Interparticle Collision of Particle Composites—Finite and Discrete Element Simulations

Three different cases of interparticle collisions were simulated to observe the fragmentation mechanisms of composite particles. It was found that the cracking mechanism in interparticle collisions is different from that observed in particle-wall collisions. By means of DEM simulations, it was observed that the primary interparticle bonds break in both tension and shear. Depending on the type of interparticle collisions, cracks and fragments are generated. In general, finer fragments are generated around the impact site of the particles and are surrounded by larger fragments. From DEM simulations, it was found for the most part that the impacting particles exhibit more damage than the stationary particles. The symmetrical loading showed almost the same amount of damage to both types of particles. The discontinuous nature of the material and the unsymmetrical loading caused unsymmetrical damages (crack patterns). In unsymmetrical loadings, the impacting particle was more damaged and had more cracks than the stationary particle.     

Relationship between static bending and compressive behaviour of particle-reinforced cement composites

Innovative particle-reinforced materials made of alumina particles and cement-based matrix were designed, manufactured and tested to evaluate the potential use of ceramic aggregates in concretes. These particle-reinforced composites were tested in three-point bending and uniaxial compression conditions to determine the influence of the shape and size of the ceramic inclusions, and the addition of silica fume on the mechanical properties. A specific methodology combining post-mortem observations with a statistical analysis of tensile failure stresses (average strength and Weibull modulus) was conducted to deduce the origin of failure for each cement-based composite (porosity or ceramic particles/matrix decohesion). A remarkable correlation is observed between bending failure stress level and the average strength measured under uniaxial compression loading. As main conclusion, addition of alumina particles in a mortar appears to strengthen or to weaken the composite depending on whether silica fume is used in the cementitious matrix.     

Micromechanical modelling of oval particulates subjected to bi-axial compression

One of the questions that still remain unanswered among researchers dealing with granular materials is how far the particle shape affects the micro-macroscopic features of granular assemblies under mechanical loading. The latest advances made with particle instrumentation allow us to capture realistic particle shapes and size distribution of powders to a fair degree of accuracy at different length scales. Industrial applications often require information on the micromechanical behaviour of granular assemblies having different particle shapes and varying surface characteristics, which still remains largely unanswered. Traditionally, simulations based on discrete element method (DEM) idealise the shape of individual particles as either circular or spherical. In the present investigation, we analyse the influence of particle shape on the shear deformation characteristics of two dimensional granular assemblies using DEM. We prepared the assemblies having nearly an identical initial packing fraction (dense), but with different basic shapes of the individual particles: (a) oval and (b) circular for comparison purposes. The granular assemblies were subjected to bi-axial compression test. We present the evolution of macroscopic strength parameters and microscopic structural/topological parameters during mechanical loading. We show that the micromechanical properties of granular systems are significantly influenced by the shape of the individual particles constituting the granular assemblies.     

DEM simulation and experimental validation for mechanical response of ellipsoidal particles under confined compression

The representation of non-spherical particles in discrete element method (DEM) has not been addressed adequately. Although the multiple sphere method (MSM) is the most popular approach to describe non-spherical particle shape, the validity of the MSM has not been established yet. The purpose of this study is to examine the validity and adequacy of the MSM. A uni-axial confined compression test was designed and set up to study the mechanical behaviour of an ellipsoidal granular assembly under vertical loading and the load transfer to the contacting boundary. Four levels of multi-sphere approximation for an axi-symmetric ellipsoidal particle were employed in DEM simulation to investigate the adequacy of multi-sphere approximation. A comparison on compression characteristics between the numerical and experimental results was made and discussed in this paper. Most of the compared physical properties showed reasonable agreement, indicating that capturing the key linear dimensions of a non-spherical particle may be sufficient to predict reasonable results. A small number of sub-spheres (say, N?≥?5) for representing an axi-symmetric ellipsoidal particle can give plausible results. However, the DEM simulations also produced a certain extent of discrepancy in loading stiffness with experiments. Plausible explanations are provided and require further investigation.     

