Static versus dynamic friction: the role of coherence

Abstract

A simple model for solid friction is analysed. It is based on tangential springs representinginterlocked asperities of the surfaces in contact. Each spring is given a maximal strainaccording to a probability distribution. At their maximal strain the springs breakirreversibly. Initially all springs are assumed to have zero strain, because at staticcontact local elastic stresses are expected to relax. Relative tangential motion ofthe two solids leads to a loss of coherence of the initial state: the springs getout of phase due to differences in their sizes. This mechanism alone is shown tolead to a difference between static and dynamic friction forces already. We findthat in this case the ratio of the static and dynamic coefficients decreases withincreasing relative width of the probability distribution, and has a lower bound of1 and an upperbound of 2.