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Granular Media (granular + media)
Selected Abstracts,Arching' effect in elastic polycrystals: implications for the variability of fatigue livesFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 4 2002S. Pommier ABSTRACT The paper deals with a study of heterogeneous stress and strain distribution in polycrystals in relation with elastic anisotropy of grains. A similitude with the arching effect widely studied in granular materials is proposed and this concept is extended to heterogeneous polycrystals in which the load transfer is not binary in the way it is in granular media but may vary significantly and suddenly from one grain to another according to the crystal orientation to the load direction. Experiments and 3D finite element analyses show that though the individual orientation of grains is random, the strain and stress distribution is not. A network is formed inside the polycrystal whose scale is larger than the grain size. The load percolation network consists in heavily loaded links whose direction is coincident with the direction of the principal stresses. So, the typical scale for the variability of the local stresses is not the grain size but the size of the load percolation network. Since this scale is found to be rather large in particular for iron, zinc and copper, this effect should contribute significantly to the variability of the fatigue lives of notched vs. smooth components. [source] Turbulent Flow Through Porous MediaGROUND WATER, Issue 5 2001Douglas W. Barr The pressure driving flow through porous media must be equal to the viscous resistance plus the inertial resistance. Formulas are developed for both the viscous resistance and the inertial resistance. The expression for the coefficient of permeability consists of parameters which describe the characteristics of the porous medium and the permeating fluid and which, for unconsolidated isotropic granular media, are all measurable. A procedure is proposed for testing for the occurrence of turbulence and calculating the effective permeability when it occurs. The formulas are applied to a set of data from 588 permeameter runs ranging from laminar to highly turbulent. The equations fit the data from the permeameter closely through the laminar flow conditions and quite closely through the turbulent conditions. In the turbulent range, the plotting of the data separates into three distinct lines for each of the three shapes of particles used in the tests. For the porous medium and fluid of these tests, turbulence begins at a head gradient of about 0.1. [source] Bearing capacity of shallow foundations in transversely isotropic granular mediaINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 8 2010A. Azami Abstract The main focus in this work is on the assessment of bearing capacity of a shallow foundation in an inherently anisotropic particulate medium. Both the experimental and numerical investigations are carried out using a crushed limestone with elongated angular-shaped aggregates. The experimental study involves small-scale model tests aimed at examining the variation of bearing capacity as a function of the angle of deposition of the material. In addition, the results of a series of triaxial and direct shear tests are presented and later employed to identify the material functions/parameters. The numerical part of this work is associated with the development and implementation of a constitutive framework that describes the mechanical response of transversely isotropic frictional materials. The framework is based on the elastoplasticity and accounts for the effects of strain localization and inherent anisotropy of both the deformation and strength characteristics. The results of numerical simulations are compared withthe experimental data. A parametric study is also carried out aimed at examining the influence of various simplifications in the mathematical framework on its predictive abilities. Copyright © 2009 John Wiley & Sons, Ltd. [source] Influence of inherent anisotropy on mechanical behavior of granular materials based on DEM simulationsINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 8 2010Zafar Mahmood Abstract We study the influence of inherent anisotropy, i.e. bedding angle on stress,strain behavior and shear band formation in quasi-static granular media. Plane strain biaxial tests are carried out using two-dimensional distinct element method (DEM). Oval/elliptical-shaped particles are generated by overlapping the discrete circular elements. Particle assemblies with four different bedding angles are tested. Evolution of the microstructure inside and outside the shear band and effect of bedding angle on the microstructure are investigated. Influence of bedding angle on fabric and force anisotropy is studied. It is found that by using non-circular particles, generation of large voids and excess particle rotations inside the shear band are reproduced in a quite similar manner to those of the natural granular soils, which are difficult to produce with standard DEM simulations using circular particles. Copyright © 2009 John Wiley & Sons, Ltd. [source] On the capillary stress tensor in wet granular materialsINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 10 2009L. Scholtès Abstract This paper presents a micromechanical study of unsaturated granular media in the pendular regime, based on numerical experiments using the discrete element method, compared with a microstructural elastoplastic model. Water effects are taken into account by adding capillary menisci at contacts and