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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.
The extent to which the evolution of instabilities and failure across multiple length scales can be reproduced with the aid of a bifurcation analysis is examined. We adopt an elastoplastic micropolar constitutive model, recently developed for dense cohesionless granular materials within the framework of thermomicromechanics. The internal variables and their evolution laws are conceived from a direct consideration of the dissipative mechanism of force chain buckling. The resulting constitutive law is cast entirely in terms of the particle scale properties. It thus presents a unique opportunity to test the potential of micromechanical continuum formulations to reproduce key stages in the deformation history: the development of material instabilities and failure following an initially homogeneous deformation. Progression of failure, initiating from frictional sliding and rolling at contacts, followed by the buckling of force chains, through to macroscopic strain softening and shear banding, is reproduced. Bifurcation point, marking the onset of shear banding, occurred shortly after the peak stress ratio. A wide range of material parameters was examined to show the effect of particle scale properties on the progression of failure. Model predictions on the thickness and angle of inclination of the shear band and the structural evolution inside the band, namely the latitudinal distribution of particle rotations and the angular distributions of contacts and the normal contact forces, are consistent with observations from numerical simulations and experiments. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
3.
用三维梁-颗粒模型BPM3D(beam-particlemodelinthreedimensions)对岩石类非均质脆性材料的力学性质和破坏过程进行了数值模拟。梁-颗粒模型是在离散单元法基础上,结合有限单元法中的网格模型提出的用于模拟岩石类材料损伤破坏过程的数值模型。在模型中,材料在细观层次上被离散为颗粒单元集合体,相邻颗粒单元由有限单元法中的弹脆性梁单元联结。梁单元的力学性质均按韦伯(Weibull)分布随机赋值,以模拟岩石类材料力学参数的空间变异性。材料内部裂纹通过断开梁单元来模拟。通过自动生成的非均质材料模型对岩石类材料的破坏机理进行研究。岩石类非均质脆性材料在单轴压缩状态下破坏过程细观数值模拟结果显示,岩石材料宏观破坏是由于其内部细观裂纹产生、扩展、贯通的结果。通过数值模拟结果之间的对比分析,揭示出岩石试样宏观破坏模式随细观层次上韦伯分布参数的变化而不同。与实际矿柱破坏形态的对比分析表明了模型的适用性。根据数值模拟结果对岩石类非均质材料的破坏机理进行了探讨。 相似文献
4.
This paper describes a three-dimensional random network model to evaluate the thermal conductivity of particulate materials. The model is applied to numerical assemblies of poly-dispersed spheres generated using the discrete element method (DEM). The grain size distribution of Ottawa 20–30 sand is modeled using a logistic function in the DEM assemblies to closely reproduce the gradation of physical specimens. The packing density and inter-particle contact areas controlled by confining stress are explored as variables to underscore the effects of micro- and macro-scales on the effective thermal conductivity in particulate materials. It is assumed that skeletal structure of 3D granular system consists of the web of particle bodies interconnected by thermal resistor at contacts. The inter-particle contact condition (e.g., the degree of particle separation or overlap) and the particle radii determine the thermal conductance between adjacent particles. The Gauss–Seidel method allows evaluation of the evolution of temperature variation in the linear system. Laboratory measurements of thermal conductivity of Ottawa 20–30 sand corroborate the calculated results using the proposed network model. The model is extended to explore the evolution of thermal conduction depending on the nucleation habits of secondary solid phase as an anomalous material in the pore space. The proposed network model highlights that the coordination number, packing density and the inter-particle contact condition are integrated together to dominate the heat transfer characteristics in particulate materials, and allows fundamental understanding of particle-scale mechanism in macro-scale manifestation. 相似文献
5.
A smoothed particle hydrodynamics (SPH) framework for three-dimensional dynamic soil-multibody interaction modeling is presented, where both soils and rigid bodies are discretized using SPH particles. In the framework, soils are modeled using the Drucker-Prager model, while rigid bodies are considered with a multibody dynamics solver. A hybrid contact method suitable for three-dimensional simulations is developed to model the soil-body and body-body frictionless and frictional contacts, where contact forces are calculated based on ideal plastic collision and the unit normal/tangential vectors of the actual surface. Owing to its simplicity in contact detection and accuracy in contact force calculation, the hybrid contact method can be easily incorporated into SPH. Furthermore, graphics processing unit (GPU) parallelization is utilized to improve efficiency. The presented numerical framework and the hybrid contact method are validated using several examples. Numerical results are compared with analytical solutions and results from the literature. Furthermore, two three-dimensional simulations involving dynamic soil-multibody interaction are included to demonstrate the application. 相似文献
6.
