Behavior Of Granular Assemblies Research Articles

Understanding the interplay between particle shape, grading and sample density on the behaviour of granular assemblies: A DEM approach

This study investigates the interplay between particle shape, grading and initial sample density, three of the most important factors influencing the mechanical behaviour of sheared granular assemblies. Using the discrete element method (DEM), two-dimensional assemblies of varying initial sample density, particle aspect ratio, AR\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$AR$$\\end{document}, and coefficients of uniformity, Cu\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${C}_{u}$$\\end{document}, were prepared and subjected to drained biaxial shearing until the critical state was reached. We assessed the interplay between each of these parameters by evaluating whether the effect of any given parameter on a mechanical quantity is influenced by any other parameter. Our analyses show that the effect of some of these key parameters on mechanical response, can indeed be influenced by other key parameters. The effect of the particle AR\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$AR$$\\end{document} on the peak shear strength for the initially dense assemblies differs when compared with the medium-dense assemblies. The mechanical coordination number of the assemblies at the initial state correlates with the peak strength thereby explaining the interplay between particle AR\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$AR$$\\end{document} and initial sample density on the peak shear strength. The linear relationship established between strength and dilatancy for a combination of all assemblies studied suggests that the strength-dilatancy relationship is a unique characteristic of granular assemblies. The dilatancy of the assemblies correlates strongly with the amount of contacts lost during shearing. The interplays found between particle shape, grading and initial sample density in this study show that to develop robust constitutive models for the prediction of granular material behaviour, the effects of multiple factors must be considered.Graphic abstract

Influence of dry cohesion on the micro- and macro-mechanical properties of dense polydisperse powders & grains

Understanding how cohesive granular materials behave is of interest for many industrial applications, such as pharmaceutical or food and civil engineering. Models of the behaviour of granular materials on the microscopic scale can be used to obtain macroscopic continuum relations by a micro-macro transition approach. The Discrete Element Method (DEM) is used to inspect the influence of cohesion on the micro and macro behaviour of granular assemblies by using an elasto-plastic cohesive contact model. Interestingly, we observe that frictional samples prepared with different cohesion values show a significant difference in pressure and coordination number in the jammed regime; the differences become more pronounced when packings are closer to the jamming density, i.e. the lowest density where the system is mechanically stable. Furthermore, we observe that cohesion has an influence on the jamming density for frictional samples, but there is no influence on the jamming density for frictionless samples.

Modeling Mechanical Behavior of Very Coarse Granular Materials

A novel approach has been developed to predict the mechanical behavior of very coarse granular materials with a constitutive model, which considers both grain breakage and size effect. The behavior of granular assemblies is significantly influenced by particle breakage. A critical-state double-yield-surface model incorporating the change in the critical-state line and in elastic stiffness caused by grain breakage during loading has been adopted. The amount of grain breakage was estimated by extending the size effect theory on individual grains to granular assemblies. The results from earlier studies on granular materials with parallel gradations have been usefully exploited to calibrate and to validate the model. Comparisons between experiments and simulations suggest that this approach can predict the mechanical behavior of very coarse granular materials from test results performed on a finer fraction with a homothetic gradation.

Numerical simulation of 3D semi-real-shaped granular particle assembly

Recent researches have proved that the shape of particles has significant effect on the mechanical behavior of granular materials. Therefore to model the appropriate behavior of granular assemblies, it's very ideal that they become simulated as accurate as possible. The method that is applied in this paper simulates grain shapes close to reality. In this way, shape of a particle is modeled by combining a set of overlapping spheres, which are connected to each other in a rigid way. A program that is based on Discrete Element Method (DEM) has been developed to generate different real grain shapes. In order to investigate the influence of grain shape on mechanical behavior of granular soil, four groups of grains with different angularities, including high angular, medium angular, round, and spherical, are generated. Several triaxial tests with different confining pressures and friction coefficients have been carried out on assemblies with different grain shapes. The results prove that angularity of grains considerably affects the behavior of granular material.

Behavior of Granular Assemblies at High Velocity in a Large Open Shear Box

Abstract Large open shear box tests on granular materials have been conducted to investigate the behavior of these materials at high velocity. Several modes of kinetic motion were identified from the tests: steady sliding, sliding with rolling, impulsive rolling, and steady rolling. The measured resistances in these tests of different modes were observed to be dependent on material friction, base friction, velocity, body shape, and mobilized volume. However, spectral analysis of the measured data in time series shows that there exist no periodicities in the spectra, i.e., frequency- or velocity-independent. Of practical implication for this study is that the measured resistance for constant velocity motion decreases with increasing rolling motion activity within the moving granular mass. The average coefficient of resistance or friction could be less than the coefficient of kinetic friction measured in Coulomb-type shear tests.

Model Simulation of Powder Compaction by Complex Mold Based on Deformation Behavior of Free Particles Measured by Compression Test

ABSTRACTDiscrete element method (DEM) is one of the excellent procedures for model simulation of behavior of granular assemblies. A two-dimensional DEM simulation of powder compaction by a complex mold was conducted by considering the deformation behavior of individual particles measured by a compression test of free particles. Spherical copper particles were used as a model material in this study. The particles (particle size ranges from 38 to 125microns) were individually compressed between parallel platens to measure compressive load and displacement in the loading direction. From the measured load and displacement data, force-displacement relations at contacts between the particles, and between the particles and mold wall were determined. In the simulation, the two-dimensional complex mold consisting of three stages (upper stage:3-mm wide and 1-mm deep, middle stage:2-mm wide and 1-mm deep, lower stage:1-mm wide and 1-mm deep) was charged with the copper powder consisting of spherical particles with size range of 50 to 200microns and was pressed under four press conditions. Forces at all contacts were calculated by using the determined force-displacement relations and each particle's position was traced by solving equations of motion for each particle. Pressure at each wall of the mold and density distribution in the powder compact were calculated as results of the simulation. It was found that the increase in the pressure at each wall corresponds to the increase in the density of the powder near the wall.

A discrete numerical model for granular assemblies

The distinct element method is a numerical model capable of describing the mechanical behaviour of assemblies of discs and spheres. The method is based on the use of an explicit numerical scheme in which the interaction of the particles is monitored contact by contact and the motion of the particles modelled particle by particle. The main features of the distinct element method are described. The method is validated by comparing force vector plots obtained from the computer program BALL with the corresponding plots obtained from a photoelastic analysis. The photoelastic analysis used for the comparison is the one applied to an assembly of discs by De Josselin de Jong and Verruijt (1969). The force vector diagrams obtained numerically closely resemble those obtained photoelastically. It is concluded from this comparison that the distinct element method and the program BALL are valid tools for research into the behaviour of granular assemblies. La méthode des éléments distincts est un modèle numérique capable de décrire le comportement mécanique de l'assemblage de disques et de sphères. La méthode est basée sur l'utilisation d'un système numérique explicite dans lequel l'interaction des particules est contrôlée contact par contact et le mouvement des particules simulé particule par particule. Les caracteristiques principales de la méthode des eléments distints sont décrites. La méthode est validée en comparant les tracés de vecteur de force obtenus par le programme sur ordicateur BALL avec les tracés correspondants obtanus a l'aide d'une analyse photo-élastique. L'analyse photo-élastique utilisée pour la comparaison est celle appliquée sur un assemblage de disques par De Josselin de Jong et Verruijt (1969). Les diagrammes de vecteur de force obtenus numériquement sont très voisins de ceux obtenus photo-élastiquement. Cette comparaison permet de conclure que la methode des éléments distincts et le programme BALL sont des instruments valables pour la recherche du comportement des assemblages granulaires.