Séminaire

I will provide an overview of some of the latest research from our group in Sydney. This will cover numerical, experimental, and educational tools, including details on our N-dimensional discrete element method code, our X-ray based techniques for observing granular media, and numerical tools for modelling granular materials as they segregate, crush and flow. No equations will be presented, although they can be supplied by the speaker on request.

• En savoir plus sur Benjy Marks

This seminar presents results on the effects of imposed rate of pore pressure change on the stability of rate and state frictional slip. The work is motivated by recent experiments that impose pressure at different rates. These find that the slip velocity and shear stress drops of accelerated slip events correlated with pore pressure rate rather than the magnitude of the pore pressure. Additional motivation comes from field observations suggesting that injection rate is an important factor in the occurrence of induced earthquakes in mid-continental US. Numerical simulations for a simple spring – slider model, assuming sliding is governed by rate and state friction, show that the pressure rate can control the frequency of rapid slip events. Refinement of the model indicates how the features of slip events observed in the laboratory depend on frictional parameters, rate of loading, rate and magnitude of pore pressure increase, and diffusivity.

Despite limitations on size and time scales, laboratory experiments provide a more controlled environment for understanding fundamental physical processes. Comparing the results of numerical simulations with experimental observations is the basis for understanding more complex behavior of pore pressure interaction with frictional slip. Such comparisons provide a means of examining the effects of different parameters, correlating results from different experiments and suggesting new experiments. Because such simulations provide a way of generalizing results for a range of parameters not attainable in laboratory experiment they provide insight into field observations of induced seismicity, in particular, whether slip due to fluid injection occurs and, if it does, whether it is seismic or aseismic.

• En savoir plus sur John Rudnicki

Soil behaviour has been traditionally interpreted and modelled within in the framework of continuum mechanics, which has been and will remain the most convenient approach for geotechnical design. Nonetheless, we do not ignore the particulate nature of soils and often turn towards particle-scale mechanisms to interpret experimental data and/or inform ‘continuum-scale’ constitutive models.

In granular materials, the understanding of particle scale mechanisms has been raised to a quantitative level via XCT observations and DEM modelling. Clay is lagging behind due to the difficulty of observing clay particle interactions ‘in-situ’. The majority of our speculations are based on post-mortem observations of heavily manipulated samples (SEM, MIP).

The development of experimental and numerical micromechanics of clays is strongly linked with the conceptual understanding and quantitative modelling of the clay particle energy-separation relationship. Researchers in soil mechanics tend to assume that DLVO theory provides a robust reference for their conceptual modelling. This is only partially true. DLVO theory is mainly explored for the case of i) infinite, uniformly-charged, and parallel particles and ii) particles driven by kinetic energy (colloidal state). None of these assumptions apply to ‘consolidated’ geotechnical clays. In addition, the distribution and magnitude of electrical charge is far from being understood and this has profound implications on the response of clay assembly to external mechanical loading. This presentation focuses on a number of questions that need to addressed to build the foundations of clay micromechanics.

• En savoir plus sur Alessandro Tarantino

I present two series of particle dynamics simulations to investigate the intrinsic and emergent length and time scales in granular flows. Our simulation data suggest that the rheology is controlled by at least two emerging length scales depending on the inertial nature of the flow and the roughness of boundary elements. It is also found that all  characteristic times boil down to two time scales whose ratio controls the rheological behavior (effective friction and solid fraction). These results are relevant to the definition of the representative volume element and quasi-static conditions.    
 

• En savoir plus sur Farhang Radjai

The compaction behavior of deformable grain assemblies beyond jamming remains misunderstood, and existing models that seek to find the relationship between the confining pressure P and solid fraction ϕ end up settling for empirical strategies or fitting parameters. Numerically and experimentally, we analyze the compaction of highly deformable frictional grains of different shapes and soft/rigid particle mixtures in two and three dimensions: numerically, using a coupled discrete - finite element method, the Non-Smooth Contact Dynamics Method (NSCD), and experimentally using high-resolution imaging coupled with a dedicated DIC algorithm. We characterize the evolution of the packing fraction, the elastic modulus, and the microstructure (particle rearrangement, connectivity, contact force, and particle stress distributions) as a function of the applied stresses. We show that the solid fraction evolves non-linearly from the jamming point and asymptotically tends to a maximal packing fraction, depending on the soft/rigid mixture ratio, the friction coefficient, and the particle shape. At the microscopic scale, different power-law relations are evidenced between the local grain structure and contacts, and the packing fraction and pressure, regardless of the shape, the mixture ratio, or the dimensionality (2D/3D). A significant outcome of this work is the development of a theoretical and micromechanical-based approach for the compaction of soft granular assemblies far beyond the jamming point. This latter is derived from the granular stress tensor, its limit to small deformations, and the evolution of the connectivity. Furthermore, from the expression of these well-defined quantities, we establish different compaction equations, free of ad hoc parameters, well-fitting our numerical and experimental data. These equations mainly depend on the dimensionality, where the characteristics of shape, elastic bi-dispersity, and compression geometry (uniaxial vs isotropic) are considered as input parameters. Our theoretical framework allows us to unify the compaction behavior of assemblies of soft, soft/rigid, and noncircular soft particles coherently, both in 2D and 3D, for isotropic and uniaxial compression.

