Etude du comportement hydromécanique de béton de granulats de bois par corrélation d'images numériques

Site web : http://imagerie-gc.univ-bpclermont.fr/Dans le contexte de la construction durable, les bétons de granulats de bois constituent une solution à considérer pour réduire l'impact environnemental des bâtiments. Cependant, ces bétons nécessitent encore d'être caractérisés pour optimiser leurs performances et pour proposer des recommandations sur leur utilisation. L'objectif de ce travail est d'utiliser la technique de corrélation d'images numériques pour étudier le comportement mécanique des bétons de granulats de bois. Nous avons étudié différentes formulations constituées de quantités identiques de granulats de bois et de dosages en ciment différents

Hydromechanical couplings in the clay matrix of argilite: some methodological aspects of the atomistic-to-continuum upscaling

International audienceClays are ubiquitous in civil engineering applications, from deep geological disposal of radioactive waste to petroleum engineering, all of which require a good knowledge of their mechanical properties. Swelling clays are highly sensitive to the relative humidity: increases of water content can induce significant volume increases, as well as variations of the elastic stiffness. A faithful description of the behavior of clayey materials must therefore account for the hydromechanical couplings within clays.Hydromechanical couplings within the clay matrix of argilite were investigated numerically following a multi-scale approach. At the smallest scale, the clay layers of Na-montmorillonite are characterized through Monte Carlo molecular simulations. The constitutive law of the particle (a stack of clay layers) is then derived through rheological models. Finally, the clay matrix is represented as a polycrystal made of clay particles and homogenized by means of numerical non-linear continuum mechanics methods.Throughout our investigations, it was realized that the atomistic-to-continuum scale transition raised non-trivial theoretical questions. In this poster, we propose to show how the output of molecular simulations can be rigorously fed into a continuum mechanics simulation. Although the issues raised in this poster are illustrated on the upscaling of Na-montmorillonite, they are of a methodological nature and have a much wider range of applications

Adsorption and strain: The CO2-induced swelling of coal

International audienceEnhanced coal bed methane recovery (ECBM) consists in injecting carbon dioxide in coal bed methane reservoirs in order to facilitate the recovery of the methane. The injected carbon dioxide gets adsorbed at the surface of the coal pores, which causes the coal to swell. This swelling in confined conditions leads to a closure of the coal reservoir cleat system, which hinders further injection. In this work we provide a comprehensive framework to calculate the macroscopic strains induced by adsorption in a porous medium from the molecular level. Using a thermodynamic approach we extend the realm of poromechanics to surface energy and surface stress. We then focus on how the surface stress is modified by adsorption and on how to estimate adsorption behavior with molecular simulations. The developed framework is here applied to the specific case of the swelling of CO2-injected coal, although it is relevant to any problem in which adsorption in a porous medium causes strains

Bridging Microscopic Dynamics and Hydraulic Permeability in Mechanically-Deformed Nanoporous Materials

In the field of nanoconfined fluids, there are striking examples of deformation/transport coupling in which mechanical solicitation of the confining host and dynamics of the confined fluid impact each other. While this intriguing behavior can be potentially used for practical applications (e.g. energy storage, phase separation, catalysis), the underlying mechanisms remain to be understood as they challenge existing frameworks. Here, using molecular simulations analyzed through concepts inherent to interfacial fluids, we investigate fluid flow in compliant nanoporous materials subjected to external mechanical stresses. We show that the pore mechanical properties significantly affect fluid flow as they lead to significant pore deformations and different density layering at the interface accounted for by invoking interfacial viscous effects. Despite such poromechanical effects, we show that the thermodynamic properties (i.e. adsorption) can be linked consistently to Darcy's law for the permeability by invoking a pore size definition based on the concept of Gibbs' dividing surface. In particular, regardless of the pore stiffness and applied external stress, all data can be rationalized by accounting for the fluid viscosity and slippage at the interface independent of a specific pore size definition. Using such a formalism, we establish that the intimate relation - derived using the linear response theory - between collective diffusivity and hydraulic permeability remains valid. This allows for linking consistently microscopic dynamics experiments and permeability experiments on fluid flow in compliant nanoporous materials.Comment: 50 pages total, 6 figures in the main text + 10 figures in the supporting informatio

