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.     

A combined finite‐discrete element method for simulating pharmaceutical powder tableting

The pharmaceutical powder and tableting process is simulated using a combined finite‐discrete element method and contact dynamics for irregular‐shaped particles. The particle‐scale formulation and two‐stage contact detection algorithm which has been developed for the proposed method enhances the overall calculation efficiency for particle interaction characteristics. The irregular particle shapes and random sizes are represented as a pseudo‐particle assembly having a scaled up geometry but based on the variations of real powder particles. Our simulations show that particle size, shapes and material properties have a significant influence on the behaviour of compaction and deformation. Copyright © 2005 John Wiley & Sons, Ltd.     

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.     

On the Concept of Jammed Configurations from a Structural Mechanics Perspective

Applying a method that is widely known in nonlinear structural mechanics, the paper offers an alternative approach for the jamming analysis of granular assemblies. The main advantage of the proposed approach is that deformable particles with general shapes can be handled with it, in contrast to the previous methods that are restricted to rigid grains with spherical shape. The paper first gives an overview on the existing concepts of jammed states. Then an alternative set of definitions is proposed; the definitions are based on the stability analysis of the considered assemblies. After that, a calculation method (restricted to elastic contacts) is introduced for the jamming analysis. The method is based on determining the eigenvalues of the stiffness matrix that contains the effect of the particle properties as well as the already existing contact forces that are present in the system.     

Fabric rates of elliptical particle assembly in monotonic and cyclic simple shear tests: a numerical study

This paper aims to investigate the evolutions of microscopic structures of elliptical particle assemblies in both monotonic and cyclic constant volume simple shear tests using the discrete element method. Microscopic structures, such as particle orientations, contact normals and contact forces, were obtained from the simulations. Elliptical particles with the same aspect ratio (1.4 and 1.7 respectively for the two specimens) were generated with random particle directions, compacted in layers, and then precompressed to a low pressure one-dimensionally to produce an inherently anisotropic specimen. The specimens were sheared in two perpendicular directions (shear mode I and II) in a strain-rate controlled way so that the effects of inherent anisotropy can be examined. The anisotropy of particle orientation increases and the principal direction of particle orientation rotates with the shearing of the specimen in the monotonic tests. The shear mode can affect the way fabric anisotropy rate of particle orientation responds to shear strain as a result of the initial anisotropy. The particle aspect ratio exhibits quantitative influence on some fabric rates, including particle orientation, contact normal and sliding contact normal. The fabric rates of contact normal, sliding contact normal, contact force, strong and weak contact forces fluctuate dramatically around zero after the shear strain exceeds 4 % in the monotonic tests and throughout the cyclic tests. Fabric rates of contact normals and forces are much larger than that of particle orientation. The particle orientation based fabric tensor is harder to evolve than the contact normal or contact force based because the reorientation of particles is more difficult than that of contacts.     

Re-entrainment of particles from powder structures: experimental investigations

The effectiveness of re-entrainment of particles from powder structures into air flowing through a powder aliquot placed in channel of rectangular cross-section was investigated experimentally. The shear stream of the gas causes breakage of the powder structure and particles or their clusters move to the air stream with an efficiency that depends on airflow rates and air humidity. At higher flow rates, large agglomerates are re-entrained, especially if the powder was conditioned at higher air humidity. The modifications of particles surfaces through adsorption of surfactants or electrostaticaly active substances cause the screening of the short-range cohesive forces between particles and particle processing make the re-entrainment more effective. Reduced size of particles clusters re-entrained into the air stream is also observed for modified surfaces of powder particles.     

Particle size induced heterogeneity in compacted powders: Effect of large particles

We investigated the effect of particle size distribution on heterogeneity of compacted powders. We used experiments and discrete particle based simulations to compact powders, test the mechanical strength of the compact, and study the microstructure of the compact. A metallic powder which has a wide particle size distribution was used in the experiments. We found that the compaction profile is not reproducible when particles larger than 1/6 of the die diameter are present in the powder sample. The presence of these large particles generate a highly heterogeneous inter-particle contact and bonding forces. The discrete particle simulations showed that for these heterogeneous compacts the tensile strength exhibits high variability, even for one compact if the diametrical compression force is applied along different axes. Based on these results, it is recommend that the largest particle in a powder compact should not exceed one sixth of the die diameter, which is the same as the recommendation of ASTM International D4767 - 11 for compression test of cohesive soils.     

