Shearing of a confined granular layer: tangential stress and dilatancy.
We study the behavior of a confined granular layer under shearing, in an annular cell, at low velocity. We give evidence that the response of the granular layer under shearing is described by characteristic length scales. The tangential stress reaches its steady state on the same length scale as the dilatancy. Stop-and-go experiments performed at several driving velocities show a logarithmic increase of the static friction coefficient with waiting time, followed by rejuvenation on a characteristic length of the order of the magnitude of a Hertz contact between adjacent grains. The dilatancy does not evolve during the stop, neither during the elastic reloading when the driving is resumed. There is a small variation when sliding sets anew, which corresponds to the rejuvenation of the layer, and this variation is independent of the waiting time. We argue that aging is due to the behavior of individual contacts between grains, not global evolution of the piling. Under an instantaneous increase of the velocity, the tangential stress reaches a new steady state, exhibiting velocity strengthening behavior. An increase of dilatancy is also observed. It is much larger than fluctuations in the steady state, variations in a stop and-go-experiment, but much less than for shearing of freshly poured grains. The dilatancy variation during a velocity jump is not due to structural rearrangements of the piling. The evolutions of tangential stress and dilatancy are logarithmic in the ratio of upper and lower velocities.
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