Poromechanics of Microporous Carbons: Application to Coal Swelling during Carbon Storage

International audienceCoal seams are naturally filled with natural gas. Enhanced Coal Bed Methane recovery (ECBM) is a technique which consists in injecting carbon dioxide (CO2) in coal seams in order to enhance the recovery of the methane (CH4) present in the coal seams. A major issue for the industrial development of this technique is the loss of permeability of the reservoirs during injection. In a coal bed, most of the transport of fluids occurs in a network of natural frac- tures. The loss of permeability is attributed to the closure of the fractures induced by the swelling of the coal ma- trix during the progressive replacement of CH4 by CO2. Since both fluids are mostly adsorbed in the microporous matrix of coal, this particular problem raises the funda- mental question of how adsorption impacts the mechanics of a microporous solid. In this work, we present a porome- chanical modeling valid for microporous solids under ad- sorption and we apply this modeling to the specific case of ECBM. The first section presents the theoretical derivation of general constitutive equations of poromechanics which are valid for generic pore sizes and morphologies. In the second section, we apply this general poromechanics to the specific case of CH4 adsorption in coal. We use molecu- lar simulations to calibrate the derived constitutive laws. In the third section we validate this calibration by analyz- ing results of adsorption experiments in unjacketed condi- tions. The fourth section is dedicated to the case of CO2 adsorption in coal. Finally in the last section, we use this modeling to predict the swelling of coal in the context of ECBM

Influence of diffusion and sorption on sound propagation in multiscale porous materials

This paper investigates sound propagation in multiscale sorptive porous materials saturated with a pure gas. An example of this type of materials is a packing of porous grains where a mesoscopic, microscopic, and nanoscopic scale can be identified. These scales are associated with the grain size, and the pores inside the grains of micrometre and nanometre sizes, respectively. Considering that the inner-grain and inter-granular viscous permeabilities are highly contrasted, the macroscopic description of sound propagation through the material is established by upscaling a local mesoscopic description using the two-scale asymptotic method of homogenisation for periodic media. The local mesoscopic description is given by i) an homogenised description of the inner-grain physics that accounts for fluid flow and heat conduction in the micropores, inner-grain interscale mass difussion, and sorption occurring on the walls of the nanopores, ii) the equations describing fluid flow and heat conduction in the mesoscopic fluid network, and iii) the conditions of continuity of mass flux and pressure, and negligible temperature variations on the pore boundaries. It is concluded that the effective compressibility becomes significantly affected by sorption and interscale pressure and mass diffusion processes. This leads to a decrease in sound speed and an increase in sound attenuation, particularly around the characteristic frequencies associated to the diffusion processes

Theoretical and practical differences between creep and relaxation Poisson's ratios in linear viscoelasticity

International audiencePoisson's ratio is a well-defined parameter in elasticity. For time-dependent materials , multiple definitions based on the ratios between lateral and axial deformations are available. Here, we focus ourselves on the two most widely used definitions in the time domain, which define time-dependent functions that we call relaxation Poisson's ratio and creep Poisson's ratio. Those two ratios are theoretically different, but are linked in an exact manner through an equation we derive. We show that those two functions are equal at both initial and large times and that their derivatives with respect to time also are. Based on simple rheological models for both the deviatoric and volumetric creep behaviors, we perform a parametric study and show that the difference between those two time-dependent Poisson's ratios can be significant. However, based on creep data available in the literature, we show that, for cementitious materials, this difference can be negligible or not, depending on the case

A Coupled Nanoindentation/SEM-EDS Study on Low Water/Cement Ratio Portland Cement Paste: Evidence for C–S–H/Ca(OH)2 Nanocomposites

