This paper presents a numerical technique to model soft particle materials in which the particles can undergo large deformations. It combines an implicit finite strain formalism of the Material Point Method and the Contact Dynamics method. In this framework, the large deformations of individual particles as well as their collective interactions are treated consistently. In order to reduce the computational cost, this method is parallelised using the Message Passing Interface (MPI) strategy. Using this approach, we investigate the uniaxial compaction of 2D packings composed of particles governed by a Neo-Hookean material behaviour. We consider compressibility rates ranging from fully compressible to incompressible particles. The packing deformation mechanism is a combination of both particle rearrangements and large deformations, and leads to high packing fractions beyond the jamming state. We show that the packing strength declines when the particle compressibility decreases, and the packing can deform considerably. We also discuss the evolution of the connectivity of the particles and particle deformation distributions in the packing. (C) 2018 Elsevier B.V. All rights reserved.

VL - 237 UR - https://linkinghub.elsevier.com/retrieve/pii/S0010465518303904 JO - Computer Physics Communications ER - TY - JOUR T1 - Phase separation of stable colloidal clusters JF - Physical Review Materials Y1 - 2018 A1 - Petersen, Thomas A1 - Bazant, Martin Z. A1 - Roland Jean-Marc Pellenq A1 - Franz-Josef Ulm AB -This article presents a nonequilibrium thermodynamic theory for the mean-field precipitation, aggregation, and pattern formation of colloidal clusters. A variable gradient energy coefficient and the arrest of particle diffusion upon “jamming” of cluster aggregates in the spinodal region predicts observable gel patterns that, at high intercluster attraction, form system-spanning, out-of-equilibrium networks with glasslike, quasistatic structural relaxation. For reactive systems, we incorporate the free energy landscape of stable prenucleation clusters into the Allen-Cahn reaction equation. We show that pattern formation is dominantly controlled by the Damköhler number and the stability of the clusters, which modifies the autocatalytic rate of precipitation. As clusters individually become more stable, bulk phase separation is suppressed.

VL - 2 UR - https://journals.aps.org/prmaterials/abstract/10.1103/PhysRevMaterials.2.095602 IS - 9 JO - Phys. Rev. Materials ER - TY - JOUR T1 - Poroelasticity of Methane-Loaded Mature and Immature Kerogen from Molecular Simulations JF - Langmuir Y1 - 2018 A1 - Obliger, Amaël A1 - Valdenaire, Pierre-Louis A1 - Capit, Nicolas A1 - Franz-Josef Ulm A1 - Roland Jean-Marc Pellenq A1 - Jean-Marc Leyssale KW - ADSORPTION-INDUCED DEFORMATION; ORGANIC-MATTER; COMPETITIVE ADSORPTION; NANOPOROUS MEDIA; POROUS CARBONS; GLASSY-CARBON; COAL; TRANSPORT; SORPTION; MODEL AB -While hydrocarbon expulsion from kerogen is certainly the key step in shale oil/gas recovery, the poromechanical couplings governing this desorption process, taking place under a significant pressure gradient, are still poorly understood. Especially, most molecular simulation investigations of hydrocarbon adsorption and transport in kerogen have so far been performed under the rigid matrix approximation, implying that the pore space is independent of pressure, temperature, and fluid loading, or in other words, neglecting poromechanics. Here, using two hydrogenated porous carbon models as proxies for immature and overmature kerogen, that is, highly aliphatic hydrogen-rich vs highly aromatic hydrogen-poor models, we perform an extensive molecular-dynamics-based investigation of the evolution of the poroelastic properties of those matrices with respect to temperature, external pressure, and methane loading as a prototype alkane molecule. The rigid matrix approximation is shown to hold reasonably well for overmature kerogen even though accounting for flexibility has allowed us to observe the well-known small volume contraction at low fluid loading and temperature. Our results demonstrate that immature kerogen is highly deformable. Within the ranges of conditions considered in this work, its density can double and its accessible porosity (to a methane molecule) can increase from 0 to ∼30%. We also show that these deformations are significantly nonaffine (i.e., nonhomogeneous), especially upon fluid adsorption or desorption.

