Laboratoire Navier

We study the plane shear flow of a dense assembly of dissipative disks using discrete simulation and prescribing the pressure and the shear rate. Those shear states are steady and uniform, and become intermittent in the quasistatic regime. In the limit of rigid grains, the shear state is determined by a single dimensionless number, called the inertial number I , which describes the ratio of inertial to pressure forces. Small values of I correspond to the quasistatic critical state of soil mechanics, while large values of I correspond to the fully collisional regime of kinetic theory. When I increases in the intermediate dense flow regime, we measure an approximately linear decrease of the solid fraction from the maximum packing value, and an approximately linear increase of the effective friction coefficient from the static internal friction value. From those dilatancy and friction laws, we deduce the constitutive law for dense granular flows, with a plastic Coulomb term and a viscous Bagnold term. The mechanical characteristics of the grains (restitution, friction, and elasticity) have a small influence in the dense flow regime. Finally, we show that the evolution of the relative velocity fluctuations and of the contact force anisotropy as a function of I provides a simple explanation of the friction law.

We present results on a series of two-dimensional atomistic computer simulations of amorphous systems subjected to simple shear in the athermal, quasistatic limit. The athermal quasistatic trajectories are shown to separate into smooth, reversible elastic branches which are intermittently broken by discrete catastrophic plastic events. The onset of a typical plastic event is studied with precision, and it is shown that the mode of the system which is responsible for the loss of stability has structure in real space which is consistent with a quadrupolar source acting on an elastic matrix. The plastic events themselves are shown to be composed of localized shear transformations which organize into lines of slip which span the length of the simulation cell, and a mechanism for the organization is discussed. Although within a single event there are strong spatial correlations in the deformation, we find little correlation from one event to the next, and these transient lines of slip are not to be confounded with the persistent regions of localized shear--so-called "shear bands"--found in related studies. The slip lines give rise to particular scalings with system length of various measures of event size. Strikingly, data obtained using three differing interaction potentials can be brought into quantitative agreement after a simple rescaling, emphasizing the insensitivity of the emergent plastic behavior in these disordered systems to the precise details of the underlying interactions. The results should be relevant to understanding plastic deformation in systems such as metallic glasses well below their glass temperature, soft glassy systems (such as dense emulsions), or compressed granular materials.

The archetypal feature of a viscoplastic fluid is its yield stress: If the material is not sufficiently stressed, it behaves like a solid, but once the yield stress is exceeded, the material flows like a fluid. Such behavior characterizes materials common in industries such as petroleum and chemical processing, cosmetics, and food processing and in geophysical fluid dynamics. The most common idealization of a viscoplastic fluid is the Bingham model, which has been widely used to rationalize experimental data, even though it is a crude oversimplification of true rheological behavior. The popularity of the model is in its apparent simplicity. Despite this, the sudden transition between solid-like behavior and flow introduces significant complications into the dynamics, which, as a result, has resisted much analysis. Over recent decades, theoretical developments, both analytical and computational, have provided a better understanding of the effect of the yield stress. Simultaneously, greater insight into the material behavior of real fluids has been afforded by advances in rheometry. These developments have primed us for a better understanding of the various applications in the natural and engineering sciences.

Digital fabrication has been termed the “third industrial revolution” in recent years, and promises to revolutionize the construction industry with the potential of freeform architecture, less material waste, reduced construction costs, and increased worker safety. Digital fabrication techniques and cementitious materials have only intersected in a significant way within recent years. In this letter, we review the methods of digital fabrication with concrete, including 3D printing, under the encompassing term “digital concrete”, identifying major challenges for concrete technology within this field. We additionally provide an analysis of layered extrusion, the most popular digital fabrication technique in concrete technology, identifying the importance of hydration control in its implementation.

This article offers a comprehensive overview of the underlying physics relevant to an understanding of materials processing during the various production steps in extrusion-based 3D concrete printing (3DCP). Understanding the physics governing the processes is an important step towards the purposeful design and optimization of 3DCP systems as well as their efficient and robust process control. For some processes, analytical formulas based on the relevant physics have already enabled reasonable predictions with respect to material flow behavior and buildability, especially in the case of relatively simple geometries. The existing research in the field was systematically compiled by the authors in the framework of the activities of the RILEM Technical Committee 276 “Digital fabrication with cement-based materials”. However, further research is needed to develop reliable tools for the quantitative analysis of the entire process chain. To achieve this, experimental efforts for the characterization of material properties need to go hand in hand with comprehensive numerical simulation.

Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Twitter Facebook Reddit LinkedIn Tools Icon Tools Reprints and Permissions Cite Icon Cite Search Site Citation N. Roussel, P. Coussot; “Fifty-cent rheometer” for yield stress measurements: From slump to spreading flow. J. Rheol. 1 May 2005; 49 (3): 705–718. https://doi.org/10.1122/1.1879041 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentThe Society of RheologyJournal of Rheology Search Advanced Search |Citation Search

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.

