are observed. On the theoretical side, the understanding of these phenomena is still incomplete and subject of the present project.
Frictional phenomena are commonly described using phenomenological rate-and-state friction models. Two possible mechanisms for generating slow rupture events were invoked in this framework. The first involves a non-monotonic dependence of the steady state frictional resistance on slip velocity, while the second involves spatial variation of frictional parameters and stress heterogeneities. The former mechanism is an intrinsic property of the friction law, while the latter mechanism is an extrinsic one. Laboratory measurements performed on a quasi-2D spatially homogeneous system, may suggest that the second mechanism is not necessary for the existence of slow rupture.

Project Description

In this project we investigate a friction model based on the dynamics of microcontacts at frictional interfaces. It predicts the existence of a frictional instability prior to the onset of sliding, which excites cracklike fronts whose velocity is independent of sound speed. Most importantly, the properties of these slow fronts propagating under transient inhomogeneous conditions are determined by steady state front solutions at the minimum of the sliding friction law, where a velocity-weakening behavior crosses over to a velocity-strengthening one.

Sliding on a substrate. In the left half the system is in the slip state, i.e. sliding on the substrate, whereas it is stuck on the ground in the right half. A rupture front invades the stick region. The simulation is based on a finite element simulation using our rate and state theory.

This is a joint activity with E. Brener (Research Center Jülich, Germany), E. Bouchbinder and Y. Bar Sinai (both Weizmann Institute of Science, Rehovot, Israel).

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