Interrupted shear of granularmedia

The response of a granular material during a stop-and-goshear experiment is investigated using an annular shear cell andsilicagel powders of different particle sizes. The experimentalresults are examined on the basis of the Dieterich-Rice-Ruinamodel for solid friction. In addition to making this analogywith solid friction, we describe a new instability that isobserved when restarting shear, where the powder bed is found toslip and compact for short hold times but only dilates for longhold times. The minimum hold time to restore a non-slipbehaviour has been investigated for different size particles andnormal loadings. The observed dependencies show analogiesbetween this behaviour and the sliding rearrangements seen abovethe stick-slip threshold.