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1.
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. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
2.
为了研究颗粒棱角对颗粒材料力学行为的影响,建立了具有不同棱角度的对称多面体颗粒,采用了一种简单并适合任意颗粒形状的接触本构模型,对三维离散元开源程序YADE进行了修改,研究了颗粒棱角度在模拟直剪试验中的影响以及接触力各向异性在剪切过程中的演化规律。研究结果表明,颗粒棱角度越小,颗粒间相互咬合自锁的作用越小,颗粒受剪更易转动,致使颗粒体系的剪切强度和剪胀性下降;竖向加载力越大,颗粒棱角度的影响越明显;法向接触力的各向异性在剪切过程中表现为先增后减最后趋向稳定的趋势;法向接触力的各向异性变化程度随颗粒棱角度的增大而增大。 相似文献
3.
An algorithm for detecting inter-ellipsoid contacts 总被引：1，自引：0，他引：1
An inter-ellipsoid contact detection algorithm was developed and used for simulating the behaviour of assemblies of ellipsoid-shaped particles using the well-known discrete element method (DEM). The contact algorithm was implemented in the modified version of the DEM program TRUBAL originally written to simulate the behaviour of assemblies of spheres. The modified program was used to perform deviatoric and axisymmetric compression tests on a 1000 prolate spheroid assembly in periodic space. The obtained stress–strain–dilation curves conform with the experimental evidence both qualitatively and quantitatively. 相似文献
4.
The paper provides an in-depth exploration of the role of particle crushing on particle kinematics and shear banding in sheared granular materials. As a two-dimensional approximation, a crushable granular material may be represented by an assembly of irregularly shaped polygons to include shape diversity of realistic granular materials. Particle assemblies are subjected to biaxial shearing under flexible boundary conditions. With increasing percentage of crushed particles, mesoscale deformation becomes increasingly unstable. Fragmented deformation patterns within the granular assemblies are unable to form stable and distinct shear bands. This is confirmed by the sparsity of large fluctuating velocities in highly crushable assemblies. Without generating distinct shear bands, deformation patterns and failure modes of a highly crushable assembly are similar to those of loose particle assemblies, which are regarded as diffuse deformation. High degrees of spatial association amongst the kinematical quantities confirm the key role that non-affine deformation and particle rotation play in the generation of shear bands. Therefore, particle kinematical quantities can be used to predict the onset and subsequent development of shear zones, which are generally marked by increased particle kinematic activity, such as intense particle rotation and high granular temperature. Our results indicate that shear band thickness increases, and its speed of development slows down, with increasing percentage of crushed particles. As particles crush, spatial force correlation becomes weaker, indicating a more diffuse nature of force transmission across particle contacts. 相似文献
5.
In this paper, a lattice-type model to simulate the micro-mechanical behaviour of particulate/granular media is presented. In this numerical model, a particulate assembly is simulated as a lattice/truss. Nodes located at contacts between a particle and its neighbours are linked by bars to each other. Each particle is represented by a lattice within its microstructure and particle interact through load transfer at the nodes. Constraints are prescribed at the nodes to describe active, deactivated and reactivated contacts. When a particulate assembly develops into a mechanism (deformation with zero incremental load), further deformation is simulated through a framework that describes the kinematics of the particles (sliding, rolling and rotation of particles). This framework is formed by introducing nodes at the particle centroids and linking them with bars. Bars-linking particles with a non-sliding contact are assigned large stiffnesses relative to bars linking particles with a sliding contact. Numerical tests are conducted on two-dimensional assemblies of disks, arranged as very loose and very dense packing under simple shear loading conditions. The results concord with the results of numerical tests conducted using the discrete element method and with photoelastic experiments. Additionally, the model is applied to study the effects of initial imperfections caused by particles with low elastic modulus. Copyright © 1999 John Wiley & Sons, Ltd. 相似文献
6.
This paper introduces a new generator algorithm and computer program for 3-D numerical simulation of packing configuration in a granular assemblies composed of ellipsoidal particles of different a/b aspect ratios. Each ellipsoidal particle is approximated by the revolution of an ellipse, formed by four connected arcs, about the major axis passing through its centroid. The centroid co-ordinates, major axis direction and lengths of the major and minor axes are the essential data for the packing generation and associated contact detection. The domain to be filled with particles can be a polyhedron of any shape. The packing program was coded based on a newly proposed scheme which obeys the no interpenetration kinematics of solid bodies. New contact detection algorithms for any two ellipsoids in the packing space were developed. Though simple, these algorithms effectively determine the contact condition and contact point without solving the simultaneous equations of the two ellipsoidal surfaces. Each particle's packing location, contact-point co-ordinates, and three-dimensional graphs can be created using the packing domain given boundaries, along with numbers, and geometrical information of particles to be generated. Simulation results show that this new algorithm provides an effective packing model as a required initial input for analysing the mechanics of granular material. This generation scheme potentially can explore the complex 3-D behaviours of material composed of discrete particles. Copyright © 1999 John Wiley & Sons, Ltd. 相似文献
7.
