Onset and evolution of the sliding: key role of the local contact dynamics

Stick-slip is a non-smooth transition occurring at the interface between two solids in frictional contact and in relative motion. It is associated with a variation of frictional resistance during sliding and results in a dynamic instability due to a temporal succession of phases of elastic energy storage (stick phase) followed by phases of energy release (slip phase) with an associated tangential stress drop at the contact. On the contrary, continuous (or smooth) sliding occurs when the whole interface undergoes sliding without stick phases. The onset of the instability was usually related to the difference between the static and the dynamic coefficient of friction, to a rate-weakening feature of the friction coefficient or to a variation of friction coefficient along the interface [1]. Nevertheless it has been observed [2-4] that stick-slip motion also arises in systems without such assumption, i.e. with a constant friction coefficient and no distinction between static and kinetic friction. Stick-slip in this case arises as a consequence of a dynamic instability involving the coupling between normal and tangential motion [2] or to a destabilization of the interface waves [5]. Recent literature stated that the variation of frictional resistance is rather the result of both the frictional properties at the interface and the elastic and inertial properties of the system [6-7], assuming that stick-slip configures as an interchange between the elastic energy stored into the system and its kinetic energy by means of the friction force. More generally the frictional behaviour of a system has been associated with a more complex mutual interaction between the local dynamics at the frictional interface (wave generation and propagation) and the global dynamics of the systems [8-16]. The aim of this paper is to account for local wave generation and propagation rather than just macroscopic dynamics; the transition between stick-slip and continuous sliding is investigated in function of the variations of the local contact dynamics highlighting its key role