VL - 34 UR - http://pubs.acs.org/doi/10.1021/acs.langmuir.8b02534 IS - 45 JO - Langmuir ER - TY - JOUR T1 - Potential-of-Mean-Force Approach for Molecular Dynamics–Based Resilience Assessment of Structures JF - Journal of Engineering Mechanics Y1 - 2018 A1 - Keremides, Konstantinos A1 - Abdolhosseini Qomi, Mohammad Javad A1 - Roland Jean-Marc Pellenq A1 - Franz-Josef Ulm KW - Molecular dynamics; Structural mechanics; Potential of mean force; Morse potential; Progressive structural collapse; Fragility curves AB -A molecular dynamics (MD)-based structural mechanics approach is proposed for the assessment of resilience of buildings. At the core of the approach, potentials of mean force (PMFs) suitable for structural members for both two-body (stretch) and three-body (bending) interactions are derived to define the energy states between mass points discretizing structural members. An original potential parameter calibration procedure is proposed: for close-to-equilibrium potential parameters, the procedure is based on matching measured frequency of a structure with the frequency of the molecular model. In turn, for bond-rupture parameters, it is shown that classical interatomic potential expressions, such as Morse potential, can be used to calibrate the energy content of many structural members and connections. By way of example, the MD-based structural mechanics approach is applied to a large-scale structure. Compared with classical continuum-based approaches, the added value of the method thus proposed is a rational means of determining the progressive structural collapse load of structures based on thermodynamic integration. By redefining structural mechanics within the context of statistical physics, molecular simulations, and potentials of mean force, the approach provides a powerful means of determining fragility curves required for the assessment of resilience of buildings.

VL - 144 UR - http://ascelibrary.org/doi/10.1061/%28ASCE%29EM.1943-7889.0001491 IS - 8 JO - J. Eng. Mech. ER - TY - JOUR T1 - Peridynamics simulation of the comminution of particles containing microcraks JF - EPJ Web of Conferences Y1 - 2017 A1 - Blanc, Nicolas A1 - Frank, Xavier A1 - Mayer-Laigle, Claire A1 - Farhang Radjaï A1 - Jean-Yves Delenne ED - Saeid Nezamabadi ED - Luding, S. ED - Jean-Yves Delenne AB -In this study, we rely on a ’bond-based’ peridynamic approach to investigate the strength and failure of 2D particles containing a collection of 1D microcracks. The mechanical tests were performed on disks under diametral compression. In an extensive parametric study, the distribution of microcracks was varied for different particle sizes. The evolution of yield stress with diameter and the probability of failure in terms of Weibull distributions are investigated in detail. Finally, by means of a floodfill algorithm, we analyze the variation of the mean fragment size as a function of the density of defects.

VL - 140 UR - http://www.epj-conferences.org/10.1051/epjconf/201714007018 JO - EPJ Web Conf. ER - TY - JOUR T1 - The Potential of Mean Force concept for bridging (length and time) scales in the modeling of complex porous materials JF - EPJ Web of Conferences Y1 - 2017 A1 - Katerina Ioannidou A1 - Benoit Carrier A1 - Matthieu Vandamme A1 - Roland Jean-Marc Pellenq ED - Farhang Radjaï ED - Saeid Nezamabadi ED - Luding, S. ED - Jean-Yves Delenne AB -We introduce the concept of Potential of Mean Force, PMF, as a way to implement upscaling modeling from the nano-scale to micron-scale. A PMF is a free energy function representing in an effective way the interactions between objects (cement hydrates, clay platelets, etc.) at thermodynamics conditions. The PMF is therefore the key piece of information allowing to coarse-grained Physical-Chemistry information in a meso-scale model formulation. The use of PMF offers a huge computational advantage as it allows a straight up-scaling to the meso-scale while keeping essential interactions information that are the hallmark of Physical-Chemistry processes. Such a coarse-grained modeling integrates atomistic response into inter-particle potentials that fully propagate molecular scale information all the way to the meso-scale.