The local pore spaces in granular materials tend to be aligned parallel to the major principal stress direction upon particle mobilization. Manifestation of this response has been numerically validated in our previous studies with the aid of discrete element method modeling and image processing techniques during creep and shearing. We now extend the modeling of pore geometry, constructed with spherical particles, to assemblies of particle clumps. Two-dimensional simulations are performed for both loose and dense assemblies of spherical particles and particle clumps. Each particle packing is bound by rigid or flexible walls and subjected to biaxial compression and the particle mobilization effect on the evolution of pore orientation is explored. Randomly shaped pores surrounded by adjacent particles are geometrically quantified by Delaunay tessellation and fitted with ellipses. Results show that localization is apparent in dense assemblies, in particular for clumped particle packing, while loose assemblies exhibit diffusive failure. Small pores within well-defined shear bands tend to align either parallel to the direction of the shear band or perpendicular to the major principal stress. On the other hand, small pores within the blocks and large pores have a tendency to become elongate towards the major principal stress direction. This study reveals for the first time that pore orientation is dependent upon particle shape, pore size, and assembly conditions on the pore and global scales. 相似文献
8.
Macroscopic and microscopic behaviors of granular materials under proportional strain path: a DEM study 
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下载免费PDF全文 Under the proportional strain loading path, particle assemblies may exhibit various failure modes. Besides the strain localization, the diffuse failure may also occur under certain conditions. The diffuse failure mode corresponds to a homogeneous occurrence of failure with stress states strictly included within the plastic limit condition. This paper emphasizes the influences of the density degree and the rolling resistance under the strain path. A contact model considering rolling friction is adopted in a discrete element method analysis as an approximate means to account for the effects of particle shape. Mechanical responses indicate that loose assemblies without the rolling resistance are more vulnerable to static liquefaction. A sample with a smaller initial void ratio or larger rolling friction coefficient will reinforce the stability of the structure and reduce the likelihood of failure. For microscopic properties, the evolution of coordination numbers, contact forces, force chains and the anisotropies of the assemblies are explored and discussed. Rotational resistance helps increase the shear stress of the granular material, and the microscopic parameters indicate that the assembly has a strong anisotropy and a stable structure to resist the increasing loading. 相似文献
9.
In this paper, numerical simulation of 3-dimensional assemblies of 1000 polydisperse sphere particles using Discrete Element
Method (DEM) is used to study the liquefaction behaviour of granular materials. Numerical simulations of cyclic triaxial shear
tests under undrained conditions are performed at different confining pressures under constant strain amplitude. Results obtained
in these numerical simulations indicate that with increase in confining pressure there is an increase in liquefaction resistance. 相似文献
10.
大位移剪切下钙质砂破碎演化特性 总被引：1，自引：0，他引：1
为了揭示钙质砂在大位移剪切作用下的破碎及形状演化规律,对南海钙质砂进行了系列不同剪切位移下的环剪试验。首先,利用筛分和激光粒度分析获取试验后的颗粒粒径分布,分析颗粒分布变化情况;其次,通过粒径分布对破碎进行定量分析;最后,运用图像处理技术计算颗粒圆度和扁平度,分析了颗粒形状的变化情况。试验结果表明,在不同的竖向压力下,颗粒会达到不同的稳定级配,但达到稳定所需的剪切位移相同;经历大位移剪切后,出现粒径为0.01～0.075 mm钙质砂破碎严重的现象;随剪切位移的增加,颗粒的圆度和扁平度减小。针对细小颗粒破碎严重的现象,修正了相对破碎率;修正后的相对破碎率能考虑粒径为0.01～0.075 mm颗粒发生的破碎。剪切后的钙质砂颗粒更为规则,整体轮廓趋于圆形、表面更光滑。 相似文献
11.
