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

TL;DR

This study uses a thermodynamics model to explore how friction, vibration, and shear rate influence shear banding and stick-slip instabilities in granular materials, revealing conditions that promote or suppress these phenomena.

Contribution

It introduces a comprehensive model analyzing the effects of vibrations and friction on shear localization and stability in granular materials, linking microscopic mechanisms to macroscopic behavior.

Findings

Vibrations fluidize granular networks and de-localize slip at low and intermediate shear rates.

Stick-slip occurs only in frictional grains and is confined to shear bands.

High shear rates lead to similar stress-slip responses regardless of vibrations or friction.

Abstract

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