Short-time Dynamics of Frictional Strength in Dry Friction
O. Ben-David, G. Cohen and J. Fineberg
The Racah
Institute of Physics, The Hebrew University of Jerusalem, Givat
Ram, Jerusalem 91904, Israel.
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 4msec) 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 (60msec) 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.
Key words: Contact Mechanics, Friction
Mechanisms, Static Friction, Stick-Slip, Unlubricated
Friction, Polymers (solid), fracture induced weakening