S.J. Antony

Influence of contact stiffnesses on the micromechanical characteristics of dense particulate systems subjected to shearing

When characterizing fine particles experimentally, it is a common practice to only measure the normal stiffness between particles and to assume that it is equal to the tangential stiffness. The impact of variations in the stiffness measurements upon the bulk characteristics of particulate systems remains unclear. Using computer simulations, the authors show that the variations in the contact stiffness ratio affect the micromechanical characteristics of nonspherical particulate systems more dominantly than the spherical particulate systems. Hence, attention must be paid to measure both the normal and tangential contact stiffnesses when characterizing nonspherical fine particulates to estimate their assembly characteristics.

Impact fracture of composite and homogeneous nanoagglomerates

It is not yet clear on whether the fracture characteristics of structured composite capsules and homogeneous nanoagglomerates differ significantly under impact loading conditions. Experimental measurement of impact fracture properties of such small agglomerates is difficult, due to the length and time scales associated with this problem. Using computer simulations, here we show that nanoagglomerates are subjected to normal impact loading fracture within a few nanoseconds in a brittle manner. The restitution coefficient of the nanoagglomerates varies nonlinearly with initial kinetic energy. The fracture of nanoagglomerates does not always happen at the moment when they experience the maximum wall force, but occurs after a time lag of a few nanoseconds as characterised by impact survival time (IST) and IST index. IST is dependant on the initial kinetic energy, mechanical and geometric properties of the nanoagglomerates. For identical geometries of the capsules, IST index is higher for capsules with a soft shell than for these with a hard shell, an indication of the enhanced ability of the soft nanocapsules to dissipate impact energy. The DEM simulations reported here based on theories of contact mechanics provide fundamental insights on the fracture behaviour of agglomerates--at nanoscale, the structure of the agglomerates significantly influences their breakage behaviour.

Micromechanical analysis of inclusions in particulate media using digital photo stress analysis tomography

Abstract. An experimental study aimed at sensing the stress distribution characteristics of inclusions inside particulate assemblies subjected to axial compaction is presented. The particulate assemblies are made of powders and grains, in which photoelastic inclusions are embedded along the central axis of the assemblies at different elevations. Digital photo stress analysis tomography is used to obtain the contours of maximum shear-stress distribution and the direction of major principal stress within the inclusions under the external loading. Using this, an analysis is performed for understanding the implications of using Hertz theory based on discrete element modeling for simulating stresses in relatively big inclusions surrounded by particulates. In the case of the inclusions surrounded by the grains, the location at which the peak value in maximum shear stresses occurs within the inclusions deviates from that of Hertzian analysis. This effect is dominant in the case of inclusions residing close to the loading surface. Unlike granular materials, shear-stress distribution characteristics of inclusions in powder surroundings tend to display continuum-like behavior under external compression and points to the need for a deeper understanding of the effects of the surrounding materials in particulate beds with inclusions.

Influence of particle-scale properties on the charge transfer characteristics in semiconducting particulate packing: particle-based finite element analysis

Abstract -The role of single-particle properties on the (macroscopic) charge distribution characteristics in particulate packing is not yet well understood, in spite of their extensive industrial relevance. In this paper, using computer simulations, we probe the influence of packing structure and size of the constituting particles on the charge distribution characteristics in semiconducting deterministic particulate packing. The simulations are based on the coupled particle finite element method approach (three-dimensional). We show that ordered particulate structures transfer charge more efficiently across the bed than for amorphous packing. For a given packing structure (face-centered cubic), the measure of charge transfer across the bed per unit area increases with decreasing particle size. The overall conductivity of the bed is proportional to the bead conductivity used in the packing. The ramping time for full potential across the packing is attained in just about 1 ms. The results show that the variations in the structural packing arrangement and size of the particles strongly influence the charge distribution (hopping) characteristics in particulate assemblies.

Chapter 19 Analysis of Agglomerate Breakage

Publisher Summary Agglomerates are formed by smaller particles, which have been brought together and joined to one another by a physical or chemical process. Agglomerates can break during processing or transport making them less suitable for their intended use because of formation of debris and hence quality degradation. An agglomerate breakage within a shearing bed of particles is clearly dependant on the size ratio. The breakage of large agglomerates is a crucial stage before the completion of granulation processes, as it leads to the production of a desirable size distribution of agglomerates. The parameters that influence agglomerate strength can be classified into four types: single particle properties, interparticle interactions, agglomerate properties and external parameters, such as impact angle and impact velocity. The mechanical strength of agglomerates under impact or shear deformation during handling and processing is of great interest to these industries for optimizing product specification and functionality. This difficulty arises from the degree of freedom and number of parameters that influence agglomerate structure and properties.

Equilibrium and Kinetic Properties of Self-Assembled Cu Nanoparticles: Computer Simulations

We present a study of the influence of interparticle interactions on the kinetics of self-assembly and mechanical strength properties of Cu nanoparticulate aggregates. Three types of commonly used inter-particle interaction forces have been considered to account for the attraction between particles, namely electrostatic forces, van der Waals forces and the JKR cohesion model. These models help to account for the forces generated due to surface treatment of particles, a process commonly used in fabricating composite particles. The assembly formed using the electrostatic interaction force model has 50% of the particles positively charged and the remaining particles are negatively charged. All the assemblies considered here have a polydisperse size distribution of particles. To be able to compare the bulk properties predicted between these models, the maximum force required to break the interparticle contacts (pull-off force) is kept identical in all the systems considered here. Three assemblies were generated. The assemblies were allowed to self-assemble based on the three interaction force models as mentioned above. We have studied some of the key properties of self-assembled Cu aggregates obtained by using the above mentioned models. The study shows that, although the pull-off force between particles is identical, variations in the long-range forces between particles significantly affect the structural properties and mechanical strength of the self-assembled nanoaggregates. The approach adopted here forms a basis on which to further probe the bulk behaviour of self-assembled particulates in terms of their single-particle properties.

Mechanical Failure of Grains in Sheared Granular Media: Effect of Size Ratio

Recent studies have shown that, when the size ratio of grains (ratio of size of a grain normalised to the average size of surrounding grains) exceed about five, the stress state of the grains is dominantly hydrostatic (fluid‐like) in dense granular packing subjected to shearing. This behaviour explains the reason for the retardation tendency of breakage of large particles observed in sheared dense granular media. However, most of the engineering applications dealing with sheared random particulate media, for example mixing of detergent grains, often involve particles having size ratio less than five and information on the nature of stress experienced by grains in such scenario remains unknown, an aspect addressed in the present work using Distinct Element Method (DEM) simulations. Our stress map of particles precisely defines the stress transition:‐for particles with size ratio greater than ∼3, the stress state is dominantly hydrostatic; and between ∼1.66 and 3, the stress state gradually transit form fro...