Resuspension of particles from surfaces: Technological,environmental and pharmaceutical aspects

Particles that were previously deposited on the surface of any substrate are resuspended to the surrounding environment due to complex effects. The effectiveness of resuspension depends on the geometry of deposits in the form of mono- or multilayer geometries, material properties of the particle and the substrate and the interaction of particles with external excitations. The particle leaves the substrate or its surrounding neighbors, if the external force exceeds the adhesive/cohesive force. Different types of adhesive forces, such as van der Waals interactions, capillary forces and electrical double layer forces, are considered. The adhesive effects are extended to the cases of deformable bodies with the formation of the surplus of the contact area between the particle and the substrate. Finally, the adhesion of particles to the rough surface of the substrate is considered. The next sections of this paper are devoted to the analysis of the particle detachment process due to its interaction with a flowing gas of different flow properties, including a pulsating jet flow and a developed turbulent flow. The mechanisms of the pull-off forces as well as sliding and rolling effects in the presence of asperities are also presented. Finally, the detachment of particles from the multilayer structures of deposited particles is analyzed. The paper is summarized focusing on the practical aspects of resuspension, including the techniques of cleaning of the wafer surface, environmental consequences of resuspension with health risk and the application of resuspension for the production of inhalable particles from drug powder structures.     

Study on a large-scale discrete element model for fine particles in a fluidized bed

The discrete element method (DEM) is widely used to comprehend complicated phenomena such as gas–solid flows. This is because the DEM enables us to investigate the characteristics of the granular flow at the particle level. The DEM is a Lagrangian approach where each individual particle is calculated based on Newton’s second law of motion. However, it is difficult to use the DEM to model industrial powder processes, where over a billion particles are dealt with, because the calculation cost becomes too expensive when the number of particles is huge. To solve this issue, we have developed a coarse grain model to simulate the non-cohesive particle behavior in large-scale powder systems. The coarse grain particle represents a group of original particles. Accordingly, the coarse grain model makes it possible to perform the simulations by using a smaller number of calculated particles than are physically present. As might be expected, handling of fine particles involving cohesive force is often required in industry. In the present study, we evolved the coarse grain model to simulate these fine particles. Numerical simulations were performed to show the adequacy of this model in a fluidized bed, which is a typical gas–solid flow situation. The results obtained from our model and for the original particle systems were compared in terms of the transient change of the bed height and pressure drop. The new model can simulate the original particle behavior accurately.     

The effect of interactions on the saturation remanence of particulate assemblies

The effect of interactions on the saturation remanence of assemblies of identical, uniaxially anisotropic, single-domain particles is calculated using a spatial-mean interaction field. The particle easy-axis directions are assumed known and given by a distribution function. The remanence is determined by finding the magnetization orientation functional of the particle easy-axis orientation which minimizes the total assembly energy. Curves of remanence versus interaction strength (assembly packing fraction) are shown for a) randomly oriented assemblies of spherical particles with uniaxial crystalline anisotropy only, b) randomly oriented assemblies of acicular particles with shape anisotropy, and c) oriented assemblies of acicular particles with 6:1 aspect ratio. Disregarding external sample-shape demagnetization effects, this model always yields increased remanences due to interactions. A criterion is given which predicts when external shape effects are capable of dominating the net interaction field to yield a reduction in remanence. The applicability of these results and extensions of the theory are discussed in relation to particle assemblies in magnetic tape and high coercivity CoNiP films.     