VL - 140 UR - http://www.epj-conferences.org/10.1051/epjconf/201714001009 JO - EPJ Web Conf. ER - TY - JOUR T1 - A potential-of-mean-force approach for fracture mechanics of heterogeneous materials using the lattice element method JF - Journal of the Mechanics and Physics of Solids Y1 - 2017 A1 - Hadrien Laubie A1 - Farhang Radjaï A1 - Roland Jean-Marc Pellenq A1 - Franz-Josef Ulm AB -Fracture of heterogeneous materials has emerged as a critical issue in many engineering applications, ranging from subsurface energy to biomedical applications, and requires a rational framework that allows linking local fracture processes with global fracture descriptors such as the energy release rate, fracture energy and fracture toughness. This is achieved here by means of a local and a global potential-of-mean-force (PMF) inspired Lattice Element Method (LEM) approach. In the local approach, fracture-strength criteria derived from the effective interaction potentials between mass points are shown to exhibit a scaling commensurable with the energy dissipation of fracture processes. In the global PMF-approach, fracture is considered as a sequence of equilibrium states associated with minimum potential energy states analogous to Griffith’s approach. It is found that this global approach has much in common with a Grand Canonical Monte Carlo (GCMC) approach, in which mass points are randomly removed following a maximum dissipation criterion until the energy release rate reaches the fracture energy. The duality of the two approaches is illustrated through the application of the PMF-inspired LEM for fracture propagation in a homogeneous linear elastic solid using different means of evaluating the energy release rate. Finally, by application of the method to a textbook example of fracture propagation in a heterogeneous material, it is shown that the proposed PMF-inspired LEM approach captures some well-known toughening mechanisms related to fracture energy contrast, elasticity contrast and crack deflection in the considered two-phase layered composite material.

VL - 105 JO - Journal of the Mechanics and Physics of Solids ER - TY - JOUR T1 - Production of H 2 by water radiolysis in cement paste under electron irradiation: A joint experimental and theoretical study JF - Cement and Concrete Research Y1 - 2017 A1 - Le Caer, Sophie A1 - Dezerald, Lucile A1 - Boukari, Khaoula A1 - Laine, Maxime A1 - Taupin, sébastien A1 - Kavanagh, Ryan M. A1 - Johnston, Conrad S.N. A1 - Foy, Eddy A1 - Charpentier, Thibault A1 - Konrad J. Krakowiak A1 - Roland Jean-Marc Pellenq A1 - Franz-Josef Ulm A1 - Tribello, Gareth A. A1 - Kohanoff, Jorge J. A1 - Andres Saùl AB -Long-term confinement of nuclear waste is one of the main challenges faced by the nuclear industry. Fission products such as Sr-90 and Cs-137, both beta(-) emitters known to induce serious health hazards, represent the largest fraction of nuclear waste. Cement is a good candidate to store them, provided it can resist the effects of irradiation over time. Here, we have investigated the effects of beta(-) decay on cement by performing electron irradiation experiments on different samples. We show that H-2 production in cement, the main effect of water radiolysis, depends strongly on composition and relative humidity. First-principles calculations indicate that the water-rich interlayer regions with Ca2+ ions act as electron traps that promote the formation of H-2. They also show that holes localize in water-rich regions in low Ca content samples and are then able to participate in H-2 production. This work provides new understanding of radiolysis effects in cements.

VL - 100 UR - https://linkinghub.elsevier.com/retrieve/pii/S0008884617302065 JO - Cement and Concrete Research ER - TY - JOUR T1 - Probing Interconnectivity in Hierarchical Microporous/Mesoporous Materials Using Adsorption and Nuclear Magnetic Resonance Diffusion JF - The Journal of Physical Chemistry C Y1 - 2016 A1 - Anne Galarneau A1 - Guenneau, Flavien A1 - Gedeon, Antoine A1 - Mereib, Diaa A1 - Rodriguez, Jeremy A1 - Fajula, François A1 - Benoit A. Coasne AB -Adsorption and transport in hierarchical materials are investigated by means of adsorption and nuclear magnetic resonance experiments. Using micro/mesoporous zeolites with well-defined mesoporosity, we show that adsorption at a given pressure can be described as a simple linear combination of the adsorbed amounts taken at the same pressure for the pure microporous (zeolite FAU-Y) and mesoporous (Al-MCM-41) solids. Such a quantitative decomposition allows us to demonstrate the ability of diffusion measurements by Pulsed Field Gradient Nuclear Magnetic Resonance (PFG NMR) to probe interconnectivity in hierarchical solids. On the one hand, transport in the mechanical mixtures can be described as the superimposition of diffusion in pure microporous and mesoporous solids. On the other hand, PFG NMR for the hierarchical sample provides an effective diffusivity that is intermediate between those for the pure zeolite and mesoporous silica. Furthermore, this effective diffusivity is slower than the linear combination of the two diffusivities weighted by the number of molecules present in each phase (used in the independent domain and fast-exchange theories) clearly showing interconnectivities and transfer limitations between the microporous and mesoporous domains. We also discuss the ability of combining theories such as the fast exchange model and the effective medium theory to quantitatively predict diffusion in such microporous/mesoporous materials.