Irregularly shaped (IRS) particles widely exist in many engineering and industrial fields. The macro physical and mechanical properties of the particle system are governed by the interaction between the particles in the system. The interaction between IRS particles is more complicated because of their complex geometric shape with extremely irregular and co‐existed concave and convex surfaces. These particles may interlock each other, making the sliding and friction of IRS particles more complex than that of particles with regular shape. In order to study the interaction of IRS particles more efficiently, a refined method of constructing discrete element model based on computed tomography scanning of IRS particles is proposed. Three parameters were introduced to control the accuracy and the number of packing spheres. Subsequently, the inertia tensor of the IRS particle model was optimized. Finally, laboratory and numerical open bottom cylinder tests were carried out to verify the refined modeling method. The influence of particle shape, particle position, and mesoscopic friction coefficient on the interaction of particles was also simulated. It is noteworthy that with the increase of mesoscopic friction coefficient, the fluidity of IRS particle assembly decreases, and intermittent limit equilibrium state may appear. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
7.
采用细观数值模拟方法研究散粒体的锚固效应,基于随机模拟技术生成三维多面体颗粒及其在空间中的分布,在随机散粒体不连续变形模型的基础上将砾石锚固试验进行数值实现,分析加锚散粒体材料的宏观与细观力学性能,研究加锚密度及其与颗粒粒径的关系对散粒体力学性质的影响,并探讨锚杆在散粒体材料中的作用机制。分别建立不同锚杆间距和不同颗粒粒径的数值试样,数值模拟结果表明：散粒体锚固数值试验能够较好地反映不同加锚散粒体结构的变形规律与锚固效应;散粒体材料的宏观特性与其细观组构的演化密切相关;锚杆加固散粒体的作用机制为加锚散粒体内形成压缩区,挤压加固作用提高了散粒体间的接触作用力,散粒体结构的整体性得到加强并能承受一定荷载;不考虑锚杆长度的情况下,当锚杆间距小于3倍的散粒体平均粒径时,锚杆能够有效地加固散粒体形成稳定结构。 相似文献
8.
黏土的变形主要受土颗粒之间的联结方式控制,将土颗粒之间的联结方式分为完善联结和滑动联结。在弹性变形阶段,颗粒之间的联结为完善联结,随着剪应力的增大,骨架中一部分完善联结逐渐变成滑动联结,这种转变即为损伤的演化。骨架的损伤和破坏遵循Mohr-Coulomb准则,在p-q平面中以应力点到初始损伤线和破坏线的相对距离表示损伤比,给出了一种描述骨架损伤和计算损伤演化的方法,进而提出了一种描述黏土在不排水条件下剪切变形的损伤本构模型。模型中的参数可根据常规三轴压缩试验确定,模型的形式简单,可适用于复杂的应力路径情况。对试验结果的拟合表明,该模型能较好地反映黏土在不排水条件下的剪切变形特征。 相似文献
9.
多块体形状堆石体碾压颗粒破碎数值模拟 总被引：1，自引：0，他引：1
通过6种典型堆石块体的形状近似,分别采用两种接触本构模型建立了多块体形状堆石体离散元数值模型,研究堆石体在碾压荷载作用下的颗粒破碎过程,建立颗粒破碎的量化计算方法,分析碾压前、后堆石级配曲线的变化,讨论接触本构模型和颗粒形状对块体破碎的影响。模拟结果显示,碾压荷载下堆石颗粒以张拉破碎为主,随着碾压遍数的增加,局部开始出现剪切破碎;提出的颗粒破碎量化计算方法,在大粒径范围对粒径变化幅度预测偏大,但级配曲线整体趋势与实测结果比较吻合;相比较于接触连接模型,平行连接模型与现场碾压试验结果更接近。6种块体形状的数值结果显示,随着形状系数的增加,在其他条件不变的情况下颗粒破碎率逐渐降低,其中类长方形块体在碾压荷载作用下颗粒破碎最明显。相比较于纯圆颗粒或者单一非圆颗粒,采用的6种颗粒形状建立的堆石体振动碾压离散元模型,更接近现场实际情况。 相似文献
10.
