Simulation of the effect of bond strength on the breakage pattern of agglomerates by Distinct Element Method

Chemical, pharmaceutical and food industries amongst many others use agglomerates either as intermediate or manufactured products. The mechanical strength of agglomerates under impact or shear deformation during handling and processing is of great interest to these industries for optimising product specification and functionality. The effect of surface energy on agglomerate behaviour under impact has been investigated using Distinct Element Method (DEM). Four different agglomerates were formed and impacted against a target along the direction of gravity for three different values of the surface energy (0.35, 3.5 and 35.0 J/m²). The agglomerate breakage pattern was influenced by the surface energy and a transition in the mode of failure of agglomerates was observed when the surface energy was varied. Based on the previous work, the surface energy is expressed in terms of Weber Number, We=(V-V0²)ρD/γ. Agglomerates showed extensive deformation under impact at the lowest value of surface energy (0.35 J/m²) and no evidence of fragmentation was found for any value of impact velocity. In this case the agglomerates behaved macroscopically in a ductile mode. At values of surface energy larger than 3.5 J/m² the agglomerates fragmented at the same time as local damage around the impact site occurred. This type of behaviour is typical of semi-brittle material failure. Therefore, the breakage pattern of agglomerates is influenced by the surface energy

Analysis of the flowability of cohesive powders using Distinct Element Method

Computer simulations using Distinct Element Method (DEM) have been carried out to investigate the effect of cohesion on the flowability of polydisperse particulate systems. For this purpose, two assemblies with different values of surface energy and made of 3000 spheres with the mechanical properties of glass beads were considered. The analysis of the flowability of the powders is presented in terms of the unconfined yield stress as a function of strain rate for different pre-consolidation loads. For values of the surface energy of 1.0 J/m2 and strain rates lower than 6 s− 1, the unconfined yield stress does not change significantly indicating a quasi-static behaviour of the particulate assemblies during the compression process. For larger strain rates, the unconfined yield stress varies with the power index of 1.2 of the strain rate. The influence of the pre-consolidating stress on the powder behaviour has also been investigated and a flow factor was obtained from the linear relationship between the unconfined yield stress and pre-consolidation stress. The computer simulations show qualitatively a good agreement with the experimental trends on highly cohesive powder flow behaviour

Analysis of the effect of cohesion and gravity on the bulk behaviour of powders using Discrete Element Method

Computer simulations using Distinct Element Method have been carried out to analyse the bulk behaviour of a polydisperse assembly of glass beads. For this purpose an assembly made of 3000 spheres were generated to which the mechanical properties of glass beads were assigned. The system was initially compressed isotropically at a strain rate of 1 s-1 in the absence of gravity and surface energy. Once the assembly reached a packing fraction of about 0.62, the effects of cohesion and gravity on the bulk behaviour were analysed for two different cases. In the first case only gravity was applied, whilst in the second case both gravity and surface energy were acting on the particles. The evolution of the components of the stress tensor for the case in which only gravity was applied indicated that the gravity did not appreciably affect the isotropy of the system. In contrast, the system in which surface energy was introduced became anisotropic. The concept of unconfined yield stress of bulk cohesive powders was used to analyse the effect of surface energy and strain rate. For values of surface energy of 1.0 J/m2 and of strain rate lower than 1 s-1 the unconfined yield strength did not change significantly indicating a quasi-static behaviour for the compression process. However, for values of strain rates larger than 1 s-1 the unconfined yield strength increased with the strain rate, following a power law trend with an index of 1.7

Stress and strain rate analysis of the FT4 Powder Rheometer

The Freeman FT4 Powder Rheometer has been reported to describe well the powder flow behaviour in instances where other techniques fail. We use DEM to simulate the FT4 operation for slightly cohesive large glass beads at a range of strain rates. The curved impeller is shown to be beneficial in comparison to a flat blade as the variation of shear stress across the blade is reduced. The shear stress in front of the blade correlates well with flow energy (which the device measures) for a range of tip speeds and is shown to increase approximately linearly with tip speed when operating beyond the quasi-static regime

Influence of measurement volume on predicted attrition by the distinct element method

During agitated drying and mixing processes, particle beds are exposed to shear deformation. This leads to particle attrition, the extent of which is dependent on the prevailing stresses and strains in the bed. The distributions of shear stresses and strain rates within the bed are highly non-uniform, requiring attention to localised conditions. Therefore a narrow angular sector of the bed is divided radially and vertically into a number of measurement cells, within which the stresses and strain rates are calculated throughout one rotation by the Distinct Element Method. These are then used in an empirical relationship of material breakage to predict the extent of attrition due to agitation. Here we investigate the influence of the measurement cell size on the estimated stresses and strain rates, and the subsequent effect on the predicted attrition. The measurement cell size is altered by varying the measurement sector size and the number of radial and vertical divisions within it. The median particle size is also varied to establish its influence on the predicted attrition. An increase in the average number of particles in a given cell, by varying the particle size or measurement cell dimensions, leads to a reduction in the estimated stresses and strain rates, and therefore a reduction in the predicted attrition. Comparison of the predicted attrition with the experimental breakage in the agitated vessel shows that the prediction method is accurate when the cell dimensions are comparable to the width of a naturally occurring shear band