VL - 120 UR - http://pubs.acs.org/doi/10.1021/acs.jpcc.5b10129 IS - 3 JO - J. Phys. Chem. C ER - TY - JOUR T1 - Physical Origins of Thermal Properties of Cement Paste JF - Physical Review Applied Y1 - 2015 A1 - Mohammad Javad Abdolhosseini Qomi A1 - Franz-Josef Ulm A1 - Roland Jean-Marc Pellenq AB -Despite the ever-increasing interest in multiscale porous materials, the chemophysical origin of their thermal properties at the nanoscale and its connection to the macroscale properties still remain rather obscure. In this paper, we link the atomic- and macroscopic-level thermal properties by combining tools of statistical physics and mean-field homogenization theory. We begin with analyzing the vibrational density of states of several calcium-silicate materials in the cement paste. Unlike crystalline phases, we indicate that calcium silicate hydrates (CSH) exhibit extra vibrational states at low frequencies (

The alignment of water molecules along chiral pores may activate proton/ion conduction along dipolar hydrophilic pathways. Here we show that a simple synthetic “T-channel” forms a directional pore with its carbonyl moieties solvated by chiral helical water wires. Atom-scale simulations and experimental crystallographic assays reveal a dynamical structure of water and electrolyte solutions (alkali metal chlorides) confined in these organic T-channels. Oscillations in the dipole orientation, which correspond to alternative ordering (dipole up–dipole down) of the water molecules with a period of about 4.2 Å (imposed by the distance between two successive carbonyl groups) are observed. When ions are added to the system, despite the strong Coulombic water/ion interaction, confined water remains significantly ordered in the T-channel and still exhibits surface-induced polarization. Cation permeation can be achieved through alternated hydration–dehydration occurring along strongly oriented water wires. The T-channel, which exhibits chirality with strong water orientation, provides an opportunity to unravel novel water-channel systems that share many interesting properties of biomolecular systems.

We show that Derjaguin’s theory of adsorption can be used to predict adsorption on bare and modified surfaces using parameters available to simple experiments. Using experiment and molecular simulation of adsorption of various gases on hydroxylated, methylated, and trifluoromethylated silica, this simple parametrization of Derjaguin’s model allows predicting adsorption on any functionalized surface using a minimum set of parameters such as the heat of vaporization of the adsorbate and the Henry constant of the adsorption isotherm. This general yet simple scheme constitutes a powerful tool as it avoids having to carry out tedious and complex adsorption measurements.

The elastic properties of graphene as described by the reactive empirical bond order potential are studied through uniaxial tensile tests calculations at both zero temperature, with a conjugate gradient approach, and room temperature, with molecular dynamics simulations. A perfect linear elastic behavior is observed at 0 K up to ≈0.1% strain. The Young’s modulus and Poisson’s ratio obtained with this potential are of ≈730 GPa and 0.39, respectively, with little chirality effects. These values differ significantly from former estimations, much closer to experimental values. We show that these former values have certainly been obtained by neglecting the effect of atomic relaxation, leading to a severe inaccuracy. At larger strains, an extended apparent linear domain is observed in the stress–strain curves, which is relevant to Young’s modulus calculations at finite temperature. Our molecular dynamics simulations at 300 K have allowed obtaining the following, chirality dependent, apparent Young’s moduli, 860 and 761 GPa, and Poisson’s ratios, 0.12 and 0.23, for armchair and zigzag loadings, respectively.

VL - 89 JO - Carbon ER - TY - JOUR T1 - Probing the microporosity of low-k organosilica films: MP and t-plot methods applied to ellipsometric porosimetry data JF - Microporous and Mesoporous Materials Y1 - 2015 A1 - Lépinay, Matthieu A1 - Broussous, Lucile A1 - Licitra, Christophe A1 - Bertin, François A1 - Rouessac, Vincent A1 - Ayral, Andre A1 - Benoit A. Coasne AB -Ellipsometric porosimetry (EP) experiments are performed to obtain the adsorption isotherms of water, methanol, and toluene on pristine and damaged SiOCH porous materials. The use of gaseous adsorbates with different polarities, sizes, and surface tensions enables us to probe their affinity with such organosilica surfaces. Using reference t-curves obtained from Statistical Mechanics molecular simulations, we discuss the ability of the MicroPore analysis (MP) method to accurately estimate micropore sizes from EP measurements by comparing them with the mean pore sizes obtained using positron annihilation lifetime spectroscopy (PALS) and grazing incidence small angle X-ray scattering (GISAXS). We also report accessible microporous volumes estimated from the *t*-plot method used with the reference t-curves obtained by means of molecular simulation. We show that EP characterization of microporous films combined with the MP and *t*-plot methods can be improved by taking into account the effect of the chemical nature of the pore surface on the variation of the adsorbed thickness (*t*-curve).

