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Sliding rod and evolution of friction strength

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Document pages: 4 pages

Abstract: The evolution of friction strength has important foundation and practical significance. Applications range from seismic dynamics 1, 2, 3, 4 to hard drive read write cycle 5. The friction strength is determined by the shear resistance of a large number of discrete contacts constituting the interface between two sliding bodies. The overall strength of the interface is determined by the actual contact area and the shear strength of the contact 6,7. Although the average motion of large slow sliding objects is well described by the empirical friction laws 3, 8, 9 and 10, the interface strength is a dynamic entity, which is intrinsically related to the rapid process (such as separation reattachment 11, 12, 13 and 14) and the slow process 6, 7, 13, 15 and 16 of contact area recovery. Here we show how the friction strength evolvesom 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 times16. 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.

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