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.     

Influence of relative density on granular materials behavior: DEM simulations of triaxial tests

The rheological behavior of non-cohesive soils results from the arrangement and complex geometry of the grains. Numerical models based on discrete element modeling provides an opportunity to understand these phenomena while considering the discrete elements with a similar shape to that of the grains the soil is composed of. However, dealing with realistic shapes would lead to a prohibitive calculation cost. In a macroscopic modeling approach, simplification of the discrete elements’ shape can be done as long as the model can predict experimental results. Since the intrinsic non-convex geometry property of real grains seems to play a major role on the response of the granular medium, it is thus possible to keep this geometrical feature by using cluster of spherical discrete elements, which has the advantage to reduce dramatically the computation cost. Since the porosities found experimentally could not always be obtained with the numerical model—owing to the huge difference in shape, the notion of relative density, which requires a search for minimum and maximum porosities for the model, was chosen to compare the experimental and numerical results. Comparing the numerical simulations with the experimental triaxial tests conducted with relative densities and different confining pressures shows that the model is able to predict the experimental results.     

Experimental and numerical study of cylindrical triaxial test on mono-sized glass beads under quasi-static loading condition

This paper aims at studying the shear behavior of homogeneous granular materials by conventional triaxial test. The work is performed both in laboratory tests and by discrete element method simulations. Conventional triaxial tests are performed on glass beads packing, while a cylindrical rigid wall boundary condition based on lame formula and a series of procedures are proposed to simulate the conventional triaxial test. The experimental results on dry and saturated glass beads samples have been studied to find out the effect of saturation condition on the shear behavior. The comparisons between experimental and numerical results show that the numerical model can reproduce deviatoric curves satisfactorily in experimental conditions as long as experimental sample remains cylindrical. It correctly describes the volumetric strains of a numerical sample up to the peak value. Additionally, a parametric study on the influence of main micromechanical parameters has been carried out, which has been compared to experimental tests with glass beads of different textures. The comparison highlights the significant effect of friction coefficients and rolling resistance coefficients on global behavior of granular materials.     

Analysis of the influence of crushing on the behavior of granular materials under shear

The behavior of granular materials mainly depends on the mechanical and engineering properties of particles in its structural matrix. Crushing or breakage of granular materials under compression or shear occurs when the energy available is sufficient to overcome the resistance of the material. Relatively little systematic research has been conducted regarding how to evaluate or quantify particle crushing and how it effects the engineering properties of the granular materials. The aim of this study is to investigate the effect of crushing on the bulk behavior of granular materials by using manufactured granular materials (MGM) rather than using a naturally occurring cohesionless granular material. MGM allow changing only one particle parameter, namely the “crushing strength”. Four different categories of MGM (with different crushing strength) are used to study the effect on the bulk shear strength, stiffness modulus, friction and dilatancy angle “engineering properties”. A substantial influence on the stress–strain behavior and engineering properties of granular materials is observed. Higher confining stress causes some non-uniformity (strong variations/jumps) in volumetric strain and a constant volumetric strain is not always observed under large shear deformations due to crushing, i.e. there is no critical state with flow regime (with constant volumetric strain).     

Forces in piles of granular material: an analytic and 3D DEM study

We investigate the stress distribution at the base of a conical sandpile using both analytic calculations and a three dimensional discrete element code. In particular, we study how a minimum in the normal stress can occur under the highest part of the sandpile. It is found that piles composed of particles with the same size do not show a minimum in the normal stress. A stress minimum is only observed when the piles are composed of particles with different sizes, where the particles are size segregated in an ordered, symmetric, circular fashion, around the central axis of the sandpile. If a pile is composed of particles with different sizes, where the particles are randomly distributed throughout the pile, then no stress dip is observed. These results suggest that the stress dip is due to ordered, force contacts between equiheight particles which direct stress to the outer parts of the pile. Received: 14 June 2000     

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.     

