Direct tensile tests on particulate agglomerates for the determination of tensile strength and interparticle bond forces

The importance of direct tensile tests on solid and capillary bonded particulate agglomerates is investigated and compared to compression test measurements. The properties of wet agglomerates are varied by changing the contact angle by means of functionalization of the particle surface. Process conditions are considered by variation of ambient humidity. A qualitative evaluation of the results is performed by analyzing the measured force distance curves of different tensile tests. The results are quantitatively evaluated by calculating the breakage strength, mass related breakage energy and breakage probability showing that the ratio between tensile and compressive tests is highly dependent on the adjusted parameters. Next to the process parameter effect, also the influence of agglomerate size is considered. Tensile strength data are used to estimate the single bond forces between the primary particles of the agglomerates. Tensile and compressive test results are compared to numerical results (DEM) of agglomerate breakage using an elastic stiff bond model.     

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.     

Breakage of an artificial crushable material under loading

The mechanical behaviour of granular materials depends on their grading. Crushing of particles under compression or shear modifies the grain size distribution, with a tendency for the percentage of fine material to increase. It follows that the frictional properties of the material and the critical states are modified as a consequence of the changes in grain size distribution and the available range of packing densities. This paper illustrates an extended experimental investigation of the evolution of the grading of an artificial granular material, consisting of crushed expanded clay pellets under different loading conditions. The changes of grading of the material after isotropic, one-dimensional and constant mean effective stress triaxial compression were described using a single parameter based on the ratio of the areas under the current and an ultimate cumulative particle size distribution, which were both assumed to be consistent with self similar grading with varying fractal dimension. Relative breakage was related to the total work input for unit of volume. For poorly graded samples, the observed maximum rate of breakage is practically independent of initial uniformity. Further experiments at higher confining stress are required to investigate the mechanics of breakage of better graded samples.     

Discrete element modelling of cyclic loading of crushable aggreates

This paper examines the discrete element modelling of cyclic loading of an aggregate of crushable sand grains. Each grain of sand is modelled as an agglomerate of balls bonded together. The aggregate is subjected to compaction followed by isotropic normal (plastic) compression, and then unloaded to half the maximum applied stress. The aggregate is then subjected to cyclic loading to a maximum stress ratio of 0.8. The aim of the paper is to examine the reduction of the rate of axial strain with number of cycles, and to determine the relative influences of volumetric strain and shear strain rates on the axial strain rate. In particular, the paper aims to show whether particle breakage is mainly related to the accumulation of volumetric strain. This is found to be the case, which is consistent with proposals by other authors that plastic hardening under monotonic loading is due to particle fracture.     

Influence of particle breakage on the dynamic compression responses of brittle granular materials

The dynamic compression responses of dry quartz sand are tested with a modified spilt Hopkinson pressure bar (MSHPB), and the quasi-static compression responses are tested for comparison with a material testing system. In the experiments, the axial stress–strain responses and the confining pressure of the jacket are both measured. Comparison of the dynamic and the quasi-static axial stress–strain curves indicate that dry quartz sand exhibits obvious strain-rate effects. The grain size distributions of the samples after dynamic and quasi-static loading are obtained with the laser diffractometry technique to interpret the rate effects. Quantitative analyses of the grain size distributions show that at the same stress level, the particle breakage extent under quasi-static loading is larger than that under dynamic loading. Moreover, the experimental and the theoretical relationships of the particle breakage extent versus the plastic work show that the energy efficiency in particle breakage is higher under quasi-static loading, which is the intrinsic cause of the strain-rate effects of brittle granular materials. Using the discrete element method (DEM), the energy distributions in the brittle granular material under confined compression are discussed. It is observed that the input work is mainly transformed into the frictional dissipation, and the frictional dissipation under dynamic loading is higher than that under quasi-static loading corresponding to the same breakage extent. The reason is that more fragmentation debris is produced during dynamic breakage of single grains, which promotes particle rearrangement and the corresponding frictional dissipation significantly.     

