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Molecular dynamics studies with 3D model using spherical indenter and 2D plane strain model using cylindrical indenters revealed that the optimum coating thickness maximizing the hardness for a single layered nanocoating lies between 15 nm and 20 nm. The computational time is significantly lower for the plane strain model using cylindrical indenter than for the spherical indenter. However, the calculated hardness is different in both cases due to continuum and non-continuum effects. Continuum effects result in a reduction of hardness calculated by a cylindrical indenter, while the non-continuum effects result in an increase.

Ultra-high rotational speed micro friction stir welding is the combination of micro friction stir welding (μFSW) and ultra-high rotational speed FSW to overcome the practical difficulties such as increased heat loss from the workpiece, forging force requirement and fixturing issues associated with μFSW by using ultra-high rotational speeds. This work deals with the CFD modelling of ultra-high rotational speed micro friction stir welding of AA1100 to investigate the heat generation, temperature distribution and material flow in the weld zone at ultra-high tool rotational speeds. The temperature-dependent material properties and coefficient of friction are used in this study. A partial sliding-sticking contact condition is assumed between the tool and workpiece, and the possibility of partial melting occurring at high rotational speed is incorporated using boundary conditions. The predicted temperature field agrees well with the experimentally measured temperature results. The thermo-mechanically affected zone (TMAZ) predicted in the numerical simulation is also comparable with the micrographic studies. It is observed that the contribution of plastic heat generation is more than that of frictional heat generation at high rotational speeds, and partial melting does not occur. Welding speed does not have a significant influence on the peak temperature at high rotational speeds. Micro friction stir welding can be successfully performed at ultra-high rotational speeds to overcome the disadvantages and practical difficulties associated with low rotational speeds.

Adhesion is very common when the nanoscale or microscale solids come into contact, thus playing an important role in a variety of situations of interest. Due to the special nature of these problems, computational methods are often superior to analytical and experimental techniques for problems in adhesive contact mechanics. Finite element method is probably the most commonly used tool for such computational studies, due to its versatility and robustness. A major difficulty with these studies is the extremely high computational resource requirement. In this work, we propose two lumped parameter algorithms, which can significantly alleviate this issue. These algorithms provide results which approach the correct solution from either side, thereby providing upper and lower bounds to the correct solution, and thus, an estimate of the error involved in the analysis. Further, these algorithms can easily be incorporated into commercial finite element software and hence can be used more widely. Applicability of these algorithms is illustrated by studying the adhesive interactions between an elastic sphere and a rigid half-space, including the jump-to-contact instability caused by adhesion at small separations.

Liquid layering is considered to be one of the factors contributing to the often anomalous enhancement in thermal conductivity of nanoparticle suspensions. The extent of this layering was found to be significant at lower particle sizes, as reported in an earlier work by the authors. In continuation to that work, an investigation was conducted to better understand the fundamental parameters impacting the reported anomalous enhancement in thermal conductivity of nanoparticle suspensions (nanofluids), utilizing equilibrium molecular dynamics simulations in a copper-argon system. Nanofluids containing nanoparticles of size less than 6 nm were investigated and studied analytically. The heat current auto-correlation function in the Green-Kubo formulation for thermal conductivity was decomposed into self-correlations and cross-correlations of different species and the kinetic, potential, collision and enthalpy terms of the dominant portion of the heat current vector. The presence of liquid layering around the nanoparticle was firmly established through simulations that show the dominant contribution of Ar-Ar self-correlation and the trend displayed by the kinetic-potential cross-correlation within the argon species.