• En savoir plus sur Cárdenas-Barrantes Manuel

Granular materials deform macroscopically via local slip and coordinated particle rearrangement events at the microscale. Discrete and continuum numerical models have been employed in the engineering and physics communities to capture the effects of individual particle rearrangements on the macroscopic plasticity of granular and related materials. For instance, glassy rheology models and shear transformation zone theories have been used to capture the aggregated effects of local individual rearrangement events and their interactions in colloids, metallic glasses, and granular media. A major challenge remains the quantitative validation and calibration of these models using in-situ 3D experimental data.
In this talk, we discuss recent experiments combining in-situ X-ray computed tomography (XRCT) and 3D X-ray diffraction (3DXRD) to quantify local rearrangements in deforming 3D granular materials. We focus on granular materials with hundreds to thousands of nearly-spherical sapphire or quartz particles in uniaxial, hydrostatic, and triaxial loading conditions. Using microscopic structure and per-particle stress tensor measurements at small macroscopic strain increments, we examine the statistics and history-dependence of local rearrangement events, and study the features of the structure and force network that control when and where rearrangements occur via machine learning. We find that local rearrangements obey similar statistical distributions in various loading conditions and at various stress states when properly normalized, that materials retain significant memory of local rearrangements during monotonic loading, and that local structure rather than local stress plays a dominant role in predicting when and where large rearrangement events will occur. We also discuss other results from analysis of these datasets, including a hierarchical ordering of structural and mechanical length scales.
 

• En savoir plus sur Ryan Hurley

Granular materials deform macroscopically via local slip and coordinated particle rearrangement events at the microscale. Discrete and continuum numerical models have been employed in the engineering and physics communities to capture the effects of individual particle rearrangements on the macroscopic plasticity of granular and related materials. For instance, glassy rheology models and shear transformation zone theories have been used to capture the aggregated effects of local individual rearrangement events and their interactions in colloids, metallic glasses, and granular media. A major challenge remains the quantitative validation and calibration of these models using in-situ 3D experimental data.
In this talk, we discuss recent experiments combining in-situ X-ray computed tomography (XRCT) and 3D X-ray diffraction (3DXRD) to quantify local rearrangements in deforming 3D granular materials. We focus on granular materials with hundreds to thousands of nearly-spherical sapphire or quartz particles in uniaxial, hydrostatic, and triaxial loading conditions. Using microscopic structure and per-particle stress tensor measurements at small macroscopic strain increments, we examine the statistics and history-dependence of local rearrangement events, and study the features of the structure and force network that control when and where rearrangements occur via machine learning. We find that local rearrangements obey similar statistical distributions in various loading conditions and at various stress states when properly normalized, that materials retain significant memory of local rearrangements during monotonic loading, and that local structure rather than local stress plays a dominant role in predicting when and where large rearrangement events will occur. We also discuss other results from analysis of these datasets, including a hierarchical ordering of structural and mechanical length scales.
 

• En savoir plus sur Ryan Hurley

Different types of cohesive forces, such as van der Waals forces, electrostatic forces and capillary forces occur in granular materials. For wet granular materials, capillary forces are larger than the other cohesive forces. Hence, cohesion in wet granular materials originates from capillary forces between the constituting particles.

At the particle scale, we have obtained analytical expressions for rupture distances and capillary forces of capillary bridges under volume control (where the liquid volume is kept constant) and suction control (where the pressure difference between the surrounding gas and the liquid inside the capillary bridge is kept constant). These expressions for the pendular regime are based on the governing Young-Laplace equation. We have found that the properties of capillary bridges between two spherical particles under suction and volume control are significantly different, in particular for the rupture distances.
To investigate the influence of the hydraulic loading path on the hydromechanical behaviour of wet granular materials, we have conducted Discrete Element Method (DEM) simulations. In these simulations, the properties of capillary bridges between spherical particles are represented by the newly developed expressions that are suitable for capillary bridges under volume as well as suction control. The DEM simulation results show macro-scale differences of the behaviour that originate from the interparticle capillary bridges at the micro-scale under volume and suction control.

• En savoir plus sur Chaofa Zhao

A distance, Saint-Martin-d'Hères - Domaine universitaire

J. Carlos Santamarina (Professor - KAUST) graduated from Universidad Nacional de Córdoba and completed graduate studies at the Universities of Maryland and Purdue. He taught at NYU-Polytechnic, the University of Waterloo and at Georgia Tech before joining KAUST in 2015. His research centers on the science of geomaterials and engineering solutions to address global energy challenges, with contributions from resource recovery to energy and waste geostorage. He delivered the 50th Terzaghi Lecture on Energy Geotechnology, was a British Geotechnical Association Touring Lecturer, and is member of both Argentinean National Academies. Former team members are professors at more than forty universities, researchers at national laboratories, or practicing engineers at leading organizations worldwide.

• En savoir plus sur J. Carlos Santamarina

Tomographic imaging of granular materials at the grain scale is a fantastic tool to learn about granular kinematics. There are however limitations in the temporal resolution imposed by the requirement to stop and turn. Here we study an initially simple granular system (monodisperse spherical particles) with a new technique and prove that we can acquire 3D displacements at 60 Hz.

So what was Eddy doing during the confinement? With finally a little bit of free time, and along with Benjy Marks (University of Sydney) and Stéphane Roux (ENS Paris Saclay), this long-standing work has been given a boost. Using the strong a-priori knowledge of the size and shape of particles, a positioning algorithm (position reconstruction?) using a single projection from a divergent x-ray source like the one we have in the lab, has been developed. This essentially means that factors of hundreds can be saved in the acquisition, opening the way to imaging of granular flows.

• En savoir plus sur Edward Andò