Mechanisms governing onset of sliding friction

Experimental observations on slip onset during quasistatic shearing of bimaterial interface are presented in this study. Although friction has been a subject of multi-disciplinary research for many decades, events surrounding the transition of static to dynamic friction are not well understood. In this fundamental study of dry frictional sliding in polyurethane (PU)--poly(methyl methacrylate) (PMMA) material pair, we provide experimental evidences on propagation of slow rupture waves prior to slip onset. Constant normal load of 114 N and sliding velocity of 0.4 mm/s were chosen to induce stick-slip oscillations in the PU-PMMA sliding pair. Full-field, non-invasive techniques of dynamic photoelasticity and 2D Digital Image Correlation (2D DIC) were used simultaneously to capture and analyze the propagation of rupture fronts from the leading to trailing edges. Sequence of 900 images before and after onset of slip were acquired using high-speed photography at 44,000 frames/s. Analysis of grey-scale speckles using DIC, provided information on magnitude, direction, and velocity of slip. Based on the wave and particle data, we conclude that slower sub-shear waves produced the onset of slip. Velocity of sub--shear front was found to be around 30-85 m/s, while instantaneous slip velocities reached maximum values of 0.3 m/s. Origin of these rupture fronts was traced to a point near the tip of the leading edge in PU. Our experiments clearly demonstrate the role of characteristic waves along the interface during sliding friction