The impact of elastic spheres in presence of adhesion has been extensively studied due to its wide-ranging significance. A major emphasis of such studies is the energy loss occurring during impacts. Energy loss due to elastic stress waves is of special significance, as it could be the only active energy loss mechanism in many cases. In this work, we use finite element method to study the impact of elastic spheres on a rigid half-space in presence of adhesion. Adhesion, caused by van der Waals force, is modeled as a body force. This formulation helps to avoid many of the restrictions imposed by the commonly-used surface-force model of adhesion. The energy loss in adhesive impact of elastic spheres is predominantly due to the stress waves caused by adhesion-induced instability, which is absent below a critical sphere radius. Therefore, the coefficient of restitution increases with increase in sphere radius below this critical value, while it shows a reducing trend above it, within the range of sphere radii considered in this study. Therefore, our study indicates that the variation of critical velocity for capture with sphere radius will not be monotonic, unlike presented in many of the previous studies. (C) 2017 The Authors. Published by Elsevier Ltd.

Adhesion can cause energy losses in asperities or particles coming into dynamic contact resulting in frictional dissipation, even if the deformation occurring is purely elastic. Such losses are of special significance in impact of nanoparticles and friction between surfaces under low contact pressure to hardness ratio. The objective of this work is to study the effect of adhesion during the normal impact of elastic spheres on a rigid half-space, with an emphasis on understanding the mechanism of energy loss. We use finite element method for modeling the impact phenomenon, with the adhesion due to van der Waals force and the short-range repulsion included as body forces distributed over the volume of the sphere. This approach, in contrast with commonly used surface force approximation, helps to model the interactions in a more precise way. We find that the energy loss in impact of elastic spheres is negligible unless there are adhesion-induced instabilities. Significant energy loss through elastic stress waves occurs due to jump-to-contact and jump-out-of-contact instabilities and can even result in capture of the elastic sphere on the half-space.

Nanofluids — colloidal suspensions of nanoparticles in base fluids — are known to possess superior thermal properties compared to the base fluids. Various theoretical models have been suggested to explain the often anomalous enhancement of these properties. Liquid layering around the nanoparticle is one of such reasons. The effect of the particle size on the extent of liquid layering around the nanoparticle has been investigated in the present study. Classical molecular dynamics simulations have been performed in the investigation, considering the case of a copper nanoparticle suspended in liquid argon. The results show a strong dependence of thickness of the liquid layer on the particle size, below a particle diameter of 4nm. To establish the role of liquid layering in the enhancement of thermal conductivity, simulations have been performed at constant volume fraction for different particle sizes using Green Kubo formalism. The thermal conductivity results show 100% enhancement at 3.34% volume fraction for particle size of 2nm. The results establish the dominant role played by liquid layering in the enhanced thermal conductivity of nanofluids at the low particle sizes used. Contrary to the previous findings, the molecular dynamics simulations also predict a strong dependence of the liquid layer thickness on the particle size in the case of small particles.

A discrete computational approach based on molecular dynamics (MD) simulations is proposed for evaluating the latent heat of vaporization of nanofluids. The computational algorithm, which considers the interaction of the solid and the fluid molecules, is used for obtaining the enhancement of the latent heat of a base fluid due to the suspension of nanoparticles. The method is validated by comparing the computed latent heat values of water with standard values at different saturation temperatures. Simulation of a water–platinum nanofluid system is performed, treating the volume fraction and size of nanoparticles as parameters. The trends in the variation are found to match well with experimental results on nanofluids. Discussions are also presented on the limitations of the proposed model, and on methods to overcome them.

Thermal contact conductance is used to indicate the resistance offered by a contact interface to the flow of heat. When an interface material is applied as nano-layered coatings on super-finished contacting surfaces, the possibility of size effects necessitates the use of a discrete computation method for its analysis. Hence, a methodology is proposed which utilizes Molecular Dynamics (MD) simulations to obtain the size affected thermal conductivity of the interfacial layer, which in turn characterizes the thermal contact conductance behavior. Molecular Dynamics codes have been developed, making use of Sutton-Chen many-body potential, suitable for metallic materials. The model includes the asperities at the contact interface, assuming the asperities to be of a simplified geometry. The paper also presents the validation of the codes developed, and parametric studies on the effect of temperature, number of asperities and the material used for thermal interface coating on the size-affected interfacial conductivity.Copyright © 2008 by ASME