By means of molecular dynamics simulations, we investigate the texture and local ordering in sheared packings composed of cohesionless platy particles. The morphology of large packings of platy particles in quasistatic equilibrium is complex due to the combined effects of local nematic ordering of the particles and anisotropic orientations of contacts between particles. We find that particle alignment is strongly enhanced by the degree of platyness and leads to the formation of face-connected clusters of exponentially decaying size. Interestingly, due to dynamics in continuous shearing, this ordering phenomenon emerges even in systems composed of particles of very low platyness differing only slightly from spherical shape. The number of clusters is an increasing function of platyness. However, at high platyness the proportion of face-face interactions is too low to allow for their percolation throughout the system.

VL - 37 IS - 11 JO - Eur. Phys. J. E ER - TY - Generic T1 - Pavement infrastructures footprint: The impact of pavement properties on vehicle fuel consumption T2 - Euro-C Conference Y1 - 2014 A1 - Arghavan Louhghalam A1 - Mehdi Akbarian A1 - Franz-Josef Ulm ED - Bicanic, N ED - Mang, H ED - Meschke, G AB -A novel mechanistic model based on an infinite beam on elastic foundation is developed to

quantify the impact of pavement structural and material properties on pavement deflection and consequently on

vehicle fuel consumption. The model can also account for the effect of temperature and vehicle speed on fuel

consumption. A simplified expression for evaluating the energy dissipation for practical purposes is proposed

and used to investigate the impact of various pavement design systems on fuel consumption. GPS (General Pave-

ment Studies) sections from the FHWA’s Long Term Pavement Performance program (FHWA 2011) are used

for this study. These sections consist of asphalt concrete (AC), portland cement concrete (PCC) and composite

pavements. The model quantifies the impact of temperature and vehicle speed on the fuel consumption and

confirms that those impacts are negligible for PCC and significant for AC pavements due to their viscoelasticity.

Field tests are widely used for soil characterization in geotechnical applications in spite of implementation difficulties. The light penetrometer is a well-known testing tool for fine soils, but the physical interpretation of the output data in the case of coarse granular materials is far less evident. Indeed, the data are considerably more sensitive in this case to various parameters such as fabric structure, particle shapes or the applied impact energy. In order to achieve a better understanding of the penetration process into a coarse granular material, a numerical study was performed by means of contact dynamics simulations. The penetration of a moving tip in a sample composed of irregular grain shapes was studied and the influence of the driving velocity and input energy on the penetration strength was analyzed. The results show that the latter grows with both the penetration rate and energy, despite the strong fluctuations occur due to a jamming–unjamming process in which the contact network connectivity evolves intermittently in correlation with the penetration strength. This analysis suggests that the time-averaged data provided by a penetrometer is reliable information from which the bulk strength properties of coarse granular materials can be evaluated.

VL - 55 JO - Computers and Geotechnics ER - TY - JOUR T1 - Physics and technological aspects of nanofluidics JF - Lab on a Chip Y1 - 2014 A1 - Lydéric Bocquet A1 - Tabeling, Patrick AB -From a physical perspective, nanofluidics represents an extremely rich domain. It hosts many mechanisms acting on the nanoscale, which combine together or interact with the confinement to generate new phenomena. Superfast flows in carbon nanotubes, nonlinear electrokinetic transport, slippage over smooth surfaces, nanobubble stability, *etc.* are the most striking phenomena that have been unveiled over the past few years, and some of them are still awaiting an explanation. One may anticipate that new nanofluidic effects will be discovered in the future, but at the moment, the technological barrier is high. Fabrication of nanochannels is most often a tour de force, slow and costly. However, with the accumulation of technological skills along with the use of new nanofluidic materials (like nanotubes), nanofluidics is becoming increasingly accessible to experimentalists. Among the technological challenges faced by the field, fabricating devices mimicking natural nanometric systems, such as aquaporins, ionic pumps or kidney osmotic filtering, seems the most demanding in terms of groundbreaking ideas. Nanoflow characterization remains delicate, although considerable progress has been achieved over the past years. The targeted application of nanofluidics is not only in the field of genomics and membrane science – with disruptive developments to be expected for water purification, desalination, and energy harvesting – but also for oil and gas production from unconventional reservoirs. Today, in view of the markets that are targeted, nanofluidics may well impact the industry more than microfluidics; this would represent an unexpected paradox. These successes rely on using a variety of materials and technologies, using state-of-the-art nanofabrication, or low-tech inexpensive approaches. As a whole, nanofluidics is a fascinating field that is facing considerable challenges today. It possesses a formidable potential and offers much space for creative groundbreaking ideas.