羰基铁粉改性环氧树脂/乙基纤维素微胶囊的吸波性能

以羰基铁粉为吸波剂用溶剂蒸发法制备出环氧树脂/乙基纤维素微胶囊,使用矢量网络分析仪、激光粒度分析仪、ESEM-EDS和FTIR分别表征了微胶囊的吸波性能、粒径分布、颗粒特性以及化学结构。结果表明：羰基铁粉嵌入乙基纤维素中物理结合成微胶囊壁材,羰基铁粉提高了微胶囊的吸波性能。羰基铁粉的粒径越小与电磁波相互作用的面积越大,微胶囊的吸波性能越好。频率为18 GHz时,未掺羰基铁粉的微胶囊电磁波反射损失为-1.63 dB,而掺入粒径为3 μm和0.5 μm羰基铁粉(掺量50%)的微胶囊电磁波反射损失分别为-5.08 dB和-5.44 dB,分别降低了3.45 dB和3.81 dB。掺入粒径为0.5 μm羰基铁粉的微胶囊不团聚,其微观形貌更好。     

Multi-scale study of particle flow in silos

The flow characteristics of solid particles in a silo were studied experimentally and theoretically. A multi-scale study of the particles flow was performed by means of discrete element method (DEM). The dependence of flow behaviors on the particles diameter distribution and silo geometry was analyzed to establish the spatial and statistical distributions of microdynamic variables related to flow and silo structures such as velocity, porosity, coordination number, and interaction forces between particles. The results show that the distribution of particle diameter has great effects on particles flow, and the mixing of multi-sized particles is propitious to granular flow. The geometry of silos has greater effects on granular flow than particle size distribution, and inserts can improve the flow behaviors of “funnel flow” type to “mass flow”. Linear equations can be used to describe the relationship between discharge rate and orifice size by G^2/5 vs. D_o for the same distribution of particles diameter. The flow structure of particles in the silos is spatially non-uniform, which is illustrated by spatial and statistical distributions of porosity and coordination number. Both porosity and coordination number are affected by the mode of particles packed, which is affected by the geometry of silos and particle size distribution. The distribution of contact forces between particles is spatially non-uniform too. In flat-bottomed silo, there are arched stress chains in the vicinity of the orifice under the “bridging action”, which disappeared in wedge-shaped hopper silo.     

POWDER COATING PROCESS PARAMETERS FOR A TRANSFER EFFICIENCY MODEL

The trajectories of charged powder particles in a powder coating system are governed by the electrostatic, gravitational and aerodynamic forces acting on the particles. A mathematical model of particle trajectories inside a powder coating booth must consider (1) the aerodynamic flow field, (2) particle size and charge distributions, (3) the electrostatic field distribution, and (4)the geometry of the target. Our approach is to employ a grid generation and flow solver to examine the air flow pattern and an iterative technique where the Charge Simulation Method can be used to compute the electric field strength and the Method of Characteristics can be used to compute the charge density in the gun-to-target region. The electrostatic forces due to the deposited powder layer and image charge are to be taken into account to determine if the particle will deposit on the substrate or not. The model is applied to the geometry of a high-voltage electrode consisting of a long thin rod with a hemispherical end cap and a grounded flat disk substrate. An experimental system to measure transfer efficiency, with the ability to control various parameters effecting transfer efficiency, has been developed to verify the theoretical model. The simulation results can provide valuable information concerning particle deposition and optimization of transfer efficiency. This paper describes (1) system parameters involved in modeling the transfer efficiency, (2) an approach to develop such a model with preliminary data on the simulation of particle track, and (3) experimental data on the real-time measurements of first pass transfer efficiency.     

An Euler–Lagrange particle approach for modeling fragments accelerated by explosive detonation

In this paper, a method is proposed for modeling explosive‐driven fragments as spherical particles with a point‐particle approach. Lagrangian particles are coupled with a multimaterial Eulerian solver that uses a three‐dimensional finite volume framework on unstructured grids. The Euler–Lagrange method provides a straightforward and inexpensive alternative to directly resolving particle surfaces or coupling with structural dynamics solvers. The importance of the drag and inviscid unsteady particle forces is shown through investigations of particles accelerated in shock tube experiments and in condensed phase explosive detonation. Numerical experiments are conducted to study the acceleration of isolated explosive‐driven particles at various locations relative to the explosive surface. The point‐particle method predicts fragment terminal velocities that are in good agreement with simulations where particles are fully resolved, while using a computational cell size that is eight times larger. It is determined that inviscid unsteady forces are dominating for particles sitting on, or embedded in, the explosive charge. The effect of explosive confinement, provided by multiple particles, is investigated through a numerical study with a cylindrical C4 charge. Decreasing particle spacing, until particles are touching, causes a 30–50% increase in particle terminal velocity and similar increase in gas impulse. Copyright © 2015 John Wiley & Sons, Ltd.     