The effects of tensile residual stress and sliding boundary on measuring the adhesion work of membrane by pull-off test

The adhesion work of flexible film, such as membrane, is always measured by pull-off test. In this kind of experiment, the tensile residual stress due to pretension during sample preparation and the sliding of the punch have a critical influence on the experimental results. Our study aims to examine the effects of tensile residual stress and sliding boundary. The analytical expressions for the effect parameters are obtained based on the energy criterion. From our quantitative analysis, we find that these effects should be taken into account in the future experiments.     

On the validity condition of the chi-squared test in 2×2 tables

A 2×2 contingency table is usually analyzed by using the chi-squared asymptotic test, with Yates' continuity correction (c=n/2, wheren is the total size of sample). This correction is the correct one when the chi-squared test is an approximation to Fisher's exact (conditional) test. When the chi-squared test is used as an approximation to Barnard's exact (unconditional) test for comparing two independent proportions (two samples of sizen ₁ andn ₂) or for contrasting independence (one sample with a size ofn), the correctionc is different (c=1 ifn ₁≠n ₂ orc=2 ifn ₁=n ₂ in the first case;c=0.5 in the second). Whatever the case, it is traditional to affirm that the asymptotic test is valid whenE>5, whereE is the minimum expected quantity. Today it is recognized that this condition is too general and may not be appropriate. In the case of Yates' correction. Martín Andrés and Herranz Tejedor (2000) proved that the validity condition must be of the typeE>E ^* —whereE ^* is a known function depending on the marginals of the table—and that checking the validity of the asymptotic test is equivalent to checking the asymmetry of the base statistic (a hypergeometric random variable). In the present article the authors prove that this argument is valid for the other two continuity corrections, and moreover, that the valueE ^* is obtained for all three cases. Given that the functionE ^* reaches an absolute maximum, it can be affirmed that the three chisquared tests referred to are valid whenE<19.2, 14.9 or 6.2 (orE>8.1, 7.7 or 3.9 ifn≤500) respectively for the three previous models (although for the first model and the two-tailed testE>0 is sufficient). This research was supported by the Dirección General de Investigación (Spain). Grant BFM2000-1472.     

三维编织复合材料弹性性能数值预测及细观应力分析

基于考虑纤维束面接触的细观结构模型, 引入周期性位移边界条件, 采用细观有限元方法建立了材料的弹性性能预报模型。模型数值结果与试件实测数据吻合较好, 证明了该模型的合理有效性。经详细分析单胞在典型工况载荷作用下的细观应力分布及变形, 表明模型体现了周期性相邻单胞表面力和位移的连续性, 能获得单胞更为合理的细观应力应变场。      

On the Cauchy principal value of the surface integral in the boundary integral equation of 3D elasticity

A simple demonstration of the existence of the Cauchy principal value (CPV) of the strongly singular surface integral in the Somigliana Identity at a non-smooth boundary point is presented. First a regularization of the strongly singular integral by analytical integration of the singular term in the radial direction in pre-image planes of smooth surface patches is carried out. Then it is shown that the sum of the angular integrals of the characteristic of the tractions of the Kelvin fundamental solution is zero, a formula for the transformation of angles between the tangent plane of a suface patch and the pre-image plane at smooth mapping of the surface patch being derived for this purpose.     

Study of the origin of shear zones in quasi-static vertical chute flows by using discrete particle simulations

We perform discrete-particle simulations of vertical chute flows in the quasi-static regime using disk-like particles. The velocity profiles show a plug flow in the central region and shear zones next to the walls approximately 6 particle diameters wide regardless of bin width, as was observed experimentally. The stress distributions are in good agreement with the predictions of the continuum mechanics equations even for small systems (15 and 20 particle diameters wide) as those studied in the present work. Large stress fluctuations in space and time have been observed, these are mainly due to the inhomogeneity of the force network. It is observed in the simulations that the wall friction does not act homogeneously but it is concentrated at certain points on the wall depending on the local arrangement of the packing. Large stress zones or arches appear at these points of the wall. It is the formation and the way these arches collapse that seems to generate the shear zones. Based on this, a simple mechanism to explain the formation of the shear zone is proposed. The simulations have revealed other interesting features of the flow, particularly the presence of macroscopic fluctuations of velocity, in which large blocks of material move together showing sudden accelerations (corresponding to the collapse of the arches) and sudden decelerations (corresponding to the formation of the arches).     