Influence of a central straight crack on the buckling behaviour of thin plates under tension,compression or shear loading

Thin-walled structural components, such as plates and shells, are used in several aerospace, naval, nuclear power plant, pressure vessels, mechanical and civil structures. Due to their high slenderness, the safety assessment of such structural components requires to carefully assess the buckling collapse which can strongly limit their bearing capacity. For very thin plate, buckling collapse can occur under shear, compression or even under tension. In the latter case, fracture or plastic failure can also take place instead of elastic instability. In the present paper, the effects of a central straight crack on the buckling collapse of rectangular elastic thin-plates—characterized by different boundary conditions, crack length and orientation—under compression, tension or shear loading are analysed. Accurate FE numerical parametric analyses have been performed to get the critical load multipliers in such loading cases. Moreover the effect of crack faces contact is examined and discussed. Some useful conclusions related to the sensitivity to cracks of the buckling loads for thin plates, especially in the case of shear stresses, are drawn. Cracked plates under tension are finally considered in order to determine the most probable collapse mechanism among fracture, plastic flow or buckling and some failure-type maps are determined.     

Compressive strength of unidirectional and crossply carbon fibre/PEEK composites

The commonly accepted production methods of composite systems generally result in departure of the plies properties from transverse isotropy due to stresses acting during fibre—matrix bond formation. This anisotropy coupled with the composite structure affects compressive loading; the ultimate stresses as well as the direction, in- or out-of-plane, of kink propagation. A unidirectional and a crossply carbon fibre/PEEK composites were compression tested at ambient and elevated temperature as well as exposed to various chemical environments. Significant disruptions in fibre—matrix interface in the crossply composite were indicated. The compression tests showed that failure occurred through in-plane and out-of-plane fibre bucking and kinking in the unidirectional and crossply composites, respectively. Failure of the longitudinal plies in the crossply laminate occurred at significantly higher compression stress than for the unidirectional composite. Compressive failure mechanisms in unidirectional and multi-directional laminates are considered.     

Numerical study on compression processes of cohesive bimodal particles and their packing structure

In this study, the compression characteristics of bimodal cohesive particles were investigated using a discrete element method (DEM) simulation. The compression and packing processes were simulated under different conditions of size ratios of 1–4 and fine particle mixing ratios of 0–0.5. The cohesive force was expressed using the surface energy proposed by the Johnson-Kendall-Roberts (JKR) cohesion model having a surface energy of 0–0.2 J/m². The calculated results demonstrated that even in the case of cohesive particles, an increase in the particle size ratio reduced the void fraction of the powder bed during the packing and compression processes. In addition, it was found that the cohesive force decreased the contact number, especially the coarse-coarse contacts, although it had little impact on the void fraction. Our DEM simulations suggested that it is necessary to evaluate the contact numbers even under similar void fractions, which will be essential in the case of different material mixtures, such as all-solid-state batteries.     

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.     

Breakage behaviour of single rice particles under compression and impact

Grain breakage is mainly caused by impact and compression load in harvest and processing. At present, the mechanism of grain breakage under loading, especially the statistics of breakage characteristics, is not clear. The analysis of breakage process of single particle provides a foundation for the understanding of breakage mechanisms. This paper aims to examine breakage behaviour of a single rice particle under compression and impact experiments. Firstly, the equivalent diameter (D_p) and moisture content (MC) of rice particles were regarded as important factors that may affect breakage. Then, by performing quasi-static compression and dynamic impact experiments under different values of D_p and MC, the detailed compression failure force, rice strength, breakage modes, breakage probability, and the breakage probability models were analyzed comprehensively. Furthermore, breakage processes of rice particles under these two breakage experiments were compared and discussed. Finally, the Weibull distribution of the compression breakage characteristics, the “non-size effect” of compression and impact breakage, the tensile failure forms, velocity threshold of impact breakage and the close relationship between the breakage characteristics under impact and compression were mainly found. The findings are useful for providing guidance for the revelation of breakage mechanism and optimizing related agricultural equipment design.     

