We present an experimental study of the onset of local frictional motion along a long, spatially extended interface that separates two PMMA blocks in dry frictional contact. At applied shear forces significantly below the static friction threshold, rapid precursory detachment fronts are excited, which propagate at near sound speeds along the interface. These fronts initiate from the interface edge and arrest prior to traversing the entire sample length. Along the fronts' path, we perform real-time measurements of the real contact area at every spatial point within the interface. In addition, the motion (slip) of the material adjacent to the interface is simultaneously measured at chosen locations. Upon their arrival at each spatial point along their path, these fronts instantaneously (within 4 μs) reduce the net contact area. Net slip is only initiated after this contact area reduction occurs. Slip is initially rapid and progresses at its initial velocity for a constant (60 μs) duration. Slip dynamics then undergo a sharp transition to velocities an order of magnitude slower, which remain nearly constant until slip arrest. We demonstrate that this scenario can be quantitatively explained by a model of interface weakening caused by instantaneous fracture-induced heating. Sustained rapid slip occurs in this weakened phase. Once the interface cools beneath its glass temperature the sharp transition to slow slip takes place. A similar fracture-induced weakening scenario might be expected in additional classes of materials.
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Acknowledgments We acknowledge the comments from A. Sagy and S. M. Rubinstein. This work, as part of the ESF EUROCORES programme FANAS, was supported by the Israel Science Foundation (grant 57/07). We also acknowledge support of the US–Israel Binational Science Foundation (grant 2006288). J. Fineberg acknowledges the support of the Max Born chair of Natural Philosophy.
- Contact mechanics
- Fracture-induced weakening
- Friction mechanisms
- Polymers (solid)
- Static friction
- Unlubricated friction