Micromechanical Modelling of Granular Materials: Effect of Particle Size and Gradation 总被引：4，自引：0，他引：4
In this paper the effects of maximum particle size, particle gradation/sorting and fabric on bulk mechanical behaviour of granular materials such as coarse grained soils and rockfills are investigated" from micromechanical considerations starting from the grain scale level, using numerical" simulations based on Discrete Element Modelling (DEM). Hydrostatic compaction and biaxial tests on 2-dimensional assemblies of discs with varying particle sizes and gradations were modelled using DEM. An examination of the constitutive behaviour of granular media considering" the particulate nature of the medium has been attempted to explain the effect of particle size and gradation. Simulation results on perfectly parallel graded assemblies indicate that with increase in the size of the particles, a marginal increase (or no increase) in the angle of internal friction is observed during biaxial loading conditions. A change to a wider gradation (keeping the minimum grain size the same) results in a decrease in the angle of internal friction and an increase in volumetric strain to a considerable extent. Based on micromechanical force and fabric parameters, the basis for the physical behaviour was established. This helps in understanding the physics of parallel gradation techniques. 相似文献
12.
The behavior of crushable rockfill sheared along different stress paths is studied using the discrete element method. Rockfill particles are modeled as breakable agglomerates, and reasonable consistency is found between the predicted and experimental results. The simulation highlights the influence of agglomerate breakage on both the macro- and micro-behavior of the assembly. The evolution of both the number and mode of agglomerate breakage during shearing is significantly influenced by the confining pressure, deviator stress ratio, and loading direction. The relationship between the deviator stress ratio on the macro-scale and the deviator fabric on the micro-scale along different stress paths is explored. 相似文献
13.
软土预压工程中,初始和诱发各向异性对软土力学性质的影响十分显著,而现有研究缺乏对初始和诱发各向异性的统一研究方法。采用离散单元法,以颗粒长宽比作为定量评价指标,构建真实形态的颗粒模型,生成5组不同沉积角的初始各向异性试样,并进行竖直和水平两方向加载的双轴模拟实验,研究了初始各向异性和诱发各向异性对软土力学特性影响;在细观层面,以颗粒为对象研究了颗粒接触形式和转动角度的变化规律,以接触为对象研究了配位数和接触法向各向异性的发展趋势,在此基础上探究抗剪强度指标与各向异性关系。结果表明:初始和诱发各向异性共同影响试样力学性质,当加载方向和软土沉积方向垂直时,土体有最大的峰值强度。颗粒接触形式中面面接触的比例随加载的进行逐渐增大,并影响着试样初始模量和抗剪强度,配位数和接触法向各向异性受颗粒接触形式的影响有不同的演化规律,并在加载后期趋于稳定;同时,初始各向异性试样相较各向同性试样有更大的黏聚力,诱发各向异性主要影响试样内摩擦角,进而影响试样抗剪强度。 相似文献
14.
A discrete element modelling of bonded granulates and investigation on the bond effect on their behaviour are very important to geomechanics. This paper presents a two‐dimensional (2‐D) discrete element theory for bonded granulates with bond rolling resistance and provides a numerical investigation into the effect of bond rolling resistance on the yielding of bonded granulates. The model consists of mechanical contact models and equations governing the motion of bonded particles. The key point of the theory is that the assumption in the original bond contact model previously proposed by the authors (55th CSCE‐ASCE Conference, Hamilton, Ont., Canada, 2002; 313–320; J. Eng. Mech. (ASCE) 2005; 131 (11):1209–1213) that bonded particles are in contact at discrete points, is here replaced by a more reliable assumption that bonded particles are in contact over a width. By making the idealization that the bond contact width is continuously distributed with the normal/tangential basic elements (BE) (each BE is composed of spring, dashpot, bond, slider or divider), we establish a bond rolling contact model together with bond normal/tangential contact models, and also relate the governing equations to local equilibrium. Only one physical parameter β needs to be introduced in the theory in comparison to the original bond discrete element model. The model has been implemented into a 2‐D distinct element method code, NS2D. Using the NS2D, a total of 86 1‐D, constant stress ratio, and biaxial compressions tests have been carried out on the bonded granular samples of different densities, bonding strengths and rolling resistances. The numerical results show that: (i) the new theory predicts a larger internal friction angle, a larger yielding stress, more brittle behaviour and larger final broken contact ratio than the original bond model; (ii) the yielding stress increases nonlinearly with the increasing value of β, and (iii) the first‐yield curve (initiation of bond breakage), which define a zone of none bond breakage and which shape and size are affected by the material density, is amplified by the bond rolling resistance in analogous to that predicted by the original bond model. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
15.
A. Anandarajah 《Engineering Geology》1997,47(4):491-323
In an assembly of clay particles placed in a fluid, each particle is typically subjected to: (1) double-layer repulsive forces; (2) van der Waals attractive forces; and (3) contact mechanical forces. The study presented here concerns an approximate, quantitative analysis of clay suspensions, with considerations to the first two - the physico-chemical forces. Using recent theories to calculate the physico-chemical forces between two clay particles in an approximate model of an assembly, the equilibrium void ratio of a clay suspension is computed. The mechanical forces are ignored in the analysis. The results serve to verify the validity of physico-chemical theories employed and help interpret experimental data more fundamentally in terms of the system variables. 相似文献
16.
