Recent Publications

Date Published Jan052018AuthorsConcrete material science: Past, present, and future innovationsCement and Concrete Research
Concrete is flying off, but it is simultaneously facing tremendous challenges in terms of environmental impact, financial needs, societal acceptance and image. Based on an historical approach of the science of concrete and reinforced concrete in particular, this paper calls for the exploration of radical changes in three key aspects of concrete use: reinforcement, binder content, and implementation methods. More precisely, it is suggested that, in parallel to the introduction of robotic fabrication methods, digital technologies may be key for the introduction several innovations like (i) rebarfree reinforcement using nonconvex granular media; (ii) compressionoptimized concrete...

Date Published Jul122017AuthorsMagnetism as indirect tool for carbon content assessment in nickel nanoparticlesJournal of Applied Physics
We report a combined experimental and theoretical study to ascertain carbon solubility in nickel nanoparticles embedded into a carbon matrix via the onepot method. This original approach is based on the experimental characterization of the magnetic properties of Ni at room temperature and Monte Carlo simulations used to calculate the magnetization as a function of C content in Ni nanoparticles. Other commonly used experimental methods fail to accurately determine the chemical analysis of these types of nanoparticles. Thus, we could assess the C content within Ni nanoparticles and it decreases from 8 to around 4 at. % with increasing temperature during the synthesis. This behavior could be related to the catalytic transformation of dissolved C in the Ni particles into graphite layers surrounding the particles at high temperature. The proposed approach is original and easy to implement experimentally since only magnetization measurements at room temperature are needed....

Date Published 09/2017AuthorsDisorderinduced stiffness degradation of highly disordered porous materialsJournal of the Mechanics and Physics of Solids
The effective mechanical behavior of multiphase solid materials is generally modeled by means of homogenization techniques that account for phase volume fractions and elastic moduli without considering the spatial distribution of the different phases. By means of extensive numerical simulations of randomly generated porous materials using the lattice element method, the role of local textural properties on the effective elastic properties of disordered porous materials is investigated and compared with different continuum micromechanicsbased models. It is found that the pronounced disorderinduced stiffness degradation originates from stress concentrations around pore clusters in highly disordered porous materials. We identify a single disorder parameter, φsa, which combines a measure of the spatial disorder of pores (the clustering index, sa) with the pore volume fraction (the porosity, φ) to scale the disorderinduced stiffness degradation. Thus, we conclude...

Date Published 08/2017AuthorsStress Transmission and Failure in Disordered Porous MediaPHYSICAL REVIEW LETTERS
By means of extensive latticeelement simulations, we investigate stress transmission and its relation with failure properties in increasingly disordered porous systems. We observe a nonGaussian broadening of stress probability density functions under tensile loading with increasing porosity and disorder, revealing a gradual transition from a state governed by singlepore stress concentration to a state controlled by multipore interactions and metric disorder. This effect is captured by the excess kurtosis of stress distributions and shown to be nicely correlated with the second moment of local porosity fluctuations, which appears thus as a (dis)order parameter for the system. By generating statistical ensembles of porous textures with varying porosity and disorder, we derive a general expression for the fracture stress as a decreasing function of porosity and disorder. Focusing on critical sites where the local stress is above the global fracture threshold, we also analyze the...

Date Published 07/2017AuthorsTopological Control on the Structural Relaxation of Atomic Networks under StressPhysical Review Letters
Upon loading, atomic networks can feature delayed irreversible relaxation. However, the effect of composition and structure on relaxation remains poorly understood. Herein, relying on accelerated molecular dynamics simulations and topological constraint theory, we investigate the relationship between atomic topology and stressinduced structural relaxation, by taking the example of creep deformations in calcium silicate hydrates (C...

