We present experimental results of the transition from steady-state sliding to oscillatory motion for a fine noncohesive powder, sheared in an annular cell. The onset of instability is compared to the Dieterich-Ruina model for solid friction. We present data showing that at low velocity and close to the transition, the major sliding jumps are preceded by a short or long period of unstable plastic yielding of the granular matter. This ambivalent behavior suggests that the jumps are initiated when the sliding overcomes a critical velocity. During the stick-slip motion, the dilatancy of the powder bed has been also observed: the slippage is associated with a compaction whose value increases with the jump in the friction coefficient.

The response of a granular material during a stop-and-go shear experiment is investigated using an annular shear cell and silicagel powders of different particle sizes. The experimental results are examined on the basis of the Dieterich-Rice-Ruina model for solid friction. In addition to making this analogy with solid friction, we describe a new instability that is observed when restarting shear, where the powder bed is found to slip and compact for short hold times but only dilates for long hold times. The minimum hold time to restore a non-slip behaviour has been investigated for different size particles and normal loadings. The observed dependencies show analogies between this behaviour and the sliding rearrangements seen above the stick-slip threshold.

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