Granular Materials Research Papers - Academia.edu (original) (raw)

PACS. 05.40.-a – Fluctuation phenomena, random processes, noise, and Brownian motion. PACS. 45.70.Cc – Static sandpiles; granular compaction. PACS. 83.80.Fg – Granular solids. Abstract. – We study the stress profile of an ordered... more

PACS. 05.40.-a – Fluctuation phenomena, random processes, noise, and Brownian motion. PACS. 45.70.Cc – Static sandpiles; granular compaction. PACS. 83.80.Fg – Granular solids. Abstract. – We study the stress profile of an ordered two-dimensional packing of beads in response to the application of a vertical overload localized at its top surface. Disorder is introduced through the Coulombic friction between the grains which gives some indeterminacy and allows the choice of one constrained random number per grain in the calculation of the contact forces. The so-called “multi-agent ” technique we use lets us deal with systems as large as 1000×1000 grains. We show that the average response profile has a double-peaked structure. At large depth z, the position of these peaks grows with cz, while their widths scales like Dz. c and D are analogous to “propagation ” and “diffusion ” coefficients. Their values depend on that of the friction coefficient µ. At small µ, we get c0 − c ∝ µ and D ∝ ...

The evolution of the microstructure of an assembly of cohesionless granular materials with associated pores, which carry the overall applied stresses through frictional contacts is a complex phenomenon. The macroscopic flow of such... more

The evolution of the microstructure of an assembly of cohesionless granular materials with associated pores, which carry the overall applied stresses through frictional contacts is a complex phenomenon. The macroscopic flow of such materials take place by the virtue of the relative rolling and sliding of the grains on the micro-scale. A new discrete element method for biaxial compression simulations of random assemblies of oval particles with mixed sizes is introduced. During the course of deformation, the new positions of the grains are determined by employing the static equilibrium equations. A key aspect of the method is that, it is formulated for ellipse cross-sectional particles, hence desirable inherent anisotropies are possible. A robust algorithm for the determination of the contact points between neighbouring grains is given. Employing the present methodology, many aspects of the behaviour of two-dimensional assemblies of oval cross-sectional rods have been successfully addressed. The effects of initial void ratio, interparticle friction angle, aspect ratio, and bedding angle on the rolling and sliding contacts are examined. The distribution of normals to the rolling and sliding contacts have different patterns and are concentrated along directions, which are approximately perpendicular to one another. On the other hand, the distribution of all contact normals (combined rolling and sliding) are close to that of rolling contacts, which confirm that rolling is the dominant mechanism. This phenomenon becomes more pronounced for higher intergranular friction angle. Characteristics of the rolling and sliding contacts are also discussed in the context of the force angle, which is the inclination of contact force with respect to the contact normal.

In 1954, R.A. Bagnold published his seminal findings on the rheological properties of liquid-solid flows. We recently completed an extensive reevaluation of Bagnold's work, and our analysis and simulations indicate that the rheological... more

In 1954, R.A. Bagnold published his seminal findings on the rheological properties of liquid-solid flows. We recently completed an extensive reevaluation of Bagnold's work, and our analysis and simulations indicate that the rheological measurements of Bagnold were affected significantly by secondary flows within the experimental apparatus. The concentric cylinder rheometer was designed by Bagnold to measure simultaneously the shear and normal forces for a wide range for solid concentrations, fluid viscosities and shear rates. As presented by Bagnold, the shear and normal forces depended linearly on the shear rate in the 'macroviscous' regime. As the grain-to-grain interactions increased in the 'grain inertia' regime, the stresses depended on the square of the shear rate and were independent of the fluid viscosity. These results, however, appear to be dictated by the design of the experimental facility. In Bagnold's experiments, the height (h) of the rheometer was relatively short compared to the spacing (t) between the rotating outer and stationary inner cylinder (h/t=4.6). Since the top and bottom end plates rotated with the outer cylinder, the flow contained two axisymmetric counter-rotating cells in which flow moved outward along the end plates and inward at the midheight of the annulus. These cells contribute significantly to the measured torque, and obscured any accurate measurements of the shear or normal stresses. Before doing the reevaluation of Bagnold's work, our research objective was to examine the effects of the interstitial fluid for flows in which the densities of the two phases were different. After reevaluating Bagnold's work, we redesigned our experimental facility to minimize secondary flow effects. Like Bagnold's facility, we use a concentric cylinder rheometer with a rotating outer wall. The inner cylinder also is able to rotate slightly but will also be restrained by flexible supports; the torque is measured from the deformation of the flexures. The normal force is measured using piezoelectric transducers that record both impacts with the surface and fluid pressure variations resulting from particle collisions. Unlike Bagnold's apparatus, the top and bottom plates of the annulus will not rotate, and the torque measurement will be measured only in the center region of the inner annulus; these changes will minimize the secondary flow effects. The experiments will cover a range of particle sizes (from d = 1.5 to 4 mm), particle concentrations (up to 55% solids concentration by volume), shear rates (gamma = 10-160/sec) and solid-to-fluid densities (rhop = 1.2 to 8). During one flight of the KC-135 we will change two parameters: the rotational speed and the fluid viscosity (mu). At one time during a flight, we plan to withdraw some of the fluid (water for example) within the annulus while injecting some fluid of a different viscosity (water-glycerin mixture). Hence, the experiments will cover flows where the particle inertia dominates the fluid effects (granular flows) to flows in which the fluid inertia dominates that of the particles (dilute suspension). The range of Stokes numbers (St=d2 gamma rhop)/mu will be from about 5 to 3000. Currently, the experimental facility has just been completed. We have calibrated the normal impact measurements using carefully controlled single particle impacts with the transducers. The torque measurements have also been calibrated by mounting the inner cylinder in such a way that we could impose a known load on the drum. We use reluctance transducers to measure the motion and deformation of the flexures and calibrate the device with the imposed load. Measurements will also be made of the fluid temperature, acceleration and rotational speed of the outer drum.. Additional information is included in the original extended abstract.