Agglomerating fluidization of group C particles: major factors of coalescence and breakup of agglomerates

An experimental study was carried out to determine the influence of different superficial gas velocities on the agglomerates of cohesive particles. The probability of agglomerate coalescence and breakup is proposed on the basis of the principle of force balance. Theoretical analysis shows that the higher superficial gas velocity and fluid density, the lower the particle cohesion, and that collisions between small and large agglomerates are advantageous for the agglomerating fluidization of cohesive particles. The average agglomerate size estimated by the model of force balance decreases with increasing superficial gas velocity, which is in agreement with experimental data.     

Bending strength of black and composite agglomerates of cork

The bending strength of black and composite agglomerates of cork is investigated. In the preparation of the agglomerates, the raw material used was black regranulate (BR) of cork. Specimens of black agglomerate (BA) with a different thickness were obtained. In the case of the composite agglomerate (CA), both the particle size of BR and the resin dosage were varied. The bending strength increased with the increase in the specimen thickness for BA and in the particle size and the resin dosage for CA. This later parameter, in particular, greatly influenced such a mechanical property of this agglomeration product of cork.     

Quantitative analysis of agglomerates levitated from particle layers in a strong electric field

The electrification, agglomeration, and levitation of particles in a strong electric field were analyzed experimentally and theoretically. Particle layers of glass, alumina, and ferrite were formed on a plate electrode and an external voltage was applied. Microscopic observations of the agglomerates levitated from the particle layers revealed that the number of primary particles constituting an agglomerate is affected by particle diameter and electrical resistance, but not by the applied electric field. The electric field distributions in the system were calculated by considering the charges and geometries of the agglomerates formed on the particle layers. The charges of the agglomerates were obtained experimentally. All forces acting on the agglomerates (i.e., gravitational forces, Coulomb forces, interaction forces between polarized particles, image forces, and gradient forces) were analyzed under different conditions, including various electric field distributions and charges of agglomerates. Furthermore, the critical conditions for the levitation of the agglomerates were evaluated using a force balance.     

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.     

A Study on the Density of Agglomerates Prepared from Cork Waste

The density of black regranulate (BR) of cork and of black agglomerate (BA) and composite agglomerate(CA) prepared from such a waste by different methods was investigated.The preparation of the agglomerates was undertaken by controlling the specimen thickness for BA and the particle size for BR and the binder dosage for CA,The mass changes produced in the oven-drying treatment at 376.15 K of the agglomerates and in their subsequent stabilization under amibient conditions were also analyzed ,The density was determined by standard methods,For BR,the bulk density first decreased and then increased with decreasing particle size,It was much lower than the apparent density of the agglomeration products of cork ,Although to case of CA ,the density followed the same variation trends as for BR,Furthermore it increased significantly with the increase in resin dosage,This resulted in a noticaeable increase in the weight loss during the oven-drying and in a significant decrease in the degree of moisture adsorption during the stabilization period of the agglomerate.     

A general approach for the characterization of fragmentation problems

A general approach for the quantitative and systematic characterization of fragmentation problems, which is based on the Weibull statistics, is presented. A model, initially developed for materials stressed under impact with respect to their breakage probability, has been successfully applied to the characterization of compressive comminution, fragmentation of nanoparticle agglomerates and destroying of adhesive bonds. The experimental results from the slow compression comminution and from the comminution by falling weight match for various materials and particle sizes exactly to the master curve deduced for impact comminution. However, the material parameters determined for compression comminution are not identical for one and the same material to that determined by impact comminution. This indicates that the two model parameters are not pure material characteristics as assumed from the impact experiments, but depend on the stress mechanism. The dependence of the fragmentation degree on the particle size and the energy input observed when impacting particles in the micrometer to millimeter range has been proven also for nanoparticle agglomerates. Furthermore, a model analogous to that for breakage has been deduced for the quantification of the failure of adhesive bonds.     

