Static friction between two solids is often represented as a macroscopic system property. The friction coefficient, which corresponds to the ratio between the friction and normal force, is reported as a function of material combination. This typically leaves the impression that the friction coefficient is a material/interface property. However, static friction is the limit at the onset of frictional sliding, which nucleates locally and propagates along the interface similar to a crack. Naturally, this nucleation and propagation process depends on local interface properties, which likely vary spatially - e.g., as a result of the real area of contact of the interface. In this work, we study the effect of randomness in the local friction properties on the globally measured static friction coefficient. First, we present results from dynamic simulations that model explicitly the nucleation and propagation of the frictional rupture front. We then develop a theoretical nucleation criterion by adapting an existing criterion to our random system. Finally, we run series of Monte-Carlo simulations to provide valuable insight into the link between the randomness of local friction properties and observed variations in macroscopic friction. Our work provides the theoretical basis for a solving an inverse problem: given the measured distribution of static friction, it provides an estimate of the stochastic properties of the frictional interface.

Onset of friction at an interface with random properties

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