Magnetic-resonance-imaging rheometrical experiments show that concentrated suspensions or emulsions cannot flow steadily at a uniform rate smaller than a critical value ( ${\stackrel{\ifmmode \dot{}\else \.{}\fi{}}{\ensuremath{\gamma}}}_{c}$). As a result, a ``liquid'' region (sheared rapidly, i.e., at a rate larger than ${\stackrel{\ifmmode \dot{}\else \.{}\fi{}}{\ensuremath{\gamma}}}_{c}$) and a ``solid'' region (static) coexist. The behavior of the fluid in the liquid region follows a simple power-law model, while the extent of the solid region increases with the degree of jamming of the material.

Mars’s seismic activity and noise have been monitored since January 2019 by the seismometer of the InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) lander. At night, Mars is extremely quiet; seismic noise is about 500 times lower than Earth’s microseismic noise at periods between 4 s and 30 s. The recorded seismic noise increases during the day due to ground deformations induced by convective atmospheric vortices and ground-transferred wind-generated lander noise. Here we constrain properties of the crust beneath InSight, using signals from atmospheric vortices and from the hammering of InSight’s Heat Flow and Physical Properties (HP3) instrument, as well as the three largest Marsquakes detected as of September 2019. From receiver function analysis, we infer that the uppermost 8–11 km of the crust is highly altered and/or fractured. We measure the crustal diffusivity and intrinsic attenuation using multiscattering analysis and find that seismic attenuation is about three times larger than on the Moon, which suggests that the crust contains small amounts of volatiles. The crust beneath the InSight lander on Mars is altered or fractured to 8–11 km depth and may bear volatiles, according to an analysis of seismic noise and wave scattering recorded by InSight’s seismometer.

This book is the first to provide a thorough description of all aspects of the physico-chemical properties of foams. It sets out what is known about their structure, their stability, and their rheology. Engineers, researchers and students will find descriptions of all the key concepts, illustrated by numerous applications, as well as experiments and exercises for the reader. A solutions manual for lecturers is available via the publisher's web site.

We investigate the flowing behavior of dense suspensions of noncolloidal particles, by coupling macroscopic rheometric experiments and local velocity and concentration measurements through magnetic resonance imaging (MRI) techniques. We find that the flow is localized at low velocities, and that the material is inhomogeneous; the local laws inferred from macroscopic rheometric observations must then be reinterpreted in light of these local observations. We show that the short time response to a velocity step allows dense suspensions to be characterized locally: they have a purely viscous behavior, without any observable influence of granular friction. In the “jammed” zone, there may be a contact network, whereas in the sheared zone there are only hydrodynamic interactions: localization consists of a change in configuration at the grain scale. From the concentration and velocity profiles, we provide for the first time local measurements of the concentration dependence of viscosity, and find a Krieger-Dougherty law η(ϕ)=η0(1−ϕ∕0.605)−2 to apply. Shear-induced migration is almost instantaneous, in contrast to most other observations, and implies that the diffusion coefficients depend strongly on the concentration. We finally propose a simple constitutive law for dense suspensions, based on a purely viscous behavior, that accounts for all the macroscopic and local observations.

Salt damage in stone results in part from crystallization of salts during drying. We study the evaporation of aqueous salt solutions and the crystallization growth for sodium sulfate and sodium chloride in model situations: evaporating droplets and evaporation from square capillaries. The results show that the interfacial properties are of key importance for where and how the crystals form. The consequences for the different forms of salt crystallization observed in practice are discussed.

In approximate Kohn-Sham density-functional theory, self-interaction manifests itself as the dependence of the energy of an orbital on its fractional occupation. This unphysical behavior translates into qualitative and quantitative errors that pervade many fundamental aspects of density-functional predictions. Here, we first examine self-interaction in terms of the discrepancy between total and partial electron removal energies, and then highlight the importance of imposing the generalized Koopmans' condition---that identifies orbital energies as opposite total electron removal energies---to resolve this discrepancy. In the process, we derive a correction to approximate functionals that, in the frozen-orbital approximation, eliminates the unphysical occupation dependence of orbital energies up to the third order in the single-particle densities. This non-Koopmans correction brings physical meaning to single-particle energies; when applied to common local or semilocal density functionals it provides results that are in excellent agreement with experimental data---with an accuracy comparable to that of GW many-body perturbation theory---while providing an explicit total energy functional that preserves or improves on the description of established structural properties.

Aqueous foams are complex fluids composed of gas bubbles tightly packed in a surfactant solution. Even though they generally consist only of Newtonian fluids, foam flow obeys nonlinear laws. This can result from nonaffine deformations of the disordered bubble packing as well as from a coupling between the surface flow in the surfactant monolayers and the bulk liquid flow in the films, channels, and nodes. A similar coupling governs the permeation of liquid through the bubble packing that is observed when foams drain due to gravity. We review the experimental state of the art as well as recent models that describe the interplay of the processes at multiple length scales involved in foam drainage and rheology.