在工程荷载范围内,不计骨架颗粒的变形,骨架的变形实际是颗粒接触面变形的总和。当剪应力达到某个临界值时,黏性土骨架中有一部分颗粒接触面开始滑动,随着剪应力的增大,出现滑动的颗粒接触面的取向范围也会扩大。将这种颗粒接触面的滑动视为一种损伤。在损伤阶段,黏性土骨架中既有未滑动的接触面,也有已滑动的接触面,宏观剪切模量是这两种接触面的剪切模量的加权平均。在 平面中,根据应力圆与颗粒起始滑动包络线的相对位置,计算出已出现了滑动的颗粒接触面的取向范围,并定义该取向范围与其所能达到的最大值（由破坏时的应力圆计算）之比为骨架的损伤比。按损伤比进行加权平均得到骨架的整体剪切模量。模型中的参数完全可以根据常规三轴试验确定,模型的形式简单,可适用于复杂的应力路径。对试验结果的拟合表明,该模型能较好地反映黏性土在固结不排水条件下剪切变形的主要特征。 相似文献
11.
颗粒物质是具有复杂力链网络的常见体系,其平衡和稳定性取决于内部力链骨架的几何结构和力学性能。为恰当刻画颗粒体系的微细观力学行为,从颗粒局部排列形态、颗粒间接触力和颗粒及外荷载的物理参数出发,展开基于二维数字图像相关技术（2D-DIC）的集中力作用下颗粒体系变形的试验。通过结合DIC的试验方法,应用牛顿力学理论及颗粒线性动量平衡获得颗粒间接触力的大小和方向,确定力链的动力学规律;得到颗粒间接触力大小分布变化并提取颗粒体系力链;提出颗粒排布的两种状态：横向传力和纵向传力状态;分析颗粒接触面法线与水平轴的夹角?、传力角?,讨论了?、? 对力链断裂破坏的影响;分析颗粒直径d与外荷载压条直径D的比值R以及压条下表面能直接接触的最大颗粒数N等参数,为描述力链的演化和破坏重构机制提供参考基础;简单阐述了力链网络在外荷载下的微细观统计效应对体系宏观力学性质的影响规律。 相似文献
12.
A coupled 3‐dimensional bonded discrete element and lattice Boltzmann method for fluid‐solid coupling in cohesive geomaterials 
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下载免费PDF全文 This paper presents a 3D bonded discrete element and lattice Boltzmann method for resolving the fluid‐solid interaction involving complicated fluid‐particle coupling in geomaterials. In the coupled technique, the solid material is treated as an assembly of bonded and/or granular particles. A bond model accounting for strain softening in normal contact is incorporated into the discrete element method to simulate the mechanical behaviour of geomaterials, whilst the fluid flow is solved by the lattice Boltzmann method based on kinetic theory and statistical mechanics. To provide a bridge between theory and application, a 3D algorithm of immersed moving boundary scheme was proposed for resolving fluid‐particle interaction. To demonstrate the applicability and accuracy of this coupled method, a benchmark called quicksand, in which particles become fluidised under the driving of upward fluid flow, is first carried out. The critical hydraulic gradient obtained from the numerical results matches the theoretical value. Then, numerical investigation of the performance of granular filters generated according to the well‐acknowledged design criteria is given. It is found that the proposed 3D technique is promising, and the instantaneous migration of the protected soils can be readily observed. Numerical results prove that the filters which comply with the design criteria can effectively alleviate or eliminate the appearance of particle erosion in dams. 相似文献
13.
In modelling particulate media, such as soils, using 3-D DDA, there are four types of contacts: sphere to sphere, sphere to boundary face, sphere to boundary edge, and sphere to boundary corner contacts. The first two were studied by the authors in a previous work (Beyabanaki and Bagtzoglou 2012). In this paper, we present a new contact model for sphere-boundary edge and sphere-boundary corner contacts in sphere-based 3-D DDA. The model includes a new algorithm to search for contacts, detect the contact types and calculate contact points. Moreover, formulas for contact sub-matrices are derived. The proposed contact model has been implemented into a sphere-based 3-D DDA program and three test cases are studied in order to verify the workability of the new contact model. The numerical results obtained demonstrate the capability of the model to deal with sphere-boundary interaction in particulate media. 相似文献
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15.
基于三维颗粒离散单元法,赋予颗粒相应的细观参数,并采用黏结发生在接触颗粒间有限范围内的模型来考虑冻土颗粒中冰的胶结作用,建立了冻结黏土三维离散元数值模型.在相同围压、不同温度和相同温度、不同围压下对冻结黏土的室内三轴试验进行数值模拟,对比了数值试验与室内测试的应力-应变曲线,两者吻合较好.数值模拟结果表明:围压增大会使得接触黏结逐渐失效,在剪切带中胶结冰的破坏区域将增大,而温度的降低则会产生相反结果,这些微观变化都将对冻结黏土的宏观力学变形产生较大影响,同时,细观参数对温度的依赖性也很明显.冻结黏土三轴试验微观变形离散元模拟思路及方法可为今后运用离散单元法研究冻土力学行为提供一定的参考. 相似文献
16.
