No abstract

The dynamics of friction have been studied for hundreds of years, yet many aspects of these everyday processes are not understood. One such aspect is the onset of frictional motion (slip). First described more than 200 years ago as the transition from static to dynamic friction, the onset of slip is central to fields as diverse as physics, tribology, mechanics of earthquakes and fracture. Here we show that the onset of frictional slip is governed by three different types of coherent crack-like fronts: these are observed by real-time visualization of the net contact area that forms the interface separating two blocks of like material. Two of these fronts, which propagate at subsonic and intersonic velocities, have been the subject of intensive recent interest. We show that a third type of front, which propagates an order of magnitude more slowly, is the dominant mechanism for the rupture of the interface. No overall motion (sliding) of the blocks occurs until either of the slower two fronts traverses the entire interface.

The way in which a frictional interface fails is critical to our fundamental understanding of failure processes in fields ranging from engineering to the study of earthquakes. Frictional motion is initiated by rupture fronts that propagate within the thin interface that separates two sheared bodies. By measuring the shear and normal stresses along the interface, together with the subsequent rapid real-contact-area dynamics, we find that the ratio of shear stress to normal stress can locally far exceed the static-friction coefficient without precipitating slip. Moreover, different modes of rupture selected by the system correspond to distinct regimes of the local stress ratio. These results indicate the key role of nonuniformity to frictional stability and dynamics with implications for the prediction, selection, and arrest of different modes of earthquakes.

Frictional processes entail the rupture of the ensemble of discrete contacts defining a frictional interface. There are a variety of views on how best to describe the onset of dry frictional motion. These range from modelling friction with a single degree of freedom, a 'friction coefficient', to theoretical treatments using dynamic fracture to account for spatial and temporal dynamics along the interface. We investigated the onset of dry frictional motion by performing simultaneous high-speed measurements of the real contact area and the strain fields in the region surrounding propagating rupture tips within the dry (nominally flat) rough interfaces formed by brittle polymer blocks. Here we show that the transition from 'static' to 'dynamic' friction is quantitatively described by classical singular solutions for the motion of a rapid shear crack. We find that these singular solutions, originally derived to describe brittle fracture, are in excellent agreement with the experiments for slow propagation, whereas some significant discrepancies arise as the rupture velocity approaches the Rayleigh wave speed. In addition, the energy dissipated in the fracture of the contacts remains nearly constant throughout the entire range in which the rupture velocity is less than the Rayleigh wave speed, whereas the size of the dissipative zone undergoes a Lorentz-like contraction as the rupture velocity approaches the Rayleigh wave speed. This coupling between friction and fracture is critical to our fundamental understanding of frictional motion and related processes, such as earthquake dynamics.

The evolution of frictional strength has great fundamental and practical importance. Applications range from earthquake dynamics to hard-drive read/write cycles. Frictional strength is governed by the resistance to shear of the large ensemble of discrete contacts that forms the interface that separates two sliding bodies. An interface's overall strength is determined by both the real contact area and the contacts' shear strength. Whereas the average motion of large, slowly sliding bodies is well-described by empirical friction laws, interface strength is a dynamic entity that is inherently related to both fast processes such as detachment/re-attachment and the slow process of contact area rejuvenation. Here we show how frictional strength evolves from extremely short to long timescales, by continuous measurements of the concurrent local evolution of the real contact area and the corresponding interface motion (slip) from the first microseconds when contact detachment occurs to large (100-second) timescales. We identify four distinct and inter-related phases of evolution. First, all of the local contact area reduction occurs within a few microseconds, on the passage of a crack-like front. This is followed by the onset of rapid slip over a characteristic time, the value of which suggests a fracture-induced reduction of contact strength before any slip occurs. This rapid slip phase culminates with a sharp transition to slip at velocities an order of magnitude slower. At slip arrest, 'ageing' immediately commences as contact area increases on a characteristic timescale determined by the system's local memory of its effective contact time before slip arrest. We show how the singular logarithmic behaviour generally associated with ageing is cut off at short times. These results provide a comprehensive picture of how frictional strength evolves from the short times and rapid slip velocities at the onset of motion to ageing at the long times following slip arrest.