We perform an extensive numerical study of avalanche behavior in a 2D LJ glass at T=0, sheared at finite strain rates $\dot\gamma$. From the finite size analysis of stress fluctuations and of transverse diffusion we show that flip-flip correlations remain relevant at all realistic strain rates. We predict that the avalanche size scales as $\dot\gamma^{-1/d}$, with $d$ the space dimension.

Carbon dioxide injection in coal seams is known to improve the methane production of the coal seam, while ensuring a safe and long-term carbon sequestration. This improvement is due to the preferential adsorption of CO(2) in coal with respect to CH(4): an injection of CO(2) thus results in a desorption of CH(4). However, this preferential adsorption is also known to cause a differential swelling of coal, which results in a significant decrease in the reservoir permeability during the injection process. Recent studies have shown that adsorption in coal micropores (few angströms in size) is the main cause of the swelling. In this work, we focus on the competitive adsorption behavior of CO(2) and CH(4) in micropores. We perform molecular simulations of adsorption with a realistic atomistic model for coal. The competitive adsorption is studied at various temperatures and pressures representative of those in geological reservoirs. With the help of a poromechanical model, we then quantify the subsequent differential swelling induced by the computed adsorption behaviors. The differential swelling is almost insensitive to the geological temperatures and pressures considered here and is proportional to the CO(2) mole fraction in the coal.

The tensile strength of soil is an important mechanical parameter that controls the development of tension cracks. In this study, randomly distributed polypropylene fibers were employed to improve soil tensile behavior. Direct tensile tests were conducted on fiber-reinforced soil specimens with different fiber contents and compacted at different water contents and dry densities. Desiccation tests were also performed to evaluate the effectiveness of fiber reinforcement in improving soil tensile cracking resistance. The tensile test results showed that fiber inclusion significantly increased the soil peak strength, reduced the postpeak strength, and changed the brittle tensile failure behavior to a more ductile one. Soil tensile strength increased with the increase in fiber content. The tensile strength of both reinforced and unreinforced specimens decreased with increasing water content and increased with increasing dry density. Moreover, a higher soil dry density showed a more positive effect in mobilizing the reinforcement benefit of fibers. Based on the fiber/soil interfacial interaction mechanisms, the fiber reinforcement benefits on soil tensile behavior were analyzed. A linear relationship was obtained between the fiber reinforcement benefit and the fiber/soil interfacial shear strength. The desiccation test results showed that fiber inclusion significantly decreased soil cracking. The surface crack reduction ratio increased while the average crack width and length decreased with increasing fiber content, suggesting that fiber reinforcement was efficient in impeding soil tensile failure.

Digital Fabrication with Concrete (DFC) encompasses 3D Concrete Printing (3DCP) and many other methods of production. DFC is emerging from an era of invention and demonstration to one where the merits of one principle over another needs to be quantified systematically. DFC technologies vary in characteristics, complexity and maturity which hampers the synthesis of research and comparisons of performance. The interdependence of design geometry, material properties and process characteristics is well recognised. Materials research has made significant progress in recent years and there have been many applications with varying design geometries demonstrated. Far less has been done to guide the definition and description of the processes used. This work takes a step forward by presenting classification and process description guidance for DFC. The approach was developed by engaging a broad cross-section of the international community through the activities of the RILEM Technical Committee 276 between 2016 and 2020.

This paper is motivated by operating self service transport systems that flourish nowadays. In cities where such systems have been set up with bikes, trucks travel to maintain a suitable number of bikes per station. It is natural to study a version of the C-delivery TSP defined by Chalasani and Motwani in which, unlike their definition, C is part of the input: each vertex v of a graph G=(V,E) has a certain amount xv of a commodity and wishes to have an amount equal to yv (we assume that and all quantities are assumed to be integers); given a vehicle of capacity C, find a minimal route that balances all vertices, that is, that allows to have an amount yv of the commodity on each vertex v. This paper presents among other things complexity results, lower bounds, approximation algorithms, and a polynomial algorithm when G is a tree.

In the present work, the shear behaviour of soils and the soil–concrete interface is investigated through direct shear tests at various temperatures. A conventional direct shear apparatus, equipped with a temperature control system, was used to test sand, clay, and the clay–concrete interface at various temperatures (5, 20, and 40 °C). These values correspond to the range of temperatures observed near thermoactive geostructures. Tests were performed at normal stress values ranging from 5 to 80 kPa. Results show that the effect of temperature on the shear strength parameters of soils and the soil–concrete interface is negligible. A softening behaviour was observed during shearing of the clay–concrete interface, which was not the case with clay specimens. The peak strength of the clay–concrete interface is smaller than the ultimate shear strength of clay.