their consequences in terms of force and water volume are studied. Simulations of triaxial compression tests are used to investigate both macro and micro-effects of a partial saturation. The results provided by the two methods appear to be in good agreement, reproducing the major trends of a partially saturated granular assembly, such as the increase in the shear strength and the hardening with suction. Moreover, a capillary stress tensor is exhibited from capillary forces by using homogenization techniques. Both macroscopic and microscopic considerations emphasize an induced anisotropy of the capillary stress tensor in relation with the pore fluid distribution inside the material. Insofar as the tensorial nature of this fluid fabric implies shear effects on the solid phase associated with suction, a comparison has been made with the standard equivalent pore pressure assumption. It is shown that water effects induce microstructural phenomena that cannot be considered at the macro level, particularly when dealing with material history. Thus, the study points out that unsaturated soil stress definitions should include, besides the macroscopic stresses such as the total stress, the microscopic interparticle stresses such as the ones resulting from capillary forces, in order to interpret more precisely the implications of the pore fluid on the mechanical behaviour of granular materials. Copyright © 2009 John Wiley & Sons, Ltd. [source] Random porosity fields and their influence on the stability of granular mediaINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 10 2008José E. Andrade Abstract It is well established that the mechanical behavior of granular media is strongly influenced by the media's microstructure. In this work, the influence of the microstructure is studied by integrating advances in the areas of geostatistics and computational plasticity, by spatially varying the porosity on samples of sand. In particular, geostatistical tools are used to characterize and simulate random porosity fields that are then fed into a nonlinear finite element model. The underlying effective mechanical response of the granular medium is governed by a newly developed elastoplastic model for sands, which readily incorporates spatial variability in the porosity field at the meso-scale. The objective of this study is to assess the influence of heterogeneities in the porosity field on the stability of sand samples. One hundred and fifty isotropic and anisotropic samples of dense sand are failed under plane-strain compression tests using Monte Carlo techniques. Results from parametric studies indicate that the axial strength of a specimen is affected by both the degree and orientation of anisotropy in heterogeneous porosity values with anisotropy orientation having a dominant effect, especially when the bands of high porosity are aligned with the natural orientation of shear banding in the specimen. Copyright © 2007 John Wiley & Sons, Ltd. [source] Liquefaction and cyclic mobility model for saturated granular mediaINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 5 2006S. López-Querol Abstract A new constitutive law for the behaviour of undrained sand subjected to dynamic loading is presented. The proposed model works for small and large strain ranges and incorporates contractive and dilative properties of the sand into the unified numerical scheme. These features allow to correctly predict liquefaction and cyclic mobility phenomena for different initial relative densities of the soil. The model has been calibrated as an element test, by using cyclic simple shear data reported in the literature. For the contractive sand behaviour a well-known endochronic densification model has been used, whereas a plastic model with a new non-associative flow rule is applied when the sand tends to dilate. Both dilatancy and flow rule are based on a new state parameter, associated to the stiffness degradation of the material as the shaking goes on. Also, the function that represents the rearrangement memory of the soil takes a zero value when the material dilates, in order to easily model the change in the internal structure. Proceeding along this kind of approach, liquefaction and cyclic mobility are modelled with the same constitutive law, within the framework of a bi-dimensional FEM coupled algorithm developed in the paper. For calibration purposes, the behaviour of the soil in a cyclic simple shear test has been simulated, in order to estimate the influence of permeability, frequency of loading, and homogeneity of the shear stress field on the laboratory data. Copyright © 2006 John Wiley & Sons, Ltd. [source] On incremental non-linearity in granular media: phenomenological and multi-scale views (Part I)INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 14 2005Félix Darve Abstract On the basis of fundamental constitutive laws such as elasticity, perfect plasticity, and pure viscosity, many elasto-viscoplastic constitutive relations have been developed since the 1970s through phenomenological approaches. In addition, a few more recent micro-mechanical models based on multi-scale approaches are now able to describe the main macroscopic features of the mechanical behaviour of granular media. The purpose of this paper is to compare a phenomenological constitutive relation and a micro-mechanical model with respect to a basic issue regularly raised about granular assemblies: the incrementally non-linear character of their behaviour. It is shown that both phenomenological and micro-mechanical models exhibit an incremental non-linearity. In addition, the