In this paper, we consider the mechanical response of granular materials and compare the predictions of a hypoplastic model with that of a recently developed dilatant double shearing model which includes the effects of fabric. We... more

In this paper, we consider the mechanical response of granular materials and compare the predictions of a hypoplastic model with that of a recently developed dilatant double shearing model which includes the effects of fabric. We implement the constitutive relations of the dilatant double shearing model and the hypoplastic model in the finite element program ABACUS/Explicit and compare their predictions in the triaxial compression and cyclic shear loading tests. Although the origins and the constitutive relations of the double shearing model and the hypoplastic model are quite different, we find that both models are capable of capturing typical behaviours of granular materials. This is significant because while hypoplasticity is phenomenological in nature, the double shearing model is based on a kinematic hypothesis and microstructural considerations, and can easily be calibrated through standard tests. Copyright © 2006 John Wiley & Sons, Ltd.

The loading of a granular material induces anisotropies of the particle arrangement (fabric) and of the material’s strength, incremental stiffness, and permeability. Thirteen measures of fabric anisotropy are developed, which are arranged... more

The loading of a granular material induces anisotropies of the particle arrangement (fabric) and of the material’s strength, incremental stiffness, and permeability. Thirteen measures of fabric anisotropy are developed, which are arranged in four categories: as preferred orientations of the particle bodies, the particle surfaces, the contact normals, and the void space. Anisotropy of the voids is described through image analysis and with Minkowski tensors. The thirteen measures of anisotropy change during loading, as determined with three-dimensional discrete element simulations of biaxial plane strain compression with constant mean stress. Assemblies with four different particle shapes were simulated. The measures of contact orientation are the most responsive to loading, and they change greatly at small strains, whereas the other measures lag the loading process and continue to change beyond the state of peak stress and even after the deviatoric stress has nearly reached a steady state. The paper implements a methodology for characterizing the incremental stiffness of a granular assembly during biaxial loading, with orthotropic loading increments that preserve the principal axes of the fabric and stiffness tensors. The linear part of the hypoplastic tangential stiffness is monitored with oedometric loading increments. This stiffness increases in the direction of the initial compressive loading but decreases in the direction of extension. Anisotropy of this stiffness is closely correlated with a particular measure of the contact fabric. Permeabilities are measured in three directions with lattice Boltzmann methods at various stages of loading and for assemblies with four particle shapes. Effective permeability is negatively correlated with the directional mean free path and is positively correlated with pore width, indicating that the anisotropy of effective permeability induced by loading is produced by changes in the directional hydraulic radius.

This paper presents a study of the load-deformation behavior of geocell-stabilized subballast subjected to cyclic loading using a novel track process simulation apparatus. The tests were conducted at frequencies varying from 10 to 30 Hz.... more

This paper presents a study of the load-deformation behavior of geocell-stabilized subballast subjected to cyclic loading using a novel track process simulation apparatus. The tests were conducted at frequencies varying from 10 to 30 Hz. This frequency range is generally representative of Australian standard gauge trains operating up to 160 km=h. The discrete-element method (DEM) was also used to model geocell-reinforced subballast under plane strain conditions. The geocell was modeled by connecting a group of small circular balls together to form the desired geometry and aperture using contact and parallel bonds. Tensile and bending tests were carried out to calibrate the model parameters adopted for simulating the geocell. To model irregularly shaped particles of subballast, clusters of bonded circular balls were used. The simulated load-deformation curves of the geocell-reinforced subballast assembly at varying cyclic load cycles were in good agreement with the experimental observations. The results indicated that the geocell decreased the vertical and lateral deformation of subballast assemblies at any given frequency. Furthermore, the DEM can also provide insight into the distribution of contact force chains, and average contact normal and shear force distributions, which cannot be determined experimentally.