Percolating contact subnetworks on the edge of isostaticity

We search for a percolating, strong subnetwork of contacts in a quasi-statically deforming, frictional granular material. Of specific interest in this study is that subnetwork which contributes to the majority of the total deviator stress and is, or is on the edge of being, isostatic. We argue that a subnetwork derived from the minimal spanning trees of a graph—optimized to include as many elastic contacts as possible and which bear normal contact forces above a given threshold delivers such a network. Moreover adding the strong 3-force-cycles to the spanning tree introduces a level of redundancy required to achieve a network that is almost if not isostatic. Results are shown for assemblies of non-uniformly sized circular particles under biaxial compression, in two-dimensions: a discrete element (DEM) simulation of monotonic loading under constant confining pressure, and cyclic loading of photoelastic disks under constant volume.     

DEM investigations of failure mode of sands under oedometric loading

It will be practically useful to explore the evolutions of the failure modes of sand grains within a sand specimen subject to compression for the particle breakage research. This paper attempts to deal with this challenge by conducting a discrete element method (DEM) simulation study on oedometric compression of two kinds of sands (spherical and non-spherical particles). In this study, particle morphologies reconstructed by the spherical harmonic (SH) analysis were created using the agglomerate method, and the micro-parameters used to define the contact model and the properties of walls and balls were adopted based on the single particle crushing tests. The effects of particle shape on the crushing behavior of granular materials and on the evolutions of failure modes of sand grains were captured, and the experimental data was used to evaluate the feasibility and reliability of the proposed DEM modelling strategy. The simulation results show that particle shape affects not only the number, type and orientation of cracks but also the evolution of the particle failure modes. The failure mode of chipping is the most common way to crush for both spherical and non-spherical particles. The particles that have less aspect ratio, sphericity and convexity are more likely to experience the failure mode of comminution. These findings shed light on the key role of particle shape in the investigation of the failure mode of sand grains and facilitate a better understanding of grain-scale behavior of granular materials.     

Micromechanical behaviour of Si-based particulate assemblies: A comparative study using DEM and atomistic simulations

In this paper, we investigate the micromechanical behaviour of Si-based particulate systems subjected to tri-axial compression loading. The investigations are based on three-dimensional discrete element modelling (DEM) and simulations. At first, we compare the variation of mean compressive stress for a silicon assembly subjected to tri-axial compression, predicted at two different scales: at the particulate scale, using the DEM simulation (mono-dispersed particulates) and at the atomistic scale using the molecular dynamics (MD) simulation results for silicon mono-crystal reported by Mylvaganam and Zhang (2003) [K. Mylvaganam, L. Zhang, Key Eng. Mater. 233–236 (2003) 615–620]. Both the simulation methods considered the silicon assembly subjected to an identical (tri-axial) loading condition. We observed a good qualitative agreement between the DEM and MD simulation results for the mean compressive stress when the assembly was subjected to small volumetric strain. However, at large volumetric strain, the mean stress of the silicon assembly predicted from MD simulation did not scale-up with the DEM results. This discrepancy could be due to that MD simulation is only valid for particle contacts, which are independent of one another and does not consider the inherent ‘discrete’ nature of particulates and the induced anisotropy prevailing at particulate scale. The micromechanical behaviour of particulate assemblies strongly depends on the inherent discrete nature of the particles, their single-particle properties and the induced anisotropy during mechanical loading. At the second stage, using DEM, we present the evolution of macroscopic compressive stress and several micromechanical features for four cases of the commonly used Si based poly-dispersed particulate assemblies (Si, SiC,Si₃N₄ and SiO₂) under tri-axial compression loading. We also present the evolution of several other phenomena occurring at particulate scale, such as the energy dissipation characteristics due to sliding contacts and the features of fabric structures developed during mechanical loading. The study shows that the single-particle properties of the Si based assemblies considered here significantly affect the micromechanical behaviour of the assemblies and DEM is a powerful tool to get insights on the internal behaviour of discrete particulates under mechanical loading.     