Numerical simulation of powder flow during spreading in additive manufacturing

Additive manufacturing (AM) has attracted increasing attention in a wide range of applications, due to its ability for rapid manufacturing of complex shapes directly from a Computer-Aided Design (3D CAD) output. One of the manufacturing methods is based on powder processing, where a thin bed is formed to which an energy beam is applied to sinter and melt the powder. A major bottleneck in this method is associated with powder spreading, as its dynamics is sensitive to powder properties, machine design and operation conditions, such as speed of spreading. The effects of gap height and blade spreading speed on the evolving shear band and mass flow rate through the gap have been simulated by Discrete Element Method, using the most realistic physical and mechanical properties of the particles. It is shown that the particle velocity in the powder heap in front of the blade could well be described by a universal curve given by the Gauss error function. The mass flow rate through the gap increases linearly with the gap height. There exist two flow regimes with the increase of the blade spreading speed. Initially, the mass flow rate has a linear dependence on the blade speed, but eventually approaches an asymptotic value, implying a limit beyond which the mass flow rate cannot be further increased. This has an important implication on the speed of spreading

Influence of measurement cell size on predicted attrition by the Distinct Element Method

During agitated drying and mixing processes, particle beds are exposed to shear deformation. This leads to particle attrition, the extent of which is dependent on the prevailing stresses and strains in the bed. The distributions of shear stresses and strain rates within the bed are highly non-uniform, requiring attention to localised conditions. Therefore a narrow angular sector of the bed is divided radially and vertically into a number of measurement cells, within which the stresses and strain rates are calculated throughout one rotation by the Distinct Element Method. These are then used in an empirical relationship of material breakage to predict the extent of attrition due to agitation. Here we investigate the influence of the measurement cell size on the estimated stresses and strain rates, and the subsequent effect on the predicted attrition. The measurement cell size is altered by varying the measurement sector size and the number of radial and vertical divisions within it. The median particle size is also varied to establish its influence on the predicted attrition. An increase in the average number of particles in a given cell, by varying the particle size or measurement cell dimensions, leads to a reduction in the estimated stresses and strain rates, and therefore a reduction in the predicted attrition. Comparison of the predicted attrition with the experimental breakage in the agitated vessel shows that the prediction method is accurate when the cell dimensions are comparable to the width of a naturally occurring shear band

Word naming slows picture naming but does not affect cumulative semantic interference

Two experiments are reported which investigate the effect of processing words prior to naming target pictures. In Experiment 1, participants named (read aloud) sequences of five printed prime words and five target pictures from the same semantic category, and also sequences of five prime words from a different unrelated semantic category to the five related target pictures. Picture and words were interleaved, with two unrelated filler stimuli in between prime and target stimuli (i.e. a lag of 3 between primes and targets). Results showed that across the five target picture naming trials (i.e. across ordinal position of picture), picture naming times increased linearly, replicating the cumulative semantic interference (CSI) effect (e.g., Howard, Nickels, Coltheart, & Cole-Virtue, 2006). Related prime words slowed picture naming, replicating the effects found in paired word prime and picture target studies (e.g., Tree & Hirsh, 2003). However, the naming of the five related prime words did not modify the picture naming CSI effect, with this null result converging with findings from a different word and picture design (e.g., Navarrete, Mahon, & Caramazza, 2010). In Experiment 2, participants categorised the prime word stimuli as manmade versus natural, so that words were more fully processed at a conceptual level. The interaction between word prime relatedness and ordinal position of the named target picture was significant. These results are consistent with adjustments at the conceptual level (Belke, 2013; Roelofs, 2018) which last over several trials at least. By contrast, we conclude that the distinct word-to-picture naming interference effect from Experiment 1 must originate outside of the conceptual level and outside of the mappings between semantics and lexical representations. We discuss the results with reference to recent theoretical accounts of the CSI picture naming effect and word naming models

Numerical simulation of particle flow and segregation during roller spreading process in additive manufacturing

Additive Manufacturing (AM) using powder spreading requires uniform spreading. For narrow spreader gaps, as commonly used, transient jamming and segregation could adversely affect the uniformity of the spread layer. Here, we consider the dynamics of powder spreading by roller for a gas-atomised metal powder and analyse the effects of gap height and the rotational speed of roller on the evolving particle trajectory and spread layer uniformity by Discrete Element Method. It is shown that transient jamming in narrow gaps and size segregation in the spreading heap, the latter brought about by particle convection/circulation, adversely affect the uniformity of the spread layer. The segregation extent decreases with the increase of gap height or decrease of roller rotational speed. The conditions for uniform spreading are deduced from the simulations

Effect of gas-particle interaction on roller spreading process in additive manufacturing

Powder spreading in Additive Manufacturing (AM) has been analysed extensively by the Discrete Element Method, but without considering the presence of ambient gas. For fine particles, as commonly used in AM, the gas drag could affect the quality of spread layer. Here, we consider the dynamics of powder spreading by a roller for a gas-atomised metal powder and analyse the combined effects of gas-particle interaction and interparticle adhesion on the particle flow in the heap and spread layer uniformity. In the presence of gas, the convection and circulation of particles within the heap are slowed down, and the heap repose angle becomes steeper. The amount of particles spread on the base is reduced, as compared to the case in which gas drag is not considered, but surprisingly particles with larger interparticle adhesion form a more uniform spread layer with larger total particle volume when gas drag is taken into account