A finite strain multiscale hydro-mechanical model is established via an extended Hill–Mandel condition for two-phase porous media. By assuming that the effective stress principle holds at unit cell scale, we established a micro-to-macro... more

A finite strain multiscale hydro-mechanical model is established via an extended Hill–Mandel condition for two-phase porous media. By assuming that the effective stress principle holds at unit cell scale, we established a micro-to-macro transition that links the micromechanical responses at grain scale to the macroscopic effective stress responses, while modeling the fluid phase only at the macroscopic continuum level. We propose a dual-scale semi-implicit scheme, which treats macroscopic responses implicitly and microscopic responses explicitly. The dual-scale model is shown to have good convergence rate, and is stable and robust. By inferring effective stress measure and poro-plasticity parameters, such as porosity, Biot's coefficient and Biot's modulus from micro-scale simulations, the multiscale model is able to predict effective poro-elasto-plastic responses without introducing additional phenomenological laws. The performance of the proposed framework is demonstrated via a collection of representative numerical examples. Fabric tensors of the representative elementary volumes are computed and analyzed via the anisotropic critical state theory when strain localization occurs.

The main purpose of this experimental study is to know the effect of improvement of soft soil by mixing it with granular materials on the properties of soft soil. The soil model was brought from a one construction project in Karbala,... more

The main purpose of this experimental study is to know the effect of improvement of soft soil by mixing it with granular materials on the properties of soft soil. The soil model was brought from a one construction project in Karbala, Iraq. Routine tests were carried out on the soft soil model alone, as well as for soft soil with specific percentage of granular materials. The model of cylindrical shape laboratory dimensions of 250 mm in diameter with a height of 300 mm. Eight different shear strength were used for laboratory model tests. Static loading tests were carried out on these model tests. It was found from results of model tests of improved soil that the averaged value of settlement reduction is 0.61 for all model tests, where the bearing capacity is increased by 31%.

On the basis of three methods of preparing well-differentiated triaxial Hostun HN31 sand specimens, namely moist tamping, dry pluviation and water pluviation, we highlight the influence of the reconstitution mode on the mechanical... more

On the basis of three methods of preparing well-differentiated triaxial Hostun HN31 sand specimens, namely moist tamping, dry pluviation and water pluviation, we highlight the influence of the reconstitution mode on the mechanical behaviour of sand. Piezoelectric transducers (bender elements) were installed in a triaxial cell and at the lateral edges of the sample in different directions (HH and HV) to investigate the structural anisotropy caused by each deposition mode and its evolution during undrained shearing tests, by measuring shear wave velocities. The results confirm and enrich the observations made with the scanning electron microscope and triaxial apparatus used by Benahmed, 2001; Benahmed, Canou & Dupla, 2004 on loose sand prepared using two different reconstitution procedures.A partir de trois modes de préparation bien différenciés d’éprouvettes triaxiales du sable d’Hostun HN31, à savoir le damage humide, la pluviation à sec et la pluviation sous eau, on met en évidence l’influence du mode de dépôt sur le comportement mécanique du sable. Des capteurs piézoélectriques (Bender elements) ont été placés sur les embases inférieure et supérieure de la cellule triaxiale et sur le pourtour latéral de l’éprouvette d’essai dans différentes directions (HH et HV) afin d’étudier l’anisotropie induite par chaque mode de déposition et son évolution durant les essais de cisaillement non drainé. Ceci est réalisé à travers des mesures de vitesses d’onde de cisaillement. Les résultats obtenus viennent confirmer et enrichir les observations faites avec le microscope électronique à balayage (MEB) par Benahmed, 2001; Benahmed, Canou & Dupla, 2004 sur le sable lâche préparé par deux méthodes de reconstitution différentes.

The problem of the steady-state velocity distribution in a driven inelastic Maxwell model of shaken granular material is revisited. Numerical solution of the master equation and analytical arguments show that the model has bilateral... more

The problem of the steady-state velocity distribution in a driven inelastic Maxwell model of shaken granular material is revisited. Numerical solution of the master equation and analytical arguments show that the model has bilateral exponential velocity tails [P(v) approximately e(-|v|/sqrt[D])], where D is the amplitude of the noise. Previous study of this model predicted Gaussian tails [P(v) approximately e(-av(2))].