Strength of non-spherical particles with anisotropic geometries under triaxial and shearing loading configurations

This paper presents a study on the macroscopic shear strength characteristics of granular assemblies with three- dimensional complex-shaped particles. Different assemblies are considered, with both isotropic and anisotropic particle geometries. The study is conducted using the discrete element method (DEM), with so-called sphero-polyhedral particles, and simulations of mechanical true triaxial tests for a range of Lode angles and confining pressures. The observed mathematical failure envelopes are investigated in the Haigh–Westergaard stress space, as well as on the deviatoric-mean pressure plane. It is verified that the DEM with non-spherical particles produces results that are qualitatively similar to experimental data and previous numerical results obtained with spherical elements. The simulations reproduce quite well the shear strength of assemblies of granular media, such as higher strength during compression than during extension. In contrast, by introducing anisotropy at the particle level, the shear strength parameters are greatly affected, and an isotropic failure criterion is no longer valid. It is observed that the strength of the anisotropic assembly depends on the direction of loading, as observed for real soils. Finally simulations on a virtual shearing test show how the velocity profile within the shear band is also affected by the grain’s shape.     

单螺栓修复对含冲击损伤碳纤维/环氧树脂复合材料层合板压缩承载能力的影响

开展了单钉修复对含冲击损伤碳纤维/环氧树脂复合材料层合板压缩承载能力影响的试验研究。测试了三种不同能量冲击后碳纤维/环氧树脂复合材料层合板的压缩承载能力及失效模式,测定了单螺栓对碳纤维/环氧树脂复合材料层合板压缩承载能力的修复效率,并借助数字图像相关技术(DIC)表征手段揭示了单螺栓修复对含冲击损伤结构失效行为的影响。结果表明:冲击后碳纤维/环氧树脂复合材料层合板的压缩承载能力随着冲击能量的增加而降低,冲击损伤破坏了碳纤维/环氧树脂复合材料层合板结构的对称性,并导致结构在加载初期呈非对称的局部屈曲变形特征,局部屈曲诱发并加剧分层损伤扩展;单螺栓修复能有效恢复结构的整体对称性,在一定程度上抑制含冲击损伤碳纤维/环氧树脂复合材料层合板的局部屈曲,达到可观的修复效率。该研究为复合材料紧固件修理方案的制订及修理损伤容限的定义提供一定的指导意义。      

Microstructure,deformation and failure of polymer bonded explosives

Polymer bonded explosives (PBXs) are highly particle filled composite materials comprised of explosive crystals and a polymeric binder (ca. 5–10% by weight). The microstructure and mechanical properties of two pressed PBXs with different binder systems were studied in this paper. The initial microstructure of the pressed PBXs and its evolution under different mechanical aggressions were studied, including quasi-static tension and compression, ultrasonic wave stressing and long-pulse low-velocity impact. Real-time microscopic observation of the PBXs under tension was conducted by using a scanning electron microscope equipped with a loading stage. The mechanical properties under tensile creep, quasi-static tension and compression were studied. The Brazilian test, or diametrical compression, was used to study the tensile properties. The influences of pressing pressures and temperatures, and strain rates on the mechanical properties of PBXs were analyzed. The mesoscale damage modes in initial pressed samples and the samples insulted by different mechanical aggressions, and the corresponding failure mechanisms of the PBXs under different loading conditions were analyzed.     