Simulation of dispersion and collection process of agglomerated particles in collision with fibers using discrete element method

The behavior of agglomerates in collision with fibers was simulated using the three-dimensional modified discrete element method and the influences of several factors on the fraction of collected particles were examined. Furthermore the single fiber collection efficiency for agglomerated particles was also investigated. In the case where gas velocity is quite low, agglomerates are only deformed but barely dispersed and thus collected as a single deformable particle. By contrast above some critical gas velocity, constituent particles are dispersed and at the same time partly collected on fibers. The fraction of collected particles first increases then decreases as the van der Waals attractive force between particle and fiber increases. The reason for the decrease in fraction of collected particles in strong adhesion region is that the smooth deformation of agglomerates along the fiber surface is inhibited by too strong adhesion. It was also suggested that there exists an optimum size ratio between the agglomerate size and fiber radius for the collection fraction. This is also closely related to the deformation of agglomerate along the fiber surface. In case of non-agglomerated particle collision, all the particles entering within the collision region are collected by fiber. By contrast in case of agglomerate collision, the dispersion of agglomerates as well as collection occurs in the same process and all the particles colliding with the fiber are not necessarily collected. Consequently the single fiber collection efficiency considerably decreases comparing to that for non-agglomerated particle collision.     

Binder jetting of well-controlled powder agglomerates for breakage studies

Based on the similarity between the solidification process of the Additive Manufacturing (AM) binder jetting technique and wet granulation mechanisms, binder jetting is used to print powder granules with controlled geometry and strength. Powder granules with different strengths were achieved by changing the printing parameters, including the layer thickness and saturation level. The printed powder granules were then characterised for their structural properties such as their porosity and printing accuracy. Different parameter settings were found to have a significant influence on surface roughness. The strength of powder granules was improved by increasing the print saturation level, without compromising the printed geometry. A breakage study was carried out by compression tests of granules printed with different shapes and strengths. The relationship between print setting, structure and strength was established and discussed. This study demonstrates that AM powder granules with designed shapes and well-controlled strengths may act as ideal calibration particles for a range of industrial applications.     

Oblique impact simulations of high strength agglomerates

Different types of particle compounds like concrete particles can be considered as a model material of high strength agglomerates. It is necessary to investigate and understand fracture behaviour of these agglomerates in order to avoid breakage during storage, handling and transportation. The aim of the research is to examine the comminution behaviour of high strength agglomerates during oblique impact loadings.A two dimensional finite element analysis has been carried out to understand stress pattern distributions before crack initiation. Then a two dimensional discrete element method has been applied to study the fragmentation behaviour of the agglomerates. Concrete particles of B35 strength category have been chosen to represent the high strength agglomerates.Analysis is done with oblique impact loadings at different velocities from 7.7 to 180 m/s. The stressing conditions comprise low flow rate transportation and handling to high speed impacts during fall down in bunker, stock piles, ship loading or stressing in crushers and mill operations. Particle size distributions and new surface generation have also been evaluated in the paper.It is shown that at higher velocities, particle size distributions are identical to each other regardless of impact angle. Increasing impact velocity does not necessarily produce more new surfaces after certain velocity limit.     

Evaluation of direct quadrature method of moment for the internally circulating fluidized bed simulation with ultrafine particles

In the framework of the Euler-Euler gas–solid two-fluid model, the particle population balance equation is solved by the direct quadrature method of moment. The dynamic process of ultrafine particle movement and aggregation in an internally circulating fluidized bed is simulated. The distribution of the concentration and velocity of the agglomerates in the flow process is given, and the changes of the moments in the bed are shown. The effects of different breakage coefficients and inlet gas rates on the concentration distribution of agglomerates are compared. The results show that the particle size decreases with the increase of breakage coefficient, and the time required to reach steady fluidization state increases; the higher the inlet velocity, the better the effect of circulating particles in the bed. When there is a certain gas velocity difference between the two sides, the effect of circulating particles in the bed is better.     

A Study on the Density of Agglomerates Prepared from Cork Wastes

The density of black regranulate (BR) of cork and of black agglomerate (BA) and composite agglomerate (CA) prepared fromsuch a waste by different methods was investigated. The preparation of the agglomerates was undertaken by controlling thespecimen thickness for BA and the particle size for BR and the binder dosage for CA. The mass changes produced in theoven-drying treatment at 376.15 K of the agglomerates and in their subsequent stabilization under ambient conditions werealso analyzed. The density was determined by standard methods. For BR, the bulk density first decreased and then increasedwith decreasing particle size. It was much lower than the apparent density of the agglomeration products of cork. Although toa lesser extent, the density was also lower for BA than for CA. It was higher for the smaller thickness specimens of BA. In thecase of CA, the density followed the same variation trends as for BR. Furthermore it increased significantly with the increasein resin dosage. This resulted in a not     