Railway ballast comprises unbounded discrete grains that are often used to form a load-bearing platform for tracks. Ballast degradation as trains pass over the tracks and infiltration of external fines including slurried (pumped) fine... more

Railway ballast comprises unbounded discrete grains that are often used to form a load-bearing platform for tracks. Ballast degradation as trains pass over the tracks and infiltration of external fines including slurried (pumped) fine subgrade soils are two of the main reasons for ballast fouling. Fouling causes tracks to settle and also reduces the load-bearing capacity, which is associated with a reduction in internal friction and increased lateral spreading of the ballast layer. This paper presents a study of mobilized friction angle, volumetric behavior, and associated evolutions of contact and fabric anisotropy of fouled ballast subjected to monotonic triaxial loading using a series of large-scale triaxial tests and discrete element modeling. Monotonically loaded and drained triaxial tests were carried out on ballast with levels of clay fouling that varied from 10 to 50% void contamination index (VCI) subjected to three confining pressures of 10, 30, and 60 kPa. The results showed that an increase in the level of fouling decreased the mobilized friction angle and increased the ballast dilation. The discrete element method (DEM) was used to study the mobilized friction angle and fabric anisotropy of fresh and fouled ballast by simulating actual large-scale triaxial tests. Irregular shaped grains of ballast were simulated by clumping bonded circular balls with appropriate sizes and positions together. Ballast fouling was approximately simulated in DEM by adding 1-mm particles into the pore spaces of the fresh ballast. The predicted mobilized friction angles and volumetric changes obtained from the DEM simulations agreed well with those measured in the laboratory, indicating that the peak friction angle of fouled ballast and dilation decreased as the degree of fouling increased. The DEM simulations provided an insight into the distribution of contact force chains, contact orientations, and evolution of fabric anisotropy of fresh and fouled ballast that could not be captured in the laboratory. These observations are important for a better understanding of the load-deformation behavior of fouled ballast from the perspective of micromechanics.

Numerical simulation can be a powerful tool in the research and development of any new electrostatic process. The aim of the present work is to introduce a simple mathematical model for simulating the outcome of a recently patented... more

Numerical simulation can be a powerful tool in the research and development of any new electrostatic process. The aim of the present work is to introduce a simple mathematical model for simulating the outcome of a recently patented tribo-aero-electrostatic separation process for binary mixtures of granular materials. The process is characterized by the fact that the charging of the granules is produced in a fluidized bed device, in the presence of an electric field. The mathematical model assumed that the probability of a granule to be separated can be expressed as a function of the number of impacts with granules belonging to the other class of materials. When this probability was given by a normal distribution law, the results of the simulations differed significantly from those of an experiment conducted with a granular mixture of polyamide and polycarbonate particles, in a laboratory tribo-aero-electrostatic separator. In order to improve the predictive capability of the simulations, a polynomial function derived from the regression of the experimental data was employed for expressing the same probability. Thus, it was possible to calculate the evolution in time of the mass of granules separated at the electrodes for various compositions of the granular mixture. The computed results were in good agreement with the experiments.

The output of any electrostatic separation process is strongly dependent on the effectiveness of particle charging. The aim of the present paper is to evaluate the effects of several factors that might influence the efficiency of a... more

The output of any electrostatic separation process is strongly dependent on the effectiveness of particle charging. The aim of the present paper is to evaluate the effects of several factors that might influence the efficiency of a fluidized bed tribocharger, in the presence of an electric field orthogonally oriented to the direction of the fluidization air. The experiments were carried out on several synthetic mixtures of binary granular mixtures. The velocity of the fluidization air, the duration of the tribocharging process, and the composition of the mixture were the three process variables investigated. Whenever possible, the effects of these variables on the output of the process (i.e., the mass and the charge of the granules collected at the electrodes), were evaluated using the experimental design methodology. The results of full- and composite-factorial experimental designs were analyzed with a commercial software (MODDE 5.0, Umetrics). The effectiveness of the charging process was found to depend significantly on each of these factors. The propitious aerodynamic design of the device is a prerequisite for the successful industry application of the technique.

Three-dimensional DEM simulations of size segregation in granular flows down chute are presented. Different cubic bi-disperse samples are generated by pluviation, on the rough base formed by randomly placed particles. Periodic boundaries... more

Three-dimensional DEM simulations of size segregation in granular flows down chute are presented. Different cubic bi-disperse samples are generated by pluviation, on the rough base formed by randomly placed particles. Periodic boundaries are applied to the flow direction and the two sides. Parametrical studies involving slope angle, width, volume fraction, and coefficient of friction are systemically performed. In all presented cases, steady, fully developed (SFD) state is achieved, where the kinetic energy and fractional volume distribution remain constant. From the macroscopic view, segregations are completed in the SFD state with slightly different extents and a thick layer of pure coarse grains appears on the top of the flow. The profiles of volume fractions are calculated and presented by shear layers. In addition, the trajectories of individual particles are tracked and analysed, showing clearly the contact conditions and shear history experienced by individual particles. It is found that the connectivity of small particles is generally at a lower level than that of the large ones, indicating a high probability of small particles dropping into voids under gravity is higher. On the other hand, the large particles experience a significant increase of connectivity as they migrate through the layer of small particles.