Triaxial Test Simulations with Discrete Element Method and Hydrostatic Boundaries

A new boundary condition has been developed for the discrete element method. This boundary is different from the conventional periodic, rigid, or flexible boundries. This new boundary mechanism was developed to simulate triaxial tests. The new boundary, hydrostatic boundary, simulated the chamber fluid but not the rubber membrane. When a particle (ellipsoids in our simulations) contacts the hydrostatic boundary, pressure is developed. The interaction between the particle and the boundary is calculated analytically based on geometry. This hydrostatic boundary condition was implemented into an existing ellipsoidal discrete element code. Triaxial compression drained tests were performed with both periodic and hydrostatic boundaries. The result showed an increase in friction angle over the values observed from the periodic boundary mechanism. The result also closely resembles the experimental triaxial data. Thirteen specimens were generated and were used to investigate the following variables: particle shape, specimen size, and void ratio. A unique slope of the linear relationship between friction angle and void ratio was identified for monosize specimens of different particle shapes. It is found that the friction angle decreases as the aspect ratio increases provided that the void ratio of the two specimens is the same. The friction angle is linear proportional to the coordination number for monosize specimens regardless the specimen size. Also, the specimen size does not influence the behavior of two-size specimens.     

Fabric Evolution of Ellipsoidal Arrays with Different Particle Shapes

Fabric evolution of ellipsoidal arrays under drained triaxial compression loading was obtained by the discrete element method. The ellipsoidal arrays (specimens) consist of 520 identical ellipsoidal particles. The effect of particle shape on fabric evolution was studied by using four different aspect ratios (major∕minor axis of the ellipsoidal particle) varying between 1.2 and 2. In the simulation, a large axial strain up to 80% was applied to the specimen. In addition to the typical macroscopic results, the fabric (microscopic information) of the specimen at certain stages was presented. The fabric includes the distribution of particle orientations and the distribution of branch vectors and normal vectors. Two distribution indicators quantified these distributions and one distribution indicator was identified to have direct variation with the principal stress ratio.     

Ultimate State of Samples of Ellipsoids under Different Stress Paths

The ultimate state of granular material was investigated by the discrete element method. This paper discusses a series of numerical triaxial tests on samples of two kinds of ellipsoids. Samples of three different densities were loaded along various stress paths. The ellipsoids were mixed in the proportions 50:50 by weight. The longer particles have an aspect ratio (major length/minor length) of 1.5 and the other particles have an aspect ratio of 1.2. The samples were generated by deposition under gravity to simulate the air-pluviation sample preparation technique. Then they were consolidated isotropically. Fourteen stress-path controlled triaxial tests were conducted numerically for each sample. They are sheared to the ultimate state to determine the friction angle and void ratio at the ultimate state. When samples of different densities were sheared along the same loading path, a unique ultimate state was observed. The initial density does not affect the ultimate state. However, when different loading paths are applied to the same numerical specimen, they do not always arrive at the same critical state in the void-mean stress space.     

Tests on Seismically Damaged Reinforced Concrete Structural Walls Repaired Using Fiber-Reinforced Polymers

This paper presents the results of an experimental study on the seismic performance of axially loaded reinforced concrete (RC) walls with boundary elements confined by limited transverse reinforcement. These specimens were initially subjected to axial compression loading and cyclic lateral loading to failure, and subsequently repaired and subjected to loading again. The test specimens include two low-rise walls of aspect ratio 1.125 and two medium-rise walls of aspect ratio 1.625. Results show that significant drift capacities were achieved from the strengthened walls. The performance of the repaired walls was similar to the original walls before repair in terms of the flexural behavior, shear strength, and ductility capacities. While the fiber-reinforced polymer (FRP) anchorage may undergo premature failure, it however failed only after the peak lateral strength of the repaired wall was attained. This paper demonstrates that repair of damaged RC walls using FRP is able to restore the performance of damaged RC walls while also serving as repair method of relative ease.     