Ball Impact and Crack Propagation – Simulations of Particle Compound Material

Particle compounds are the combination of various sized particles with non-uniform properties and can be considered as one of the most complicated engineering materials. The properties of the particle compounds vary in large range depending upon applications, methods of manufacturing and ratios of its compositions. Even if the method of manufacturing is same, the properties may be different because of the arrangements of ingredients. The different types of engineering agglomerates and building materials, like concretes, are some examples of the particle compounds. Similarly, the proper recycling of particle compound is very important in order to utilize the valuable aggregates from the cheaper fine matrixes. The aim of this research is to study the crack initiation and propagation in the building materials of spherically shaped concrete structures under impact loadings. A 2 Dimensional Finite Element Analysis is carried out with central impact loading condition to understand the stress pattern distributions before cracking. The Discrete Element Method (DEM) is adopted for further analysis to study the crack propagation in particle compound. Concrete spheres of diameter 150 mm with properties of B35 (35 N/mm² compressive strength) are chosen for the representation. A sphere is geometrically easier for the analysis. The assumption can be made that after some stages of loading the cube shaped concrete will be similar to sphere after losing its edges. This paper discusses the continuum and discrete approach for the analysis of crack propagation in particle compound with reference to the concrete ball. The analysis is done with central impact loading conditions in different velocities ranges between 7.7 m/s to 39 m/s. The correlations between theoretical simulations and practical experiments are also discussed.KeywordsFracture pattern, Crack propagation, Crack simulation, Air cannon, Numerical simulationThe authors would like to acknowledge German Research Foundation for the financial support.     

Application of DEM to evaluate and compare process parameters for a particle failure under different loading conditions

The paper presents an application of Discrete Element Modelling (DEM) in understanding the micro-process parameters of a particle failure under different loading conditions. A composite particle has been modelled using many primary particles to represent a quasi-homogeneous particle. Some of the examples of quasi-homogeneous particles are constituents of tablets, pellets, granules and concrete. These particles can behave differently under identical loading conditions even though they consist of same primary particles and proportions. This is a typical behaviour of such particles which is governed by the imperfections present in the particles. A DEM has been used to model the composite particle consisting of bi-modal distribution (smaller particles—matrices and larger particles—aggregates) of primary particles. The particle has been loaded under single plate compression, double plate compression and normal impact on different types of target. The single plate compression and normal impact experiments have also been performed. Process parameters like, fracture pattern, particle size distribution, liberation degree and new surface generation have been evaluated and compared. The results are applicable in understanding the particle failure under different processing operations like, transportation, handling and comminution. The results are also useful in selecting the better loading method for liberating aggregates from cheaper matrices for recycling.     

Dilute phase pneumatic conveying of whey protein isolate powders: Particle breakage and its effects on bulk properties

Breakage of dairy powder during pneumatic conveying negatively affects the end-customer properties (scoop uniformity and reconstitution). A dilute phase pneumatic conveying system was built to conduct studies into this problem using whey protein isolate powder (WPI) as the test material. Effects of conveying air velocity (V), solid loading rate (S_L), pipe bend radius (D), and initial particle size (d) on the level of attrition were experimentally studied. Four quality characteristics were measured before and after conveying: particle size distribution, tapped bulk density, flowability, and wettability. The damaged WPI agglomerates after conveying give rise to many porous holes exposed to the interstitial air. V is the most important input variable and breakage levels rise rapidly at higher airspeeds. The mean volume diameter D[4,3] decreased by around 20% using the largest airspeed of 30 m/s. Powder breakage is also very sensitive to particle size. There appears to be a threshold size below which breakage is almost negligible. By contrast, S_L and D show lesser influence on powder breakage. Reflecting the changes in particle size due to breakage, tapped bulk density increases whereas wettability decreases as a result of an increase in conveying air velocity. However, breakage does not show a significant effect on powder flowability as powder damage not only decreases particle size but also changes the particle’s surface morphology.