This paper aims at understanding and predicting how pressurized cracks propagate in anisotropic brittle solids, a situation frequently encountered in hydraulic fracturing. Special attention is paid to transverse isotropy, often used to model shale. Although the theory of linear elastic fracture mechanics of anisotropic solids is well established at present, this paper shows that the application of Muskhelishvili’s formalism to Lekhnitskii’s anisotropic complex potentials provides a powerful tool to extend the validity of the classical tools of isotropic fluid-driven crack models to the anisotropic case, provided that the appropriate elastic constants are used. These elastic constants are identified and derived in closed form for transversely isotropic solids. The constants are shown to be directly related to quantities easily measured in a laboratory at macroscopic scale through indentation tests and acoustic measurements. Moreover, several crack-kinking criteria are compared. Contrary to the isotropic case, the crack-kinking criteria are not consistent among themselves, even in the case of a pure pressure loading. The orientation at which it is easier to propagate an already existing crack is sought. A critical crack length, below which this crack orientation is the one of minimal stiffness felt by the crack, is identified.

VL - 140 IS - 12 JO - J. Eng. Mech. ER - TY - Generic T1 - Poro-chemo-fracture-mechanics ... bottom-up: Application to risk of fracture design of oil and gas cement sheath at early ages T2 - Euro-C Conference Y1 - 2014 A1 - Franz-Josef Ulm A1 - Muhannad Abuhaikal A1 - Thomas Alexander Petersen A1 - Roland Jean-Marc Pellenq ED - Bicanic, N ED - Mang, H ED - Meschke, G ED - DeBorst, R AB -With ever more challenging (T, p) environments for cementing applications in oil and gas wells, there is a need to change the classical paradigm of constitutive modeling of the early-age behavior of cementitious materials in oil-and gas well applications. Herein, we propose a bottom-up approach, which starts at the molecular scale of calcium-silicate-hydrates (C-S-H), and propagates this understanding via incremental poroelastic upscaling methods to the meso-and macroscale, and ultimately to the scale of engineering applications. With a clear focus on engineering risk-of-fracture evaluations, we show how to integrate this bottom-up approach with fracture mechanics.

JF - Euro-C Conference PB - CRC PRESS-TAYLOR & FRANCIS GROUP CY - MAR 24-27, 2014, St Anton am Alberg, AUSTRIA VL - COMPUTATIONAL MODELLING OF CONCRETE STRUCTURES, VOL 1 SN - 978-1-138-02641-4; 978-1-315-76203-6 ER - TY - JOUR T1 - Poroelastic Theory Applied to the Adsorption-Induced Deformation of Vitreous Silica JF - The Journal of Physical Chemistry B Y1 - 2014 A1 - Benoit A. Coasne A1 - Weigel, Coralie A1 - Polian, Alain A1 - Kint, Mathieu A1 - Rouquette, Jérôme A1 - Haines, Julien A1 - Foret, Marie A1 - Vacher, Rene A1 - Ruffle, Benoit AB -When vitreous silica is submitted to high pressures under a helium atmosphere, the change in volume observed is much smaller than expected from its elastic properties. It results from helium penetration into the interstitial free volume of the glass network. We present here the results of concurrent spectroscopic experiments using either helium or neon and molecular simulations relating the amount of gas adsorbed to the strain of the network. We show that a generalized poromechanical approach, describing the elastic properties of microporous materials upon adsorption, can be applied successfully to silica glass in which the free volume exists only at the subnanometer scale. In that picture, the adsorption-induced deformation accounts for the small apparent compressibility of silica observed in experiments.

