Séminaire

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

Saint-Martin-d'Hères - Domaine universitaire

Introducing statistical mechanics framework into granular mechanics has been attempted both by physicists and mechanicians in recent decades. The talk overviews its recent outcomes and try to connects the two perspectives from physics and mechanics sides.

• En savoir plus sur Takashi Matsushima

Saint-Martin-d'Hères - Domaine universitaire

Granular media and concrete are the ubiquitous natural and construction materials. X-ray tomography (XRT) has been used throughout the past three decades to qualitatively and quantitatively study the kinematics of deformation in these materials from meso- to microscales. However, quantifying the microscopic mechanisms of stress transmission and energy dissipation in these materials has traditionally been limited to 2D model materials or 3D numerical simulations. Such information can provide insight into the validity of microscopic contact laws, the statistics of forces and energy dissipation, and the pre- and post-failure load sharing within the materials’ microstructures.

In this talk, I will discuss experiments employing in-situ XRT and 3D X-ray diffraction (3DXRD) measurements during the deformation of 3D granular materials and cement. Using 3DXRD, each particle in the granular materials and cements acts as a stress gauge, providing both the local elastic stress tensors and the orientations of material points within the microstructure. I will highlight experiments with granular materials in which we quantitatively measured inter-particle forces, constitutive law parameters, per-particle fracture stresses, and energy dissipation mechanisms at each inter-particle contact. I will highlight an experiment on micro-concrete in which we quantified the stress distribution throughout the microstructure before and after fracture. I will discuss ongoing and future research directions in geomaterials with combined XRT and 3DXRD.

• En savoir plus sur Ryan Hurley

Coupled Thermo-Hydro-Mechanical (THM) processes in particulate materials play an important role in a wide range of disciplines ranging from geomechanics to powder technologies. Despite an abundance of continuum based numerical approaches for simulating THM processes, the computational (and monetary) expense remains a prohibitive factor. During this presentation, a computationally efficient open-sourced discrete element based numerical method dedicated to the simulation of heat transfer and associated THM couplings in granular media is presented. Additionally, the numerical framework is validated analytically, numerically, and experimentally. The audience can expect to learn about the combination of heat transfer models comprising the conductive and advective heat transfer schemes as well as their mechanical couplings. In brief, the framework builds upon an existing pore finite volume (PFV) scheme to add conductive and advective heat transfer processes. Conductive heat transfer is modeled explicitly between and within solid and fluid phases: across DEM particle contacts, between adjacent tetrahedral pores, and between pores and incident particles. Meanwhile, advective heat transfer is added to the existing implicit fluid flow scheme by estimating mass energy flux from pressure induced fluid fluxes. In addition to the heat transfer model, a thermo-mechanical coupling is presented by considering pore space volume changes based on the thermal expansion of particles and fluid.

• En savoir plus sur Robert Caulk

Our research on the influence of vegetation on soil mechanical and hydrological properties will be discussed at scales from the single root too the hill-slope.



Vegetation can reinforce soil slopes through both direct mechanical reinforcement and hydrological reinforcement, as plants generate suctions that increase effective stress between soil particles. We have studied relevant mechanical and hydrological processes at a range of scales: measuring the mechanical properties of individual plant roots; the reinforcement that roots offer to intact soil cores; the reinforcement of scaled model slopes within a geotechnical centrifuge subject to simulated rainfall events; and the monitoring of both mechanical and hydrological reinforcement of field-scale slopes. The effects of plant roots on changing soil structure and hydrology with time will also be considered. This talk will consider what we can learn from experiments and modelling at each scale, and where significant gaps still exist in our understanding. The potential to improve slope management by appropriate choice of species and management regime will be discussed, together with problems that may be caused when vegetation is removed and not replanted promptly.

 

• En savoir plus sur Glyn Bengough

Diatomaceous soils, which are frequent in lacustrine or marine deposits, emerged as a singular type of materials showing unusual geotechnical properties characterized by high values of Atterberg limits, high natural water content, and high compressibility but complemented with high shear strength. The high liquid limit accompanied by high friction angles seems contradictory to the classical soil mechanics. However, several studies have shown that the unusual geotechnical properties of diatomaceous soils are related to the micromechanical features of the diatoms.



This investigation considers two diatoms species (Aulacoseira granulate and Centric Coscinodiscus) and identifies the effect of each type of diatom into the overall macroscopic behaviour of the soils. The study involves several tests of Atterberg limits as well as oedometric and triaxial tests on samples having different proportions of diatoms of the two different species. Results evidence that the shape of the diatom at the micro scale has observable trends in the macroscopic response of the soils.

Slides of the presentation (PDF, 9.64 Mo)

• En savoir plus sur Bernardo Caicedo

Earthquake surface fault rupture can cause substantial structural damage and loss of life in near-fault regions. This hazard may be effectively modeled as a large-strain boundary-displacement problem governed by the fundamental granular nature of soil. Tremendous insights into the rupture mechanisms of granular soils are possible when soil is examined as a particulate medium with the characteristics of individual particles modeled numerically with the discrete element method (DEM). In this study, the interaction of a propagating fault rupture surface with a building foundation, called fault rupture-soil-foundation interaction (FR-SFI), is simulated in three dimensions using DEM with irregularly-shaped particles to capture the non-spherical nature of sand grains. High-performance computing simulations of free-field surface fault rupture and FR-SFI are validated against the results of geotechnical centrifuge experiments. Parametric analysis of the effects of soil density, modeled directly with particle assemblages having different void ratios, on FR-SFI is then performed with foundations of different contact pressures at different locations. Through quantitative analyses of inter-particle contact forces, particle rotations, particle displacements, and changes in void ratios, DEM provides insights on particle responses not possible with conventional continuum methods.

• En savoir plus sur Barbara Mazzolai