Effect of boundary vibration on the frictional behavior of a dense sheared granular layer

Abstract

We report results of 3D Discrete Element Method (DEM) simulations aiming at investigating the role of the boundary vibration in inducing frictional weakening in sheared granular layers. We study the role of different vibration amplitudes applied at various shear stress levels, for a granular layer in the stick-slip regime and in the steady-sliding regime. Results are reported in terms of friction drops and kinetic energy release associated with frictional weakening events. We find that larger vibration amplitude induces larger frictional weakening events. The results show evidence of a threshold below which no induced frictional weakening takes place. Friction drop size 2 is found to be dependent on the shear stress at the time of vibration. A significant increase in the ratio between the number of slipping contacts to the number of sticking contacts in the granular layer is observed for large vibration amplitudes. These vibration-induced contact rearrangements enhance particle mobilization and induces a friction drop and kinetic energy release. This observation provides some insight into the grain-scale mechanisms of frictional weakening by boundary vibration in a dense sheared granular layer. In addition to characterizing the basic physics of vibration induced shear weakening, we are attempting to understand how a fault fails in the earth under seismic wave forcing. This is the well know phenomenon of dynamic earthquake triggering. We believe that the granular physics are key to this understanding.

Stick-slip instabilities in sheared granular flow: The role of friction and acoustic vibrations

“…To further characterize vibration-induced shear weakening and dynamic triggering, and to identify intervals of increased seismic hazard, it will be useful to investigate the effect of varying the noise strength ρ as well as the vibration amplitude and frequency. Ferdowsi et al [10,11] have found in their simulations evidence of an amplitude threshold below which vibrations do not cause significant frictional weakening. It will be interesting to examine if the STZ theory can capture this observation.…”

“…In the context of granular earth materials, an important example of phenomena observed in nature and in simulations, and captured in the present STZ model, is the dynamic triggering of catastrophic events [10,11,19,49], known to occur in the presence of a granular gouge layer. Specifically, we found that vibrations reduce the shear strength and clock-advance the first slip event upon loading.…”

“…These are in addition to addressing hysteretic phenomena and long-time relaxation upon ceasing acoustic vibrations or in unsheared granular media [10,11,19], as well as an extensive characterization of stick-slip statistics such as dependence of the magnitude of stress drop and inter-event period on the vibration intensity ρ and other control parameters. While our results provide crucial input for refining the constitutive description of dense granular flow, we call for extensive experimental studies to validate our predictions and further constrain material parameters.…”

“…Figure 7 shows that vibrations trigger the onset of slip, in the sense that vibrations clock-advance the first slip event at a reduced shear stress following the initial loading. This provides a possible explanation for dynamic earthquake triggering, the observation that seismic waves radiated by an earthquake can trigger catastrophic events elsewhere [11,19,49]. The triggering mechanism is associated with weakening the threshold for slip events, rather than increasing the load.…”

“…Such instabilities are often observed in laboratory experiments [1][2][3][4][5][6][7][8][9]. At much larger scales, these instabilities may explain how aftershocks can be triggered by acoustic waves from earthquakes elsewhere [10][11][12][13][14][15]. In the context of industrial processing, stick-slip could result in structural damage and problems in mixing, causing major problems in the quality control of products [16][17][18].…”