VL - 118 UR - http://pubs.acs.org/doi/10.1021/jp5094383 IS - 49 JO - J. Phys. Chem. B ER - TY - JOUR T1 - Predicting the settlement of coarse granular materials under vertical loading JF - Scientific Reports Y1 - 2014 A1 - Quezada, Juan Carlos A1 - Saussine, Gilles A1 - Breul, Pierre A1 - Farhang Radjaï AB -Granular materials are widely used in industrial processes despite their complex and poorly understood mechanical behaviour both in static and dynamic regimes. A prototypical example is the settlement and compaction of a granular bed under vibrational loading. The elementary mechanisms of this process are still unclear and there is presently no established theory or methodology to predict the settlement and its statistical variability. By means of a parametric study, carried out on a full-scale track, and a critical analysis of density relaxation laws, we introduce a novel settlement model in coarse granular materials under cyclic loading. Our extensive experimental data indicate that the settlement process is governed by three independent parameters strongly correlated with the vibration intensity and initial packing fraction. We show that the mean settlement is well predicted by the model with its parameter values extracted from experimental data.

Granular materials are both pressure-dependent and density-dependent materials and exhibit a broad range of intricate behaviours due to their discrete nature, dissipative interactions and generic structural disorder^{1}. The packing fraction may vary as a result of particle rearrangements induced by shearing or vibrations and it leads to dramatic changes in the structure and mechanical response of a granular material^{2}^{,3}^{,4}^{,5}^{,6}^{,7}^{,8}. A long-time logarithmic relaxation law of the packing fraction is systematically observed in experiments^{9}^{,10}. In simple compaction models, this behaviour is attributed to the exponentially increasing time for the particles to reach a new configuration of lower packing fraction. The case of settlement under cyclic loading has, however, been much less investigated. The settlement of granular bed occurs due to both compaction and side-wise spreading. An important industrial example is the railway ballast, which undergoes gradual settlement under the static and dynamic overloads induced by train traffics^{11}^{,12}^{,13}^{,14}. The readjustment of differential settlements requires costly operations on fast-train railways. For this reason, an improved understanding of the parameters governing the settlement process is a critical technological challenge for new developments in this field.

In this paper, we show that the total settlement *τ _{N}* under vertical cyclic loading is governed by a logarithmic relaxation law as a function of the number N of cycles:

where the three fitting parameters *τ*_{∞}, *B* and *N*_{0} can be evaluated from the loading parameters, namely the frequency *ω* (related to the train speed for ballast) and initial packing fraction of the material. Our experimental correlations between model and loading parameters show consistently that *τ*_{∞} and *B* depend on the dimensionless loading intensity Γ = (*Aω*^{2})/(*pd*^{2}/*m* + *g*), where *A* is the vibration amplitude, *p* is the confining pressure (under the sleeper for ballast), *d* is the average particle diameter, *m* is the average particle mass and *g* is gravitational acceleration. We also find that the parameter *N*_{0} is linked to the initial packing fraction of the material. In fact, this parameter controls the initial settlement rate, and it was systematically determined by means of a light penetrometer in our experiments on ballast material.

In this article, we review how pressure effects in pores affect both the physics of the confined fluid and the properties of the host porous material. Molecular simulations in which high-pressure effects were observed are first discussed; we will see how the strong dependence on bulk phase pressure of the freezing temperature of a fluid confined in nanopores can be explained by important variations of the pressure within the pore. We then discuss recent works in which direct calculations of the pressure tensor of fluids confined in pores provide evidence for large pressure enhancements. Finally, practical applications of these pressure effects in which gas adsorption in microporous solids (pore size <2nm) was found to enhance their mechanical properties by increasing the elastic modulus by a factor 4 are discussed.

VL - 40 IS - 7-9 ER - TY - JOUR T1 - Packings of irregular polyhedral particles: Strength, structure, and effects of angularity JF - Physical Review E Y1 - 2013 A1 - Emilien Azéma A1 - Farhang Radjaï A1 - Dubois, Frédéric AB -We present a systematic numerical investigation of the shear strength and structure of granular packings composed of irregular polyhedral particles. The angularity of the particles is varied by increasing the number of faces from 8 (octahedronlike shape) to 596. We find that the shear strength increases with angularity up to a maximum value and saturates as the particles become more angular (below 46 faces). At the same time, the packing fraction increases to a peak value but declines for more angular particles. We analyze the connectivity and anisotropy of the microstructure by considering both the contacts and branch vectors joining particle centers. The increase of the shear strength with angularity is shown to be due to a net increase of the fabric and force anisotropies but at higher particle angularity a rapid falloff of the fabric anisotropy is compensated by an increase of force anisotropy, leading thus to the saturation of shear strength.