Contraction, Dilation, and Failure of Sand in Triaxial, Torsional, and Rotational Shear Tests

The response of a saturated fine sand (Nevada sand No. 120) with relative density Dr ≈ 70% in drained and undrained conventional triaxial compression and extension tests and undrained cyclic shear tests in a hollow cylinder apparatus with rotation of the stress directions was studied. It was observed that the peak mobilized friction angle for this dilatant material was different in undrained and drained tests; the difference is attributed to the fact that the rate of dilation is smaller in an undrained test than it is in a drained test. Consistent with the findings of others, the material is more resistant to undrained cyclic loading for triaxial compression than for triaxial extension. In rotational shear tests in which the second invariant of the deviatoric stress tensor is held constant, the shear stress path (after being normalized by the mean normal effective stress) approached an envelope that is comparable but not identical in shape to a Mohr-Coulomb failure surface. As the stress path approached the envelope, the shear end deviatoric strains continued to increase in an unsymmetrical smooth spiral path. During the rotational shear tests, the direction of the deviatoric strain-rate vector (deviatoric strain increment divided by the magnitude of change in Lode angle) was observed to be about midway between the deviatoric stress increment vector and the normal to a Mohr-Coulomb failure surface in the deviatoric plane. The stress ratio at the transition from contractive to dilative behavior (i.e., “phase transformation”) was also observed to depend on the direction of the stress path; therefore this stress ratio is not a fundamental property. Results from torsional hollow cylinder tests with rotation of stress directions are presented in new graphical formats to help understand and interpret the fundamental soil behavior.     

Stress Path Testing of an Anisotropic Sandstone

The Berea sandstone used in this study is transversely isotropic with respect to elastic response, with P-wave velocities of 2,160?m/s normal to bedding and 2,290?m/s parallel to bedding, a variation of only 6%. Triaxial compression and extension tests involving failure by loading and unloading were performed along the two directions of symmetry. With axial stress applied parallel to bedding, the internal friction angle was approximately 55° for compression and extension, indicating no intermediate stress effect for the linear Mohr-Coulomb criterion. However, for axial stress normal to bedding, the friction angle in compression was 50°, whereas in extension it was 44°. This anomalous behavior was attributed to strength anisotropy of the sandstone.     

Influence of Stress Ratio and Stress Path on Behavior of Loose Decomposed Granite

This paper presents results from four series of triaxial compression tests of loosely compacted decomposed granite (DG) or silty sand on both isotropically and anisotropically consolidated specimens. These tests included undrained tests, drained tests with constant deviator stress, and a decreasing mean effective stress path. The silty sand possessed high compressibility during isotropic compression. The observed high compressibility is probably attributed to the loose soil structure created by using the moist tamping method and the presence of crushable feldspar in the soil. Static liquefaction behavior and the so-called “reversed” sand behavior were observed in all undrained tests. This “reversed” sand behavior can be readily explained by the high compressibility of DG leading to the nonparallel and converging nature of the initial state line and the critical state line. Preshearing resulted in a more brittle response in the postpeak behavior. The higher the initial stress ratio (ηc), the smaller the ductility. Structural collapse of DG was observed. This collapse is characterized by a sudden large increase in both the axial and contractive volumetric strains. The mobilized angles of friction at collapse range from 31.8° to 38.7°, which are smaller than the critical state angle (?col′), but higher than the mobilized friction angle of the instability line (28.1°) determined by the isotropically consolidated undrained tests. A trilinear approximate relationship can be found between ?col′ and ηc and a liquefaction potential index is introduced to provide a simple preliminary design parameter for static liquefaction and instability prone slopes.     

Evolution of Sand Microstructure during Shear

Quantitative measurements of the local void ratio distribution are used to demonstrate how the microstructure throughout dilatant triaxial specimens of uniform fine quartz sand evolves during drained axial compression loading. Shear-induced increases in the mean of the local void ratio distribution initiate at the center of the specimen and migrate toward the ends of the specimen as axial strain increases. At any given strain, the mean of the local void ratio distribution is largest near the center of the specimen, reflecting the influence of end platen and membrane restraining effects. The results provide direct quantitative microstructure-based evidence that global or macro response, as conventionally used in interpreting specimen behavior, can be misleading as to the true material response. Implications of the test results on practical issues such as the location of local strain measurement systems are noted.     