堆石料流变的主要机理是由于水位变化、降雨入渗、日晒雨淋等环境因素导致堆石料发生性质劣化,与此同时,颗粒发生高接触应力-破碎和重新排列-应力释放、调整和转移,这一过程由于颗粒的持续劣化而不断重复。在考虑颗粒破碎的堆石体不连续变形分析方法(SGDD)中,引入颗粒强度劣化模型反映颗粒强度随外界环境的持续劣化。应用该方法进行堆石料三轴流变数值试验,模拟结果与室内试验所观察到的规律一致,表明考虑流变效应的SGDD方法抓住堆石料流变的主要机理,适合模拟堆石料的流变变形这一复杂的、非线性演化问题。数值试验结果表明,堆石料随外界环境的劣化程度、劣化速率、母岩强度对宏观流变变形有较大影响。 相似文献
17.
A method is proposed to calculate the distribution of energy during the quasi-static confined comminution of particulate assemblies. The work input, calculated by integrating the load-displacement curve, is written as the sum of the elastic deformation energy, the breakage energy and the redistribution energy. Experimental results obtained on samples subjected to compression stresses ranging between 0.4 and 92 MPa are used to calibrate the model. The elastic energy stored in the samples is obtained by simulating the compression test on the final particle size distributions (PSDs) with the discrete element method and by extracting the contact forces. A PSD evolution law is proposed to account for particle breakage. The PSD is related to the total particle surface in the sample, which allows calculating the breakage energy. The redistribution energy, which comprises the kinetic energy of particles being rearranged and the friction energy dissipated at contacts, is obtained by subtracting the elastic energy and breakage energy from the work input. Results show that: (1) at least 60% of the work input is dissipated by particle redistribution; (2) the fraction of elastic deformation energy increases, and the fraction of redistribution energy decreases as the compression stress increases; (3) the breakage energy accounts for less than 5% of the total input energy, and this value is independent of the compressive stress; (4) the energy dissipated by redistribution is between 14 and 30 times larger than the breakage energy. 相似文献
18.
Fu Longlong Zhou Shunhua Guo Peijun Tian Zhekan Zheng Yuexiao 《Acta Geotechnica》2021,16(5):1527-1545
Loads transfer in ballast track through contacts among randomly distributed ballast particles and have strong heterogeneity. Since the size ratio between ballast track and ballast particles is generally small, using averaged stress to describe the internal mechanical state in ballast track faces practical difficulties. For example, particle movements and high local concentration stress tend to be ignored. The inter-particle contact stress is crucial to evaluate the particle behaviors, such as abrasion, movements, and furtherly the performance of ballast track. However, the contact stress on ballast particles is hard to predict or measure. We conduct a full size model test to investigate the dynamic characteristics of longitudinal stress on ballast particles as well as different lateral regions under vertical cyclic loads with various loading magnitudes and frequencies. An obvious seesaw effect of longitudinal contact stress is observed: the stresses at some contact areas have the same phase with applied cyclic load while at other contact areas have an opposite phase. The seesaw effect of contact stress is then used to evaluate the rotational movements of ballast particles. The variation of contact area and stress of the ballast particles with loading magnitudes demonstrates that the rigid contact assumption is appropriate when analyzing the contact behavior of ballast particles. The cumulative probability distribution of contact stress with stress level can be described by an inversely proportional function, based on which the maximum contact stress can be estimated according to the longitudinal average stress. Besides, the lateral dispersion angle of the vertical loads in the ballast track is about 35°, which is independent of the given loading magnitudes and frequencies.
相似文献19.
A robust contact theory can be regarded as key to three dimensional discontinuous deformation analyses (3D-DDA). Not only must this theory provide an efficient algorithm to judge the type and location of contacts but also it must be able to present comprehensive formulations for every kind of contact (open, sliding and locked contact). There are six types of contact in three dimensional discontinuous deformation analyses (vertex-to-vertex, vertex-to-edge, vertex-to-face, edge-to-edge, edge-to-face and face-to-face) that can be converted to vertex-to-face and edge-to-edge contacts. This paper presents a new model of edge-to-edge contact to three dimensional discontinuous deformation analyses (3D-DDA). This new model considers both kinds of edge-to-edge contact (cross-over and parallel edge-to-edge contact) and presents a criterion for inter-penetration. Sub matrices of normal and shear spring and friction force are derived by geometrical analysis and penalty method. This new model is implemented in a 3D-DDA computer programme, and the example results demonstrate the validity of the model. 相似文献
20.
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