Investigation of anti-plane shear behavior of a Griffith permeable crack in functionally graded piezoelectric materials by use of the non-local theory

In this paper, the non-local theory of elasticity is applied to obtain the behavior of a Griffith crack in functionally graded piezoelectric materials under the anti-plane shear loading for the permeable electric boundary conditions. To make the analysis tractable, it is assumed that the material properties vary exponentially with coordinate vertical to the crack. By means of the Fourier transform, the problem can be solved with the help of a pair of dual-integral equations that the unknown variable is the jump of the displacement across the crack surfaces. These equations are solved by use of the Schmidt method. Numerical examples are provided. Unlike the classical elasticity solutions, it is found that no stress and electric displacement singularities are present near the crack tips. The non-local elastic solutions yield a finite hoop stress at the crack tips, thus allows us to using the maximum stress as a fracture criterion. The finite hoop stresses at the crack tips depend on the crack length, the functionally graded parameter and the lattice parameter of the materials, respectively.     

3D numerical simulation of the behaviour of a spherical particle suspended in a Newtonian fluid and submitted to a simple shear

The 3D flow around a rigid spherical particle suspended in a Newtonian fluid and submitted to simple shear is numerically studied using Rem3D^® finite element code. The sphere motion is imposed by a sticking contact between the sphere and the fluid. The effect of the particle size as compared with the finite dimension of the shear cell was investigated. The direct calculations show that 3D modelling is necessary to correctly predict the sphere behaviour. The proximity of the particle and the cell walls strongly affects the flow velocities, the sphere motion (increase of the rotation period of the sphere) and the stress field (change of orientation angle and increase of maximal local stresses).     

3,4-乙撑二氧噻吩的原位化学氧化受限聚合动力学研究

利用紫外-可见光谱,分别研究了3,4-乙撑二氧噻吩在45、65和85℃条件下的原位化学氧化受限聚合动力学规律。结果表明,温度的升高可以非常显著地加快单体的聚合速度,反应体系在45、65和85℃下完成聚合反应的时间分别为600、240和8min。温度对聚合反应的进程也有较显著的影响,在相对较低温度下（45和65℃）,反应进程基本相同,都有比较明显的3个阶段（反应开始、稳定反应和反应终了阶段）,而在相对较高温度下（85℃）,反应进程则更简单,只有快速反应阶段和反应终了阶段。这些动力学数据可以为3,4-乙撑二氧噻吩在某些应用领域如固体电容器制备工艺中确定聚合温度和时间等工艺条件提供参考依据。     

陶瓷微滤膜洗涤碱法制备Al(OH)3中杂质Na+的研究

采用无机陶瓷微滤膜,以错流过滤方式对工业上碱法生产的Al(OH)3粉体浆料进行Na^ 离子的清洗过滤研究．实验考察了操作压力、错流速率、浆料浓度等工艺参数的影响,确立了适合于超细粉体膜清洗过滤的工艺条件．经过陶瓷膜洗涤过滤过程,Al(OH)3粉体粒子仍保持其原始形貌,但粉体的分散性和纯度获得大幅度提高,膜清洗后的Al(OH)3粉体中Na2O含量降为57mg／kg;粒径尺寸约为50nm．研究结果表明,膜清洗过滤效率、操作方式以及产品质量等方面均明显优于目前工业生产上普遍应用的板框压滤机,为无机陶瓷微滤膜技术应用于碱法制备Al(OH)3工艺中的洗涤过滤提供了科学与工程化依据．