Séminaire

Séminaire Le 3 novembre 2022 A set of triaxial compression tests on partially saturated dense sands to clarify the microscopic characteristics and their link to the macroscopic responses is presented. Constant suction tests (CS tests) and constant water content tests (CW tests) are conducted under low confining pressure to observe microscopic and macroscopic behaviors of the sands associated with dilative shear bands. X-ray micro-tomography and image analysis techniques are applied to investigate the continuity as a defined index to evaluate the morphology of the pore water, the number of liquid bridges and the principal curvature of the air–water interface, etc.

The relationship between the microscopic observation and overall specimen-scale behaviour is also discussed. The tendency of decreasing curvature corresponds to that of decreasing suction in the CW test. The peak deviator stress is higher in the CS test than in the CW test when the pore water is initially discontinuous, whereas it is identical between the two tests when the pore water is initially continuous. The residual stress is lower in the CW test than in the CS test, independent of the initial water retention states. The macroscopic responses at the different initial water retention states are qualitatively identical between poorly graded sand and well-graded sand

• En savoir plus sur Yosuke Higo

Séminaire Le 17 novembre 2022 Cementitious materials and geomaterials are considered as complex porous civil engineering materials. Understanding the behaviour of such materials requires a multiscale hierarchical study of mechanisms occurring across multiple length scales (nano→micro→macro) where every phenomenon in a given scale can be understood through looking at the scale below. The molecular configuration and the bonding mechanism at the nano-scale define the interactions inside a multi-phase system (such as cement) at the micro-scale that characterize the material response to external mechanical loading and to chemical/physical attacks from the outside environment at the macro-scale.

We are interested in developing upscaling approaches based on Molecular Dynamics (MD), Monte Carlo (MC) and Density Theory functional (DFT) methods to study the multi-physics multiscale behaviour of geomaterials considering the multiscale features of the pore structure. Characterizing the structure of geomaterials at the nano-scale is not only important to predict the chemical, thermal and mechanical behaviour but also to efficiently optimize/manipulate the composition of the material in order to enhance the mechanical performance of civil engineering materials at the macro scale.

• En savoir plus sur Laanaiya Majdouline

Séminaire Le 8 septembre 2022 The action of a fluid flow on a granular material is a common situation in nature and in many industrial processes. Numerous studies, based on both experiments and modelling, have led to a better understanding and description of the fluid-granular interaction in situations such as surface erosion or fluidization. The hydro-mechanical behaviour of cemented soils, and more particularly of cemented granular soils, remains less known although this type of material is also frequently encountered in sedimentary rocks (sandstones, conglomerates, breccia…) or engineering materials (mortars, concrete, asphalt…).
To address this problem, we consider here more specifically grains that are artificially cemented by creating, from an external solid phase, adhesive bridges whose tensile strength can be varied. This allows a gradual transition from the pure granular case to weakly cemented ones. In this presentation, I will report some recent studies carried out for various situations, such as impinging jet erosion, surface erosion, localized injection, with, almost systematically, an approach combining and comparing experiments on model systems with numerical simulations involving coupled Lattice Boltzmann (LBM) and Discrete Elements (DEM) methods.
 

• En savoir plus sur Philippe Pierre

Séminaire Le 15 juin 2022

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

Séminaire Le 12 mai 2022
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

Séminaire Le 15 décembre 2021 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

Séminaire Le 25 novembre 2021 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

Séminaire Le 28 octobre 2021 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

Séminaire Le 28 avril 2021 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

Séminaire Le 29 avril 2021 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