  The onset of frictional instabilities, e.g. earthquakes nucleation, is
intimately related to velocity-weakening friction, in which the frictional
resistance of interfaces decreases with increasing slip velocity. While this
frictional response has been studied extensively, less attention has been given
to steady-state velocity-strengthening friction, in spite of its potential
importance for various aspects of frictional phenomena such as the propagation
speed of interfacial rupture fronts and the amount of stored energy released by
them. In this note we suggest that a crossover from steady-state
velocity-weakening friction at small slip velocities to steady-state
velocity-strengthening friction at higher velocities might be a generic feature
of dry friction. We further argue that while thermally activated rheology
naturally gives rise to logarithmic steady-state velocity-strengthening
friction, a crossover to stronger-than-logarithmic strengthening might take
place at higher slip velocities, possibly accompanied by a change in the
dominant dissipation mechanism. We sketch a few physical mechanisms that may
account for the crossover to stronger-than-logarithmic steady-state
velocity-strengthening and compile a rather extensive set of experimental data
available in the literature, lending support to these ideas.