VL - 8713 IS - 6 JO - Phys. Rev. E ER - TY - Generic T1 - Penetration strength of coarse granular materials from DEM simulations T2 - 7th International Conference on Micromechanics of Granular Media (Powders and Grains) Y1 - 2013 A1 - Quezada, Juan Carlos A1 - Saussine, Gilles A1 - Breul, Pierre A1 - Farhang Radjaï ED - Yu, A ED - Dong, K ED - Yang, R AB -Field tests are widely used for soil characterization in geotechnical applications in spite of implementation difficulties. The light penetrometer test is a well-known testing tool for fine soils, but the physical interpretation of the output data in the case of coarse granular materials is far less evident. In fact, the data are considerably more sensitive to various parameters such as fabric structure, particles shape or the applied impact energy. In order to achieve a better understanding of the underlying phenomena, we performed a numerical study by means contact dynamics DEM simulations. We consider the penetration of a moving tip into a sample composed of irregular grain shapes and we analyze the influence of the driving velocity and applied energy on the penetration strength. We find that the latter grows with both the penetration rate and energy. Force fluctuations on the tip involve a jamming-unjamming process. The typology of contact network and inter-granular friction play a major role in the fluctuations and measured values of the cone penetration strength.

JF - 7th International Conference on Micromechanics of Granular Media (Powders and Grains) PB - AIP CY - JUL 08-12 2013 Sydney, AUSTRALIA VL - Book Series: AIP Conference Proceedings POWDERS AND GRAINS 2013 ER - TY - JOUR T1 - Particle shape dependence in 2D granular media JF - EPL (Europhysics Letters) Y1 - 2012 A1 - Baptiste Saint-Cyr A1 - Krzysztof Szarf A1 - Charles Voivret A1 - Emilien Azéma A1 - Vincent Richefeu A1 - Jean-Yves Delenne A1 - Gael Combe A1 - Cécile Nouguier-Lehon A1 - Pascal Villard A1 - Philippe Sornay A1 - Marie Chaze A1 - Farhang Radjaï AB -Particle shape is a key to the space-filling and strength properties of granular matter. We consider a shape parameter

Microporous media, i.e., porous media made of pores with a nanometer size, are important for a variety of applications, for instance for sequestration of carbon dioxide in coal, or for storage of hydrogen in metal-organic frameworks. In a pore of nanometer size, fluid molecules are not in their bulk state anymore since they interact with the atoms of the solid: they are said to be in an adsorbed state. For such microporous media, conventional poromechanics breaks down.

In this work we derive poroelastic constitutive equations which are valid for a generic porous medium, i.e., even for a porous medium with pores of nanometer size. The complete determination of the poromechanical behavior of a microporous medium requires knowing how the amount of fluid adsorbed depends on both the fluid bulk pressure and the strain of the medium. The derived constitutive equations are validated with the help of molecular simulations on one-dimensional microporous media. Even when a microporous medium behaves linearly in the absence of any fluid (i.e., its bulk modulus does not depend on strain), we show that fluid adsorption can induce non-linear behavior (i.e., its drained bulk modulus can then depend significantly on strain). We also show that adsorption can lead to an apparent Biot coefficient of the microporous medium greater than unity or smaller than zero.

The poromechanical response of a microporous medium to adsorption significantly depends on the pore size distribution. Indeed, the commensurability (i.e., the ratio of the size of the pores to that of the fluid molecules) proves to play a major role. For a one-dimensional model of micropores with a variety of pore sizes, molecular simulations show that the amount of adsorbed fluid depends linearly on the strain of the medium. We derive linearized constitutive equations which are valid when such a linear dependence of the adsorbed amount of fluid on the strain is observed.

As an application, the case of methane and coal is considered. Molecular simulations of an adsorption of methane on a microporous realistic model for coal are performed with a flexible solid skeleton. The applicability of the set of linearized constitutive equations to this case is discussed and the results are shown to be consistent with swelling data measured during a classical adsorption experiment.

VL - 60 IS - 4 ER -