The discrete element method with deformable particles

This work presents a new original formulation of the discrete element method (DEM) with deformable cylindrical particles. Uniform stress and strain fields are assumed to be induced in the particles under the action of contact forces. Particle deformation obtained by strain integration is taken into account in the evaluation of interparticle contact forces. The deformability of a particle yields a nonlocal contact model, it leads to the formation of new contacts, it changes the distribution of contact forces in the particle assembly, and it affects the macroscopic response of the particulate material. A numerical algorithm for the deformable DEM (DDEM) has been developed and implemented in the DEM program DEMPack. The new formulation implies only small modifications of the standard DEM algorithm. The DDEM algorithm has been verified on simple examples of an unconfined uniaxial compression of a rectangular specimen discretized with regularly spaced equal bonded particles and a square specimen represented with an irregular configuration of nonuniform‐sized bonded particles. The numerical results have been verified by a comparison with equivalent finite element method results and available analytical solutions. The micro‐macro relationships for elastic parameters have been obtained. The results have proved to have enhanced the modeling capabilities of the DDEM with respect to the standard DEM.     

横肋波纹钢板-方钢管混凝土组合柱双向偏压力学性能研究及承载力计算

为研究横肋波纹钢板-方钢管混凝土组合柱的双向偏压力学性能,对两根横肋波纹钢板-方钢管混凝土组合柱试件展开了双向偏压加载试验,探究了不同偏心距对试件在双向偏压荷载下的荷载-变形曲线、破坏模态和截面应变发展状况的影响,分析了波纹板与钢管、混凝土之间的相互作用机理.在试验基础上运用有限元模拟,分析了偏心距、加载角度、钢管厚度...     

Influence of aggregate deformation and contact behaviour on discrete particle modelling of fracture of concrete

The discrete element method, DEM, has been used in fracture studies of non-homogeneous continuous media adopting circular or spherical particles. A 2D circular rigid DEM formulation developed with the purpose of modelling concrete is described and evaluated in uniaxial tensile and compression tests. According to this model, the aggregate can be modelled either as a rigid macro-particle or as a deformable group of particles. The inter-particle contacts can either be assumed as brittle or follow a given bilinear softening curve. It is shown that aggregate deformability, together with the consideration of pure friction contacts working under compression, increases the fracture energy in compression, leading to a better agreement with concrete tests. The softening contact model, by adding a higher capability of load redistribution, is shown to give a better agreement than the brittle model under tensile loading. The recognized crack mechanisms of the brittle model (tensile splitting, branching, bridging) are also present with softening.     

Modeling progressive delamination of laminated composites by discrete element method

Discrete element method (DEM) was used to model progressive delamination of fiber reinforced composite laminates. The anisotropic composite plies were constructed through a hexagonal packing of particle elements. Contacts between the particles were represented by parallel bonds with the verified normal and shear elastic properties. The ply interface was characterized by a contact softening model with a bilinear elastic behavior which is similar to the cohesive zone model in the continuum mechanics. DCB, ELS and FRMM tests were simulated by the DEM model to assess its capability of modeling mode I, mode II and mix mode fracture of delamination, respectively. Good agreements were observed between the DEM and existing numerical and experimental results of loading curves, which confirmed that the DEM model can be used to simulate initiation and propagation of composite delamination, with more insights into microscopic material behavior, such as damage extension and plastic zone.     

A meso-level approach to the 3D numerical analysis of cracking and fracture of concrete materials

A meso‐mechanical model for the numerical analysis of concrete specimens in 3D has been recently proposed. In this approach, concrete is represented as a composite material with the larger aggregates embedded in a mortar‐plus‐aggregates matrix. Both continuum‐type components are considered linear elastic, while the possibilities of failure are provided with the systematic use of zero‐thickness interface elements equipped with a cohesive fracture constitutive law. These elements are inserted along all potential crack planes in the mesh a priori of the analysis. In this paper, the basic features of the model are summarized, and then results of calculations are presented, which include uniaxial tension and compression loading of 14‐aggregate cubical specimen along X, Y and Z axes. The results confirm the consistency of the approach with physical phenomena and well‐known features of concrete behaviour, and show low scatter when different loading directions are considered. Those cases can also be considered as different specimens subjected to the same type of loading.