Cohesion of lactose powders at low consolidation stresses

The flow characteristics of a powder system are known to be influenced by particle size distribution, particularly the content of fine particles, and interparticle forces. This paper reports an investigation that has identified and quantified links between physical properties, viz size distribution, bulk density and particle density, and cohesion in compacted beds of powder. An annular shear cell was used in the determination of the cohesion of cohesive and free-flowing milled lactose powders at low consolidation stresses in the range 0.31–4.85 kPa and under ambient conditions. Following consideration of the compaction and shearing processes, it was postulated and confirmed that cohesion could be expressed as a function of powder surface area per unit volume and dimensionless preconsolidation stress. It was shown that care is needed in the measurement of surface–volume mean diameter when applying correlations developed from the experimental data.     

Discrete element modeling of solid formation during electrophoretic deposition

Electrophoretic deposition (EPD) of colloidal ZrO₂ ceramic powder was examined with respect to the internal colloidal forces and the external electrical field. The influence of electrolytic dissociation of water close to the deposition electrode (cathode) on the electrostatic interaction between the particles and the local electric field is discussed. The discrete element method (DEM) was used to get an insight into the kinetics determining particle packing and density gradient microstructures. The simulation indicates that high particle concentrations combined with low electric field strength result in coagulated flocs and a low packing density in the deposit layer. Tentative phase diagrams for various colloidal forces and electrical field strengths were established.     

数字成像颗粒分析仪在聚丙烯粉料测试中的应用

为了考察CAMSIZER数字成像颗粒分析仪用于聚丙烯粉料测试的可行性,采用CAMSIZER数字成像颗粒分析仪对聚丙烯粉料样品的粒度分布进行了测试,考察了测试结果的重复性,并对该仪器在粒度分布测试同时获得颗粒球形度数据和颗粒照片的功能进行了验证。结果表明:粒度分布测试自动化程度高,测试速度快,结果重复性好,与筛分法比较,工作量大大降低;在进行粒度分布测试的同时,可获得颗粒的球形度数据和颗粒投影照片。     

Aluminium powder particles produced by SAMD technique: typical characteristics and correlations between processing conditions and powder size

Abstract

The present paper consists of two parts. In the first part the principles of a new method of metal powder production, termed 'solid assisted melt disintegration (SAMD)' are discussed and the typical characteristics of the produced powder are outlined. In the second part the effects of some processing parameters on the size distribution and mean diameter of the powder are reported. The SAMD method involves mixing solid particles (i.e. alumina) with the liquid aluminium alloy aided by mechanical agitation. The shear force induced by the impeller is transferred to the metal via the non-wetting solid medium and results in melt disintegration. The resulting mixture of aluminium droplets and alumina particles are subsequently cooled in air and screened through 300 μm sieve to separate alumina from solidified aluminium powder particles. The SAMD technique has demonstrated the capability to produce a wide particle size distribution. The small sized particles (i.e. <53 μm) exhibited irregular shapes, but larger ones were mostly spherical. These powder particles were dense (pore free) without attached satellite particles and exhibited a relatively coarse microstructure. The processing parameters investigated include the size of Al₂O₃ particles, Al₂O₃/Al weight ratio, stirring speed and stirring time. It was concluded that there exists an optimum value for each of the aforementioned parameters corresponding to a minimum in the mean particle size.     

Numerical modeling the bonding mechanism of HVOF sprayed particles

During high velocity oxy-fuel (HVOF) thermal spraying, most powder particles remain in solid state prior to the formation of coating. A finite element (FE) model is developed to study the impact of thermally sprayed solid particles on substrates and to establish the critical particle impact parameters needed for adequate bonding. The particles are given the properties of widely used WC-Co powder for HVOF thermally sprayed coatings. The numerical results indicate that in HVOF process the kinetic energy of the particle prior to impact plays the most dominant role on particle stress localization and melting of the particle/substrate interfacial region. Both the shear-instability theory and an energy-based method are used to establish the critical impact parameters for HVOF sprayed particles, and it is found that only WC-Co particles smaller than 40 μm have sufficient kinetic and thermal energy for successful bonding.