A low water/cement ratio (w/c=0.20) hydrated Portland cement paste was analyzed by grid-indentation coupled with ex situ scanning electron microscope-energy-dispersive X-ray spectra (SEM-EDS) analysis at each indentation point. Because finite element and Monte-Carlo simulations showed that the microvolumes probed by each method are of comparable size (approximately 2 μm), the mechanical information provided by nanoindentation was directly comparable to the chemical information provided by SEM-EDS. This coupled approach provided the opportunity to determine whether the local indentation response was a result of a single- or a multiphase response—the latter being shown predominant in the highly concentrated w/c=0.20 hydrated cement paste. Results indicate that, in the selected microvolumes where C–S–H and nanoscale Ca(OH)2 (CH) are present, increasing fractions of CH increase the local indentation modulus (and hardness), yielding values above those reported for high-density (HD) C–S–H. Micromechanical analyses show that C–S–H and CH are associated, not merely as a simple biphase mixture, but as an intimate nanocomposite where nanoscale CH reinforces C–S–H by partially filling the latter's gel pores. The paper discusses the mechanism of forming the C–S–H/CH nanocomposite, as well as the impact of nanocomposites on various macroscopic properties of concrete (e.g., shrinkage, expansion). On a general level, this study illustrates how a coupled nanoindentation/X-ray microanalysis/micromechanics approach can provide otherwise inaccessible information on the nanomechanical properties of highly heterogeneous composites with intermixing at length scales smaller than the stress field in a nanoindentation experiment

Internal states, stress-strain behavior and elasticity in oedometrically compressed model granular materials

International audienceThe behaviour of a model granular material (an assembly of slightly poly-disperse spherical beads, with Hertz-Mindlin elastic and frictional contacts) subjected to one dimensional (oedometric) compressions is studied by DEM simulations. We systematically investigate the influence of the (idealized) packing process on the microstructure and stresses in the initial, weakly confined equilibrium state. Such characteristics as density (ranging from maximally dense to moderately loose), coordination number (which might vary independently of solid fraction, especially in dense systems), fabric and stress anisotropies are monitored in oedometric loading cycles in which the major principal stress varies by up to 5 orders of magnitude. The evolution of the solid fraction (or the void ratio) versus the imposed vertical (principal) stress as observed in the loading and unloading paths, like in the case of isotropic compression [2] and unlike laboratory tests on sands, the behaviour shows only very limited plastic strain and is very nearly reversible in dense samples (which tend nevertheless to lose contacts in a loading cycle if the initial coordination number was large). The irreversibility observed in sands should thus be attributed to plasticity or damage within inter granular contacts. The anisotropy of the microstructure is described by the angular distributions of contacts and forces. It is explicitly linked to the stresses in the loading history, by semi-quantitative relations. One of the important characteristics measured during the compression is the ratio of lateral to controlled ('vertical') stress, K0. We discuss conditions in which K0 might be regarded as constant. We calculate, via a static (matrix) method [1], the complete tensor of elastic moduli, expressing response to very small stress increments about the transversely isotropic equilibrium states along the loading path

A new method for measuring grain displacements in granular materials by X-ray computed tomography

International audienceWe aim to measure the individual grain displacements in a granular material under constant load (creep). X-ray computed tomography imaging provides images of the granular medium microstructure during the experiment, and discrete volumetric image correlation (DV-DIC) [1] allows the determination of the grain individual rigid body motion from the reconstructed tomography images. However, for short-term creep, and time-resolved experiments in general, the sample evolutions can be very quick and occur before the full tomography scan is complete. This constitutes a serious limitation of standard experimental procedures for the investigation of the micromechanics of the creep of granular media at the grain scale. We present a new method for measuring grain displacements, that overcomes the above-mentioned limitation. Indeed, in a granular material, assuming no breakage occurs, each grain undergoes a rigid body motion. Therefore, the displacement field reduces to a set of six degrees of freedom per grain. This suggests that the information contained in a full set of projections (necessary to perform an accurate 3D reconstruction) is excessively redundant for the determination of the grain displacements. Our method requires only few projections of the sample at its current state, thus reducing dramatically the acquisition time. Displacements are estimated from the projections directly, without 3D reconstruction. Our method is formulated as an inverse problem. A forward model based on Beer-Lambert's law is developed to efficiently perform numerical projections. The grain displacements are estimated by fitting the numerical projections to experimental projections of the current state of the sample. We also study the sensitivity of the estimated displacements to image noise, both numerically and through a theoretical model which highlights the influence of the setup parameters on the measurements. The method has been validated and its accuracy assessed against 2D and 3D numerical experiments on virtual microstructures