DEM analysis of undrained cyclic behavior and resistance of saturated dense sand without stress reversals

Granular Matter - Rockfill is a common irregular granular material used in most dam construction projects. The purpose of this paper is to investigate the distribution and size-dependent properties...     

DEM simulation of shear vibrational fluidization of granular material

Fluidization of dry granular material is the transition from a solid state to a liquid state when sufficient energy is applied during vibration. This behavior is important because it is closely related to deformations of geotechnical structures during an earthquake. The scientific challenge lies in the understanding on how strain localization is related to the fluidization zone during the entire shearing process. Despite the importance of the mechanical behavior of granular material during fluidization, it cannot be easily characterized using traditional direct shear test. In this paper, 2D DEM model is firstly conduct, shear vibrational fluidization is defined for dry granular material, and the discrete element method has been used to simulate the direct shear test on granular material under vibrational loading during shearing. The peak, residual and vibro-residual shear strength envelopes have been obtained from the numerical simulations. Three distinct zones have been identified in the upper shear box based on the observed changes in volumetric strain before vibration. During vibration, fluidization occurs in the three zones with the characteristics that the shear stress, porosity, volumetric strain, and the coordination number drop to relatively lower values. During vibration, material becomes denser than the critical state, and strain localization has been relieved. Densification of the material at the shear zone leads to a strengthening of the material which increases the shearing resistance after vibration. Furthermore, a comparison of the 2D and 3D simulations is performed. Results reveal that the motion of particles in the out-of-plane direction in the 3D simulations lead to smoother shear stress and more consistent with the experimental result.     

DEM study of the shear behavior and formation of shear band in biaxial test

This paper aims to study the shear behavior of granular matter by DEM simulations. Granular samples are prepared by automatic clump generation algorithm to create particles of irregular shapes. Simulations of the biaxial test with membrane boundary condition are used to test the shear behavior of samples. A new method for computing sample volume in membrane boundary condition is proposed. Deviatoric stress and volumetric strain curves are plotted to describe contracting-dilatancy of granular materials during the shearing stage. Formation of the shear band is studied from particle rotation and particle displacement fields. The influence of confining pressure, initial porosity, and friction coefficient on the development of shear band are studied. Lower confining pressure, higher initial porosity can be resulted in later formation of shear bands.     

Study of shear behavior of granular materials by 3D DEM simulation of the triaxial test in the membrane boundary condition

This paper studies the shear behavior of granular materials by using a three-dimensional (3D) discrete element method (DEM) simulation of the triaxial test. The experimental triaxial tests were conducted on glass beads samples for verification. DEM simulations of the triaxial test were carried out in the membrane boundary condition consisting of 37,989 membrane particles. A new method that divides the irregular sample shape into two parts of cones and parts of three-dimensional simplexes is used to follow the volume change of irregular deformation of samples. The free rotatable upper platen is considered during the shearing process, which influences the shear behavior of samples especially in the residual stage and formations of a single shear band or X-shape shear band. The confining pressures have been demonstrated to influence the rotation angle and angular velocity of the upper platen. Moreover, the timing of replacing a rigid wall boundary condition with the membrane boundary condition is investigated, which affects the porosity of samples before shearing and the mechanical strength. The DEM model in the membrane boundary condition reflects well the evolution of irregular sample deformation and shear band in the shearing process. From the perspective of micro structures, the normal force decreases and the tangential stress increases during the shearing stage. This study greatly improves the accuracy of DEM simulations of the triaxial test in the membrane boundary condition.     

DEM simulation of the shear behaviour of breakable granular materials with various angularities

Particle shape is an important factor that affects particle breakage and the mechanical behaviour of granular materials. This report explored the effect of angularity on the mechanical behaviour of breakable granular materials under triaxial tests. Various angular particles are generated using the quasi-spherical polyhedron method. The angularity α is defined as the mean exterior angle of touching faces in a particle model. A breakable particle is constructed as an aggregate composed of coplanar and glued Voronoi polyhedra. After being prepared under the densest conditions, all assemblies were subjected to triaxial compression until a critical state was reached. The macroscopic characteristics, including the shear strength and dilatancy response, were investigated. Then, particle breakage characteristics, including the extent of particle breakage, breakage pattern and correlation between the particle breakage and energy input, were evaluated. Furthermore, the microscopic characteristics, including the contact force and fabric anisotropy, were examined to probe the microscopic origins of the shear strength. As α increases, the peak shear strength increases first and then remains constant, while the critical shear strength generally increases. Assemblies with larger angularity tend to cause more serious particle breakage. The relative breakage is linearly correlated with α under shear loading. Compared with unbreakable particles, the peak shear strength and the critical volumetric strain decline, and the degree of decline linearly increases with increasing α.     