DEM study of fabric features governing undrained post-liquefaction shear deformation of sand 总被引：3，自引：3，他引：0
In an effort to study undrained post-liquefaction shear deformation of sand, the discrete element method (DEM) is adopted to conduct undrained cyclic biaxial compression simulations on granular assemblies consisting of 2D circular particles. The simulations are able to successfully reproduce the generation and eventual saturation of shear strain through the series of liquefaction states that the material experiences during cyclic loading after the initial liquefaction. DEM simulations with different deviatoric stress amplitudes and initial mean effective stresses on samples with different void ratios and loading histories are carried out to investigate the relationship between various mechanics- or fabric-related variables and post-liquefaction shear strain development. It is found that well-known metrics such as deviatoric stress amplitude, initial mean effective stress, void ratio, contact normal fabric anisotropy intensity, and coordination number, are not adequately correlated to the observed shear strain development and, therefore, could not possibly be used for its prediction. A new fabric entity, namely the Mean Neighboring Particle Distance (MNPD), is introduced to reflect the space arrangement of particles. It is found that the MNPD has an extremely strong and definitive relationship with the post-liquefaction shear strain development, showing MNPD’s potential role as a parameter governing post-liquefaction behavior of sand. 相似文献
17.
尺寸效应是岩石力学领域研究的难点与热点。本文利用自主研发的大尺寸刚性试验机,开展了最大样品尺寸为400 mm×400 mm×800 mm大理岩、闪长岩和凝灰岩的单轴抗压强度试验,从等高宽比条件下岩石强度的尺寸效应和不同高宽比条件下岩石强度变化两个方面开展了岩石强度尺寸效应研究,结果表明:相同高宽比条件下,岩石强度随样品尺寸的增大呈对数形式减小,且逐渐趋于定值;不同高宽比情况,岩石长宽为200 mm×200 mm,高度分别为200 mm,400 mm,600 mm和800 mm,岩石单轴抗压强度随高宽比的增加表现出先减小后增大的变化规律,对应的破坏形式表现为复杂劈裂、劈裂和剪切破坏。本文试验结果为丰富岩石尺寸效应研究提供了基础数据,同时对实际工程中岩体强度尺寸效应的修正具有一定参考价值。 相似文献
18.
Cemented coarse-grained alluvium is present in a vast area of Tehran city, Iran including its suburbs. This deposit consists of gravely sand to sandy gravel with some cobbles and is dominantly cemented by carbonaceous materials. In order to understand the mechanical behaviour of this soil, a series of triaxial compression tests were performed on uncemented, artificially cemented and destructured samples. Hydrated lime was used as the cementation agent for sample preparation to model the Tehran deposit. The tests were performed on cemented samples after an appropriate time for curing. The tests on cemented samples show that a shear zone appears as the shear stress approaches the peak shear strength. During shearing these samples undergo dilation at confining stress lower than 1000kPa. However, the uncemented and destructured samples show contraction during shearing. Peak shear strength is followed by strain softening for all cemented samples. The shear strength increases with increasing cement content but the influence of the cementation decreases as the confining stress increases. With increasing cementation the stress-strain behaviour of samples tend towards the behaviour expected of high-density soils. Test results indicate that the failure envelope for cemented samples is curved and not linear. 相似文献
19.
Acta Geotechnica - This study explored the characteristics of local responses in 2D assemblies of elliptical particles when subjected to shearing. A biaxial shearing system was designed for this... 相似文献
20.
This paper analyses the influence of grain shape and angularity on the behaviour of granular materials from a two‐dimensional analysis by means of a discrete element method (Contact Dynamics). Different shapes of grains have been studied (circular, isotropic polygonal and elongated polygonal shapes) as well as different initial states (density) and directions of loading with respect to the initial fabric. Simulations of biaxial tests clearly show that the behaviour of samples with isotropic particles can be dissociated from that of samples with anisotropic particles. Indeed, for isotropic particles, angularity just tends to strengthen the behaviour of samples and slow down either local or global phenomena. One of the main results concerns the existence of a critical state for isotropic grains characterized by an angle of friction at the critical state, a critical void ratio and also a critical anisotropy. This critical state seems meaningless for elongated grains and the behaviour of samples generated with such particles is highly dependent on the direction of loading with respect to the initial fabric. The study of local variables related to fabric and particle orientation gives more information. In particular, the coincidence of the principal axes of the fabric tensor with those of the stress tensor is sudden for isotropic particles. On the contrary, this process is gradually initiated for elongated particles. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献