Carmeliet

Vibration-induced unjamming of sheared granular media: investigation by 3D Discrete Element Method

modeling and simulation

This Research project aims at investigating the role of mechanical vibration in triggering the abrupt transition from a solid-like (jammed state) to a fluid-like (unjammed state) behavior of sheared granular layers. We specifically focus on understanding the underlying grain-scale processes.
This Research project is motivated by an ongoing collaboration between the investigators, the Los Alamos National Laboratory (LANL) and the Pennsylvania State University (PSU), USA.
The collaborators from PSU and LANL have been using a specific laboratory setup that was developed at PSU for investigating granular friction and its role in the physics of earthquakes.
, a layer of rock debris produced by wear during the tectonic relative sliding that generates earthquakes.i.e.In this setup a sheared granular layer is manipulated to mimic many different types of seismic events and dynamics, in particular stick-slip, which is the lab equivalent of one type of earthquake dynamics. The stick stage is associated to deformation of the tectonic blocks and accumulation of elastic energy. The slip stage corresponds to the earthquake event, accompanied by the release of the accumulated elastic energy. The laboratory granular layer mimics a geologic fault gouge,
Operating the PSU setup in the stick-slip dynamic regime, LANL/PSU researchers have found that lab analogs of earthquakes can be triggered upon subjecting the granular layer to transient elastic waves.
This lab phenomenology is similar to what has been observed just since the 1990s by seismologists: seismic waves, radiated by an earthquake, can trigger, later in time, the sudden transition from a solid-like to a fluid-like behavior for another geological fault at a different, far away, geographical location. This phenomenon is called “dynamic earthquake triggering” and remains a compelling mystery since its basic physical mechanisms are not yet understood. However, understanding dynamic earthquake triggering is of primary practical importance for improving seismic risk maps and for the prevention of other types of natural hazards, like snow avalanches and landslides.
The laboratory-scale experiments have some strong limitations: they do not give access to grain-scale measurements, thus they do not allow for understanding which grain-scale process is at the source of dynamic triggering of slip. In addition, the unjamming of sheared granular layers is a very complicated physical process of paramount importance in the field of Soft Condensed Matter and applied Materials Science. Indeed, unjamming is observed not only with granular media made of rigid macroscopic particles but also with colloids, suspensions, metallic glasses, other types of amorphous molecular solids and in applications at very small scales like particulate-based nanotribology. Unjamming of sheared granular media is not yet fully understood and remains a challenge for basic Materials Science because it involves both solid and fluid behavior, amorphous materials and dynamic phase transitions.
.Molecular Dynamics simulations based upon approach Computational Materials ScienceMotivated by the PSU/LANL experiments and by their limitations, we propose to contribute to investigating vibration-induced unjamming of sheared granular layers using a typical
We want to develop a computational model of the granular layers sheared in the PSU experimental apparatus and to simulate its behavior when perturbed by vibration.
Our major goal consists in unveiling the grain-scale mechanisms at the basis of dynamic triggering.
The project will consist of two main types of activities.
(1) 3D DEM modeling of sheared granular layers (a) in absence of applied vibration (reference datasets) and (b) in presence of it. No study has yet address modeling dynamic triggering with a such a configuration like the PSU’s one. We will focus our efforts onto (I) the implementation of realistic vibration source models by the use of elastodynamics simulations. (II) We will analyze the most relevant particle-scale parameters and processes to be included into the modeling for achieving better agreement with the observed lab phenomenology. We will constrain our models by laboratory datasets available by the LANL/PSU collaborators, who are leading investigators in the field (see articles in Nature 437: 71-874 (2005) and Nature 451: 57-61 (2008)).
(2) Development and implementation of measure functions for characterizing the grain-scale dynamics accompanying dynamic triggering. We have performed till now initial 2D studies showing that specific mesoscopic measures are needed to describe the highly spatially heterogeneous strain tensor fields that evolve during shear, their perturbation by applied vibration and its relation to the macroscopic slip events.
From a practical point of view, this project will provide fundamental help in deciphering the physics of earthquake nucleation and, thus, it will help with the prevention of natural hazards, like earthquake triggering by natural seismic events or even by human activities, e.g., enhanced geothermal energy production by hydro-fracturing (December 2006 Basel earthquake as an example) and landslide/snow avalanche initiation by construction or mining works.
However, the project will have a broader impact in improving the understanding of the dynamic behavior of granular and amorphous solids that exhibit metastable states in between the solid-like and fluid-like phases.

For further information please contact: Michele Griffa

Publications

  • GRIFFA M, DAUB E G, GUYER R A, JOHNSON P A, MARONE C, CARMELIET J: Vibration-induced slip in sheared granular layers and the micromechanics of dynamic earthquake triggering. Europhysics Letters: 96 (online 2011), 2012 DOI
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