Sensitive and fast force measurements are performed on sheared granular layers undergoing stick-slip motion, along with simultaneous optical imaging. A full study has been done for spherical glass particles with a 20% size distribution. Stick-slip motion due to repetitive fluidization of the granular layer occurs for low driving velocities. Between major slip events, slight creep occurs that is highly variable from one event to the next. The effects of varying the stiffness k of the driving system and the driving velocity V are studied in detail. The stick-slip motion is almost periodic for spherical particles over a wide range of parameters, whereas it becomes irregular when k is large and Vis relatively small. At larger V, the motion becomes smoother and is affected by the inertia of the upper plate bounding the layer. Measurements of the period and amplitude of the relative motion are presented as a function of V. At a critical value V, a transition to continuous Sliding motion occurs. The transition is discontinuous for k not too large, and large fluctuations occur in the neighborhood of the transition. The time dependence of the instantaneous velocity of the upper plate and the frictional force produced by the granular layer are determined within individual slipping events. The frictional force is found to be a multivalued function of the instantaneous velocity during slip, with pronounced hysteresis and a sudden drop just prior to resticking. Measurements of vertical displacement reveal a very small dilation of the material (about one-tenth of the mean particle size in a layer 20 particles deep) associated with each slip event; the dilation reaches its maximum amplitude close to the time of maximum acceleration. Finally, optical imaging reveals that localized microscopic rearrangements precede land follow) each macroscopic slip event; their number is highly variable and the accumulation of these local displacements is associated with macroscopic creep. The behavior of smooth particles is contrasted qualitatively with that of rough particles. [References: 37]
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