Date Published 07/2017AuthorsCrystalchemistry control of the mechanical properties of 2:1 clay mineralsApplied Clay Science
Clay minerals are the main constituents of the clay matrix of a wide variety of sedimentary deposits. When subjected to burial, some of these minerals undergo phase transitions accompanied by atomic substitutions which ultimately impact the cohesive interactions between their constitutive layers. The most common among such transitions is smectite illitization, which is also highly relevant for oil and gas exploration and production from source rocks. The impact of this transition on the mechanical properties of clay minerals as well as clay bearing rocks remains, however, to be properly addressed. To this end, a set of macroscopic single 2:1 clay minerals (pyrophyllite, talc, vermiculite, phlogopite, muscovite, and clintonite) representative of the two octahedral fillings and the variation in surface charge densities was investigated. A hybrid experimentalmodeling approach is proposed, which combines nanoindentation in orthogonal directions (x1 and x3directions...

Date Published 07/2017AuthorsViscoinertial regime of immersed granular flowsPhysical Review E
By means of extensive coupled molecular dynamics–lattice Boltzmann simulations, accounting for grain dynamics and subparticle resolution of the fluid phase, we analyze steady inertial granular flows sheared by a viscous fluid. We show that, for a broad range of system parameters (shear rate, confining stress, fluid viscosity, and relative fluidgrain density), the frictional strength and packing fraction can be described by a modified inertial number incorporating the fluid effect. In a dual viscous description, the effective viscosity diverges as the inverse square of the difference between the packing fraction and its jamming value, as observed in experiments. We also find that the fabric and force anisotropies extracted from the contact network are well described by the modified inertial number, thus providing clear evidence for the role of these key structural parameters in dense suspensions.

Date Published 04/2017AuthorsModeling Granular Materials: CenturyLong Research across ScalesJournal of Engineering Mechanics
Granular materials are the most recurrent form of solidstate matter on Earth. They challenge researchers and engineers in various fields not only because they occur with a broad variety of grain sizes, shapes and interactions in nature and industry, but also because they show a rich panoply of mechanical states. Despite this polymorphism, all these different types of soils, powders, granules, ores, pharmaceutical products, etc., are instances of the granular matter with the same least common denominator of being sandlike (psammoid in Greek), i.e., solid grains interacting via frictional contacts. This review describes milestone contributions to the field of granular materials since the early elasticplastic models developed for soils in the 1950s. The research on granular materials has grown into a vast multidisciplinary field in the 1980s with increasing focus on the microstructure and owing to new experimental tools and discrete simulation methods. It turns out that the granular...

Date Published 03/2017AuthorsMolecular Modeling and Adsorption Properties of Ordered SilicaTemplated CMK Mesoporous CarbonsLangmuir
Realistic molecular models of silicatemplated CMK1, CMK3, and CMK5 carbon materials have been developed by using carbon rods and carbon pipes that were obtained by adsorbing carbon in a model MCM41 pore. The interactions between the carbon atoms with the silica matrix were described using the PNTraz potential, and the interaction between the carbon atoms was calculated by the reactive empirical bond order (REBO) potential. Carbon rods and pipes with different thicknesses were obtained by changing the silica–carbon interaction strength, the temperature, and the chemical potential of carbon vapor adsorption. These equilibrium structures were further used to obtain the atomic models of CMK1, CMK3, and CMK5 materials using the same symmetry as found in TEM pictures. These models are further refined and made more realistic by adding interconnections between the carbon rods and carbon pipes. We calculated the geometric pore size distribution of...

Date Published 02/2017AuthorsModeling soft granular materialsGranular Matter
Softgrain materials such as clays and other colloidal pastes share the common feature of being composed of grains that can undergo large deformations without rupture. For the simulation of such materials, we present two alternative methods: (1) an implicit formulation of the material point method (MPM), in which each grain is discretized as a collection of material points, and (2) the bonded particle model (BPM), in which each soft grain is modeled as an aggregate of rigid particles using the contact dynamics method. In the MPM, a linear elastic behavior is used for the grains. In order to allow the aggregates in the BPM to deform without breaking, we use longrange centertocenter attraction forces between the primary particles belonging to each grain together with steric repulsion at their contact points. We show that these interactions lead to a plastic behavior of the grains. Using both methods, we analyze the uniaxial compaction of 2D soft granular packings. This process is...