Effect of Particle Shape on Interface Behavior of DEM‐Simulated Granular Materials

This paper presents a detailed computational investigation of the effect of particle shape on the interface shear behavior of granular materials. The discrete element method (DEM) using clusters to model rough particles is used, expanding the procedure introduced in an earlier paper by Jensen et al. [1]. Seven new cluster shapes (i.e., particle configurations) of varying degrees of roughness are presented herein, and numerical experiments simulating ring shear tests are made using these clusters. From these simulations, the effect of particle shape on void ratio (e) and interface angle of friction between soil and structure surface (δ) is reported. Particle shape characteristics include roundness, angularity, and surface roughness. The results of numerical simulations using the newly formed cluster shapes are in very good qualitative agreement with laboratory tests. Simulation results showed that the void ratio of a particle mass increased as the angularity or roughness of the particles increased. They also showed an increase in interface shear strength between perfectly round DEM particles and the more angular cluster shapes, but no systematic correlations with the various definitions of particle shape parameters was found. It may be necessary to use greater accuracy in modeling the size and shape distributions of a natural medium to further investigate the influence of particle shape on interface friction. The simulations also successfully reflected the relationship between interface friction angle and structure surface roughness as demonstrated in recent physical experiments. The simulations comparing initially “dense” media to initially “loose” media demonstrated behavior that is similar to the behavior of a natural sandy soil observed in experiments.     

Sintered copper–graphite powder compacts forindustrial applications

Abstract

Studies were made on copper/graphite based powders and sintered compacts for industrial applications. The dependence of particle shape on friction in the powder mass, compression ratio, and electrical receptivity of powder metallurgy components was studied using near spherical precipitated copper powders and angular or flakelike powders generated by mechanical comminution. Results reveal that powders with particles that are nearly spherical in shape have lower friction, lower compression ratios, and higher electrical resistivities in sintered compacts than powders with acicular or flakelike particles. Also, the effects produced by the small additions of lead and zinc (up to 2·5 wt-%) on the electrical resistivity and hardness of sintered copper–graphite compacts are also presented, and the influence of variation of briguetting pressure is discussed.     

Effect of Particle Grading on the Response of an Idealized Granular Assemblage

The effects of particle-size distribution on a granular assemblage’s mechanical response were studied through a series of numerical triaxial tests using the three-dimensional (3D) discrete-element method. An assemblage was formed by spherical particles of various sizes. A simple linear contact model was adopted with the crucial consideration of varying contact stiffness with particle diameter. Numerical triaxial tests were mimicked by imposing axial compression under constant lateral pressure and constant volume condition, respectively. It was found that an assemblage with a wider particle grading gives more contractive response and behaves toward strain hardening upon shearing. Its critical state locates at a lower position in a void ratio versus mean normal stress plot. Nevertheless, no obvious difference in the critical stress ratio was shown. Model constants in a simple but efficient phenomenologically based granular material model within the framework of critical-state soil mechanics were calibrated from the numerical test results. Results show that some model constants exhibit linear variation with the coefficient of uniformity whereas others are almost independent of particle grading. This investigation provides an opportunity to better understand the implications and meanings of model constants in a phenomenologically based model from the microscale perspective.     

Examination of the Response of Regularly Packed Specimens of Spherical Particles Using Physical Tests and Discrete Element Simulations

Significant insight into the response of granular materials can be gained by coupling accurately controlled physical tests with complementary discrete element simulations. This paper discusses a series of triaxial and plane strain laboratory compression tests on steel spheres with face-centered-cubic and rhombic packings, as well as discrete element simulations of these tests. The tests were performed on specimens of uniform-sized steel balls and on specimens of steel balls with specified distributions of ball diameters. The packing configurations are ideal and differ considerably from real sand specimens, however, studies of such idealized granular materials can yield considerable insight into the response of granular materials and the capability of discrete element simulations to capture the response. The differences in response for the two packing configurations considered illustrate the importance of fabric. The numerical simulations captured the observed laboratory response well if the particle configurations, particle sizes, and boundary conditions were accurately represented. However, the postpeak response is more difficult to capture, and it is shown to be sensitive to the coefficient of friction assumed along the specimen boundaries. The simulations of the tests on the nonuniform-sized specimens demonstrated a clear correlation between strength and coordination number.     