Simple shear simulation of 3D irregularly-shaped particles by image-based DEM

This paper describes an image-based DEM taking account of the irregular shape of solid particles in a direct manner. A micro X-Ray CT at SPring-8 (a synchrotron radiation facility in Japan) was employed to obtain high quality CT images for detecting correctly the shape of particles. The digitized 3D shape data of each particle was automatically obtained by an original image-processing technique. The particle shape was modeled by a cluster of several spherical elements using dynamic optimization method. The accuracy of the modeling can be controlled by the number of elements forming each particle. Using such modeled particles a series of simple shear simulations were performed for the specimens with various void ratios. It was found that 10-element model can quantitatively reproduce the shear behavior of relatively-dense specimens. On the other hand, in order to well simulate the packing structure and the shear behavior of loose specimens, 10-element model is found to be insufficient and more accurate model would be necessary. This result implies that the overall grain shape that is relevant to the moment transmission between grains is important in densely-packed granular assembly, while small surface angularity plays considerable role in loosely-packed granular assembly.     

DEM simulation on the one-dimensional compression behavior of various shaped crushable granular materials

In order to investigate the effects of particle shape on the compression behavior of granular materials, a series of simulations was conducted using a two-dimensional discrete element method employing moment springs. Fracturable granular assemblies were constructed from particles of the same shape and size. The range of possible particle shapes includes disk, ellipse and hexagon, with different aspect ratios. Simulations of single particle crushing tests on elliptical particles showed that crushing could be classified into three types: cleavage destruction, bending fracture and edge abrasion, depending on the manner of compression. A series of simulations of one-dimensional compression tests was then conducted on six types of crushable particle assemblies; the three types of crushing mentioned above were also observed, but their rates of occurrence depended on the particle shape. Cleavage destruction was mainly observed with circular and elliptical particles; bending fracture was observed only with elongated particles; edge abrasion was frequently observed with angular particles. Despite the difference in crushing type, all samples, when subjected to intense compression, converged to a critical grading with unique void ratio, grain size distribution and aspect ratio, with a similar distribution of number of contact points.     

Study of the shear behavior of binary granular materials by DEM simulations and experimental triaxial tests

This paper aims at studying the shear behavior of mixtures of fine and coarse particles by classical triaxial tests. The work is performed both on experimental tests and computer simulations by discrete element method. The comparisons between experimental and simulation results on monosized and binary samples show that the DEM model can reproduce deviatoric curves satisfactorily in experimental conditions. The shear behavior of monosized and binary systems with the same initial void ratio differs significantly, suggesting that the state of compaction of the system is more influential than the initial void ratio. Comparison between compacted and uncompacted samples confirms that compaction increases the shear strength of granular matter. At the particle scale, the coordination number decreases with the augmentation of the volume fraction of coarse particles. The average rotation velocity of fine particles is higher than coarse particles, but their particle stress tensor is smaller than coarse ones.     

Examining the mechanisms of sand creep using DEM simulations

In this study, DEM simulations of triaxial creep tests on dense and loose sand samples were carried out to examine the micromechanics involved during creep. The simulated creep responses reproduce qualitatively the published experimental results. During the primary creep, the creep stress is gradually borne by the contact normal forces instead of contact tangential forces so that the columnar particle structures can be formed. This process also leads to a continuous decrease in the creep rate. The columnar structures eventually are completely formed and the creep rate reaches a minimum. However, the structures become meta-stable and susceptible to buckling. This explains why a sand packing does not show an extended period of secondary creep in the experiment. Buckling of the columnar structures also gives rise to maximum dilatancy and a sharp transition of the major fabric orientation of weak forces from horizontal to vertical. The continuous buckling process of columnar structures increases the creep rate and sliding ratios of contacts during the tertiary creep. In addition, the trend of contact tangential forces decreasing and contact normal forces increasing is reversed. Finally creep rupture occurs as the creep stress–strain line intersects the complete stress–strain curve. All the creep samples follow their original volume-change tendency to continue their dilation or contraction response during creep.     

DEM of triaxial tests on crushable sand

This paper presents simulations of high-pressure triaxial shear tests on a crushable sand. The discrete element method is used, featuring a large number of particles and avoiding the use of agglomerates. The triaxial model features a flexible membrane, therefore allowing realistic deformation, and a simple breakage mechanism is implemented using the octahedral shear stress induced in the particles. The simulations show that particle crushing is essential to replicate the realistic behaviour of sand (in particular the volumetric contraction) in high-pressure shear tests. The general effects of crushing during shear are explored, including its effects on critical states, and the influence of particle strength and confining pressure on the degree of crushing are discussed.     

DEM of triaxial tests on crushable cemented sand

Using the discrete element method, triaxial simulations of cemented sand consisting of crushable particles are presented. The triaxial model used features a flexible membrane, allowing realistic deformation to occur, and cementation is modelled using inter-particle bonds. The effects of particle crushing are explored, as is the influence of cementation on the behaviour of the soil. An insight to the effects that cementation has on the degree of crushing is presented.     

DEM simulation of the packing of cylindrical particles

Granular Matter - Frustration arises for a broad class of physical systems where confinement (geometric) or the presence of a perturbation (kinematic) prevents equilibration to a minimum energy...     