multi-scale approach reveals that the macroscopic incremental non-linearity could stem from the change in the regime of local contacts between particles (from plastic regime to elastic regime) in terms of the incremental macroscopic loading direction. Copyright © 2005 John Wiley & Sons, Ltd. [source] Micromechanical modelling of monotonic drained and undrained shear behaviour of granular media using three-dimensional DEMINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 12 2002Thallak. Abstract In this paper, numerical simulation results of isotropic compression and triaxial static shear tests under drained and undrained stress paths on polydisperse assembly of loose and dense spheres are presented. An examination of the micromechanical behaviour of loose and dense assemblies under drained and undrained conditions, considering the particulate nature of granular materials, has been carried out to explain micromechanically the granular material behaviour at the grain scale level. The numerical simulations have been carried out using a discrete element model (DEM) which considers a 1000 sphere particle polydisperse assembly with periodic space representing an infinite three-dimensional space. In this paper, we present how DEM simulations can contribute to developments in constitutive modelling of granular materials through micromechanical approach using information on microstructure evolution. A series of numerical tests are performed using DEM on 3-D assemblages of spheres to study the evolution of the internal variables such as average co-ordination number and induced anisotropy during deformation along with the macroscopic behaviour of the assemblage in drained and undrained shear tests. In a qualitative sense, the macroscopic stress,strain results and stress path evolution in these simulations using 3-D assemblies demonstrate that DEM simulations are capable of reproducing realistic compression and shear behaviour of granular materials. Copyright © 2002 John Wiley & Sons, Ltd. [source] Simulation of the dynamics of depth filtration of non-Brownian particlesAICHE JOURNAL, Issue 4 2001V. N. Burganos A new simulator for flow of aqueous suspensions and deposition of non-Brownian particles in granular media can predict the pattern of deposition and concomitant reduction in permeability as functions of depth, time and system parameters. The porous structure of the granular medium represented as a 3-D network of constricted pores considers the converging,diverging character of flow within pores. Using Lagrangian-type simulation the particle deposition rate was calculated. Gravity and drag, as well as hydrodynamic and physicochemical interactions between suspended particles and pore walls, were considered in calculating 3-D particle trajectories. Deposit configurations were computed, and the evolution of the pore structure was simulated at discrete time steps. Changes in the pore geometry and nature of the collector surface affect flow and trajectory computations directly. Clusters of deposited particles were allowed to become reentrained if exposed to shear stress higher than a critical value. Reentrained clusters, which moved through downstream pores, might redeposit downstream at suitable sites and cause clogging of sufficiently narrow pores. Particle clusters clogging pores have a finite permeability, which significantly affects the system's transient behavior. Clogged pores act as collectors of solitary particles and of reentrained clusters, and substantially affect the transient behavior of the filter. The loss of permeability was monitored by calculating pore and network hydraulic conductance at each time step. Numerical results for the loss of permeability, temporal evolution of filter efficiency, and specific deposit profiles are based on suspension flow simulations in a typical granular porous medium. [source] Applications of Markov Chains in Particulate Process Engineering: A ReviewTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 6 2004Henri Berthiaux Abstract Processes involving particles, are known to exhibit extremely unpredictable behaviour, mainly due to the mesoscopic nature of granular media. Understanding particulate processes, not only for intellectual satisfaction, but also for process design and operation, basically requires a systems approach in modelling. Because they combine simplicity and flexibility, the stochastic models based on the Markov chain theory are very valuable mathematical tools to this respect. However, they are still largely ignored by the whole core of chemical engineering researchers. This motivates the existence of this review paper, in which we examine the three traditional issues: mixing and transport, separation and transformation. Les procédés faisant intervenir des particules sont connus pour avoir un comportement extrêmement non prévisible, principalement à cause de la nature mésoscopique des milieux granulaires. Comprendre les procédés particulaires, non seulement pour la satisfaction intellectuelle, mais aussi pour la conception et le fonctionnement des procédés, nécessite en fait une approche des systèmes dans la modélisation. Parce qu'ils allient simplicité et flexibilité, les modèles stochastiques basés sur la théorie des chaînes de Markov sont des outils mathématiques à cet égard très valables. Cependant, ceux-ci sont encore largement ignorés par une majorité de chercheurs en génie chimique. Cette lacune motive le présent article, dans lequel nous examinons les trois thèmes traditionnels : mélange et transport, séparation, transformation. [source] | |||||||||||||