Effect of Microfabric on Shear Behavior of Kaolin Clay

The influence of geometric arrangement of platelets (microfabric) on the mechanical behavior of kaolin clay is investigated using lubricated end triaxial testing on solid cylindrical specimens. A series of compression and extension tests under drained and undrained conditions were performed on clay specimens with different microfabric for overconsolidated ratio values of 1 and 10. The solid cylindrical specimens with dispersed and flocculated microfabric were produced in the laboratory using slurry consolidation technique under K0 condition. Based on the experimental observations, it is evident that microfabric strongly affects the mechanical behavior of kaolin clay, such as its stress–strain relationship, effective stress ratio, shear strength, excess pore-pressure evolution, and volumetric response. The influence of confining pressure on clay specimens with dispersed and flocculated microfabric is also studied in this research. This study shows that the microfabric can change the basic nature of clay. For example the normally consolidated kaolin clay shows its dilative nature during shearing for dispersed microfabric and contractive nature for flocculated microfabric.     

Fabric Study of Granular Materials after Compaction

Numerous micromechanical models have been developed based on assemblies of spherical particles with certain fabric distributions. Most of these distributions are hypothetical, and only very few of them can be determined experimentally. This paper presents a study to provide some useful fabric information for granular material. The discrete element method is used to study the microscopic information for granular materials after compaction. Specimens with 520 identical ellipsoidal elements are generated and compressed under different conditions. Up to six different aspect ratios are used to study their effect on the compression process. Two different compression methods and five different microfrictions between particles are used. The fabric of the specimens after compaction, including the total number of contacts, the distribution of particle orientations, the distribution of branch vectors, the distribution of the length of branch vectors, and the spatial distribution of a similar length of branch vector, is presented. The relations between these fabrics and particle shape, microfriction, and the compression process are also developed.     

Triaxial Compression of Sand Reinforced with Fibers

Results from drained triaxial compression tests on specimens of fiber-reinforced sand are reported. It is evident that the addition of a small amount of synthetic fibers increases the failure stress of the composite. This effect, however, is associated with a drop in initial stiffness and an increase in strain to failure. Steel fibers did not reduce initial stiffness of the composite. The increase in failure stress can be as much as 70% at a fiber concentration of 2% (by volume) and an aspect ratio of 85. The reinforcement benefit increases with an increase in fiber concentration and aspect ratio, but it also depends on the relative size of the grains and fiber length. A larger reinforcement effect in terms of the peak shear stress was found in fine sand, compared to coarse sand, when the fiber concentration was small (0.5%). This trend was reversed for a larger fiber concentration (1.5%). A model for prediction of the failure stress in triaxial compression was developed. The failure envelope has two segments: a linear part associated with fiber slip, and a nonlinear one related to yielding of the fiber material. The analysis indicates that yielding of fibers occurs well beyond the stress range encountered in practice. The concept of a macroscopic internal friction angle was introduced to describe the failure criterion of a fiber-reinforced sand. This concept is a straightforward way to include fiber reinforcement in stability analyses of earth structures.     

Investigation of iron ore particle size and shape on green pellet quality

ABSTRACT

Particle size and shape of iron ore concentrate are effective parameters in the production of quality green pellets. In this research, the effect of particle morphology on green pellet quality was studied. It was concluded that pellet quality improved with increasing specific surface area. Drop number and green compression strength of pellets ground by HPGR were found to be superior over those ground in the ball mill. The chief reasons were related to particle shape and the fraction of fine particles. Smaller particle size results in a higher order of bonding between particles and therefore the formation of a stronger system. Also, the rougher and less circular shape of particles resulted in higher strength. According to the breakage model in the particle-moisture system, pellet failure occurs because of crack propagation in the solid–liquid interface. The effects of roughness on capillary force and wettability have been explained mathematically. In addition, the effects of roughness and aspect ratio on breakage path and increase of required energy for pellet failure have been modelled.     