天然气水合物合成实验

为提高天然气水合物的生产效率及储气密度,在专门设计的水合物合成实验装置上,进行了纯甲烷水合物的合成实验。实验结果表明:对于纯净甲烷水合物,压力越高,合成速率越大;但当压力大于5 MPa时,压力的提高对生成速率的影响不大。水合物合成前抽真空时间越长,生成的水合物吸收的气体量越大,表明抽真空可以排出水中溶解的气体,提高水合物的储气密度。     

Quantifying the effects of elongation and flatness on the shear behavior of realistic 3D rock aggregates based on DEM modeling

It is significant for industrial production and engineering practice to study the macro and micromechanical behaviors of realistic particles in nature. Based on the rock aggregates database obtained by 3D scanning, this study investigated the effect of particle shape on the shear behaviors of particles by discrete element method (DEM) modeling. First, 1200 rock particle models were acquired by white-light scanning, and the elongation index (EI) and flatness index (FI) of the 1200 particles were calculated. After initial dense samples were created for particles with specific EI and FI values by the isotropic compression method, all the samples were sheared in drained triaxial compression tests under a quasi-static condition. Then, the mechanical behaviors of the samples at the peak and critical states were analyzed. Meanwhile, the evolution of internal mechanical behaviors during the shearing of samples with different EI and FI values was evaluated. Finally, through the analysis of the stress-force-fabric relationship, the underlying mechanism explaining why the macroscale mechanical behaviors of samples were dominated by particle shape was revealed from the perspective of fabric anisotropy.     

Micro-scale modeling of anisotropy effects on undrained behavior of granular soils

This paper presents a micro-scale modeling of fabric anisotropy effects on the mechanical behavior of granular assembly under undrained conditions using discrete element method. The initial fabrics of the numerical samples engendered from the deposition under gravity are measured, quantified and compared, where the gravitational field can be applied in different directions to generate varying anisotropy orientations. The samples are sheared under undrained biaxial compression, and identical testing conditions are applied, with samples having nearly the same anisotropy intensities, but with different anisotropy directions. The macroscopic behaviors are discussed for the samples, such as the dilatancy characteristics and responses at the critical state. And the associated microstructure changes are further examined, in terms of the variables in the particulate scale, with the focus on the fabric evolution up to a large deformation reaching the critical state. The numerical analysis results compare reasonably well with available experimental data. It is also observed that at critical state, in addition to the requirements by classical critical state theory, a unique fabric structure has also been developed, and might be independent of its initial fabric. This observation is coincided with the recent theoretical achievement of anisotropic critical state theory. Finally, a general framework is introduced for quantifying and modeling the anisotropy effects.     

Thermal behavior and molecular simulation of liquid crystalline polymers containing a pentamethylenic spacer

The paper presents a study concerning the thermal stability and molecular simulation of some aromatic polyethers, containing a pentamethylenic spacer. The polymers were synthesised using a phase transfer catalysis technique, starting from 1,5-dichoropentane and different bisphenols: 4,4^′-dihidroxyazobenzene, 4,4^′-dihidroxydiphenyl and bisphenol A. For the investigated polymers the molecular simulation was performed prior the synthesis in order to predict the possibility of liquid crystalline behavior. Molecular simulation was also used as a complementary analysis method for a better understanding of the thermal behavior. Thermogravimetric analysis, in static air atmosphere, with a heating rate of 10 °C/min, was used. Cerius² and Hyperchem programs were used to perform the molecular simulations. All the polymers present a good thermostability with weight loss being up to 300 °C. The kinetic characteristics suggest a complex degradation mechanism, based on successive reactions. The inter-chain interaction estimated using the polar surface and the chain conformation do not significantly influence the polymer thermostabilities. A comparison between simulated and experimental values of the isotropisation temperature and temperature corresponding to 50% weight loss was performed.     

Simulating macroscopic behavior of self-compacting mixtures with DEM

Development of proper rheological models and suitable numerical methods are necessary for a thorough understanding of the basic flow properties of fresh mortar or concrete. Main challenge for models is to find a quantitative correlation between the model parameters and the properties and proportions of the mix ingredients. This paper presents a modeling approach for the rheological behavior of fresh self-compacting mixtures using a Discrete Element Method (DEM). The employed method is based on a conceptual idea where the grain-paste-grain interactions are explicitly described as an interactive two-phase paste-bridge system. Each mixture is considered to be an assembly of mutually interacting “grain-paste-grain” systems which can be characterized according to the mix composition with help of the “excess paste theory”. Macroscopic slump flow predictions are evaluated by laboratory tests. Simulations and experimental test results show good agreement.     

DEM simulation of diametrical compression test on particle compounds

A 2 Dimensional discrete element analysis is carried out with diametrical stressing condition to understand the fracture behaviour of particle compounds. The new surface generation and particle size distributions are also analysed to study an efficiency of the crushing system. Concrete spheres of 150 mm diameter with properties of B35 (35 N/mm² compressive strength) are chosen to represent particle compounds. The paper discusses the discrete element approach for crack propagation analysis and their correlations in particle compounds.