Localization and instability in sheared granular materials: Role of friction and vibration

摘要

Shear banding and stick-slip instabilities have been long observed in sheared granular materials. Yet, theirmicroscopic underpinnings, interdependencies, and variability under different loading conditions have not beenfully explored. Here we use a nonequilibrium thermodynamics model, the Shear Transformation Zone theory, toinvestigate the dynamics of strain localization and its connection to stability of sliding in sheared, dry, granularmaterials.We consider frictional and frictionless grains as well as the presence and absence of acoustic vibrations.Our results suggest that at low and intermediate strain rates, persistent shear bands develop only in the absenceof vibrations. Vibrations tend to fluidize the granular network and delocalize slip at these rates. Stick-slip isobserved only for frictional grains, and it is confined to the shear band. At high strain rates, stick-slip disappearsand the different systems exhibit similar stress-slip response. Changing the vibration intensity, duration or timeof application alters the system response and may cause long-lasting rheological changes. We analyze theseobservations in terms of possible transitions between rate strengthening and rate weakening response facilitatedby a competition between shear-induced dilation and vibration-induced compaction.We discuss the implicationsof our results on dynamic triggering, quiescence, and strength evolution in gouge-filled fault zones.