烧结矿颗粒DEM参数的测定与标定

摘要：为了获取烧结矿颗粒在离散元法中所涉及的关键运动参数,通过实验确定了不同粒径范围的颗粒密度、弹性模量、泊松比、弹性恢复系数以及静摩擦因数。基于Hertz Mindlin接触模型,通过实验和离散单元法仿真对比堆积的静止角,得到了适用于球形颗粒模拟状态下,烧结矿的滚动摩擦因数。结果表明：烧结矿密度、弹性模量及颗粒间动摩擦因数与粒径大小近似正线性相关;颗粒间弹性恢复系数与静摩擦因数随粒径增大逐渐减小。通过转鼓实验,对比了混合粒径状态下模拟和实验形成的自由表面,表明实验结果与计算结果吻合较好,误差在5%以内,进而表明所测离散单元参数准确程度较高,可用于后续烧结矿布料及余热竖罐内下移的模拟研究。     

The effect of post-injury spinal position on canal occlusion in a cervical spine burst fracture model

STUDY DESIGN: The canal space of burst-fractured, human cervical spine specimens was monitored to determine the extent to which spinal position affected post-injury occlusion. OBJECTIVE: To test the null hypothesis that there is no difference in spinal canal occlusion as a function of spinal positioning for a burst-fractured cervical spine model. SUMMARY OF BACKGROUND DATA: Although previous studies have documented the effect of spinal positioning on canal geometry in intact cadaver spines, to the authors' knowledge, none has examined this relationship specifically in a burst fracture model. METHODS: Eight human cervical spine specimens (levels C1 to T3) were fractured by axial impact, and the resulting burst injuries were documented using post-injury radiographs and computed tomography scans. Canal occlusion was measured using a custom transducer in which water was circulated through a section of flexible tygon tubing that was passed through the spinal canal. Any impingement on the tubing produced a rise in fluid pressure that was monitored with a pressure transducer. Each spine was positioned in flexion, extension, lateral (and off-axis) bending, axial rotation, traction, and compression, while canal occlusion and angular position were monitored. Occlusion values for each position were compared with measurements taken with the spine in neutral position. RESULTS: Compared with neutral position, compression, extension, and extension combined with lateral bending significantly increased canal occlusion, whereas flexion decreased the extent of occlusion. In extension, the observed mechanism of occlusion was ligamentum flavum bulge caused by ligament laxity resulting from reduced vertebral body height. CONCLUSIONS: Increased compression of the spinal cord after injury may lead to more extensive neurologic loss. This study demonstrated that placing a burst-fractured cervical spine into either extension or compression significantly increased canal occlusion as compared with occlusion in a neutral position.     

崩落法采矿中放出体流动特性的影响因素

基于颗粒元理论和PFC3D程序,构建具有矿石散体细观力学性质的放矿模型,开展崩落法采矿中放出体流动特性影响因素研究.运用统计学知识确定对放出体流动特性有显著影响的主要因素及其敏感性,得出其与放出体流动特性的关系,并通过已有研究结论与模拟结果的对比分析,验证了基于PFC3D程序的放矿模型在放出体流动特性影响因素研究中的适宜性与可靠性.研究表明:放出体颗粒形状、摩擦系数及放矿口尺寸三种因素是显著影响崩落矿岩流动特性的重要参数.在放矿初始阶段,放矿口尺寸对放出体形态影响最大,其次为摩擦系数,颗粒形状对其影响最小;在之后的放矿过程中,颗粒形状对放出体形态影响最大,其次为放矿口尺寸和摩擦系数.散体颗粒形状越不规则、散体内摩擦角越大以及放矿口尺寸越小则放矿越困难.      

Behavior of Compacted Soil-Fly Ash-Carbide Lime Mixtures

Unconfined compression tests, Brazilian tensile tests, and saturated drained triaxial compression tests with local strain measurement were carried out to evaluate the stress-strain behavior of a sandy soil improved through the addition of carbide lime and fly ash. The effects of initial and pozzolanic reactions were investigated. The addition of carbide lime to the soil-fly ash mixture caused short-term changes due to initial reactions, inducing increases in the friction angle, in the cohesive intercept, and in the average modulus. Such improvement might be of fundamental importance to allow site workability and speeding construction purposes. In addition, under the effect of initial reactions, the maximum triaxial stiffness occurred for specimens molded on the dry side of the optimum moisture content, while the maximum strength occurred at the optimum moisture content. After 28 days, pozzolanic reactions magnified brittleness and further increased triaxial peak strength and stiffness; the maximum triaxial strength and stiffness occurred on the dry side of the optimum moisture content.