Search

Additive manufacturing (AM) is usually defined as the process of joining materials to make objects from 3D model data, typically layer upon layer, as opposed to subtractive manufacturing methodologies, which remove material (American Society for Testing and Materials, ASTM). We refer the reader to the recent overview article by (Huang et al. in Journal of manufacturing science and engineering 137:014001–1, 2015) [1] on the wide array of activities in the manufacturing community in this area.

One key aspect of many additive manufacturing processes is the deposition of heated mixtures of particulate materials onto surfaces, which then bond and cool, leading to complex microstructures and possible residual stresses. The overall objective of this work is to construct a straightforward computational approach that researchers in the field can easily implement and use as a numerically-efficient simulation and design tool. Specifically because multifield coupling is present, a recursive, staggered, temporally-adaptive, finite difference time domain scheme is developed to resolve the internal microstructural thermal and mechanical fields, accounting for the simultaneous elasto-plasticity and damage. The time step adaptation allows the numerical scheme to iteratively resolve the changing physical fields by refining the time-steps during phases of the process when the system is undergoing large changes on a relatively small time-scale and can also enlarge the time-steps when the processes are relatively slow. The spatial discretization grids are uniform and dense. The deposited microstructure is embedded into spatial discretization. The regular grid allows one to generate a matrix-free iterative formulation which is amenable to rapid computation and minimal memory requirements, making it ideal for laptop computation. Numerical examples are provided to illustrate the approach. This formulation is useful for material scientists who seek ways to deposit such materials while simultaneously avoiding inadvertent excessive residual stresses.

Summary Virtually all engineering applications involve the use of piping, conduits, and channels. In the petroleum industry, piping systems are extensively used in upstream and downstream processes. These piping systems often carry fluids that are corrosive, which leads to wear, cavitation, and cracking. The replacement of damaged piping systems can be quite expensive, both in terms of capital costs and in operational downtime. This motivates the present research on noninvasive healing of cracked piping systems. In this investigation, we propose to develop computational models for characterizing noninvasive repair strategies involving electromagnetically guided particles. The objective is to heal industrial-piping systems noninvasively, from the exterior of the system, during operation, resulting in no downtime and minimal relative cost. The particle accumulation at a target location is controlled by external electromagnetic/mechanical means. There are two primary effects that play a role for guiding the particles to the solid-fluid-interface/wall: mechanical shear caused by the fluid flow, and an electrical or magnetic force. In this work we develop and study a relationship that characterizes contributions of both, and ascertain how this relationship scales with characteristic physical parameters. Characteristic nondimensional parameters that describe system behavior are derived, and their role in design is illustrated. A detailed, fully 3D discrete-element-simulation framework is presented, and illustrated by use of a model problem of magnetically guided particles. The detailed particle behavior is considered to be regulated by three effects: the field strength, the mass-flow rate, and the wall interactions.

This work presents a computer simulation framework based on discrete element method to analyze manufacturing processes that comprise a loosely flowing stream of particles in a carrier fluid being deposited on a target surface. The individual particulate dynamics under the combined action of particle collisions, fluid-particle interactions, particle-surface contact and adhesive interactions is simulated, and aggregated to obtain global system behavior. A model for deposition which incorporates the effect of surface energy, impact velocity and particle size, is developed. The fluid-particle interaction is modeled using appropriate spray nozzle gas velocity distributions and a one-way coupling between the phases. It is found that the particle response times and the release velocity distribution of particles have a combined effect on inter-particle collisions during the flow along the spray. It is also found that resolution of the particulate collisions close to the target surface plays an important role in characterizing the trends in the deposit pattern. Analysis of the deposit pattern using metrics defined from the particle distribution on the target surface is provided to characterize the deposition efficiency, deposit size, and scatter due to collisions.

The use of targeted particulate jets for surface modification in advanced manufacturing processes such as shot-peening are now becoming widespread. The degree of precision now demanded, in tightly confined workspaces, dictates that these processes undergo deeper scrutiny, refinement and optimization, in particular to avoid unintended excessive normal and tangential impact forces and re-impact from the rebounding jet on secondary surfaces. This work focuses on the building block of a particulate jet, namely the inelastic impact of a particle with a surface. The governing equations for a general three-dimensional inelastic impact with unilateral stick-slip conditions are derived, with the objective being to extract the particle and target characteristics which control the forces induced on impact and the resulting post-impact trajectories. Quantitative and qualitative analyses are performed for different types of surfaces and allows analysts to make informed decisions on the choices of parameters in jets, in order to reduce trial and error procedures.

Obscurants are often clouds of dispersed particulate materials whose purpose is to mask a given object. The objective of this paper is to develop a simple discrete-ray/discrete-particle model in order to enable rapid assessment of the response of an obscurant cloud to an incoming high-frequency beam. The beam is decomposed into a set of discrete rays and the obscurant is represented by a discrete set of scattering particles. Ray-tracking is used to calculate the transient propagation of the rays and the absorption of energy by the particles. Examples are given, comprised of concentrated incident beams, their propagation into the obscurant cloud, and the subsequent reflection and transmitted aggregate response. Basic system trends are computed, varying the randomly dispersed particle shapes from spherical to oblate objects, which correlate the total amount of volume and surface area material available to interact with the beam and the overall scattering response. This allows further correlation of the obscurant cloud performance to the weight of the material, which is important for portable containers of dispersible obscurants, such as smoke grenades. Specifically, the model allows for rapid quantification of the modest reduction of scattering efficiency of flakes, relative to spheres, but which have significantly less weight than spheres. Thus, there is a trade-off between the weight of the dispersed system and its scattering efficiency.

The general problem of a loosely flowing erosive granular jet undergoing impact with a compliant surface is common in many manufacturing processes, and also in the operating environment of a variety of machine parts. This paper presents a three-dimensional, collision-driven discrete particle simulation framework for investigating the dynamics of a jet of erosive particles impacting a surface with a specified porosity and compliance. The framework is capable of handling repeated collisions between incoming particles and rebounding particles, and between particles and surfaces. It is also capable of performing a coupled simultaneous calculation of sub-surface stresses in the material, assuming a certain porosity. Well illustrated numerical examples are presented with detailed analysis for investigations on the mechanics and energetics of the interfering collisions in eroding jets close to the target surface, on the effect of such interference on the material erosion, and on the evolving stress levels and potential damage zones under the action of impact. Particularly, the assumption of considering first-order collisions between oncoming and rebounding jet particles is re-examined. The influence of repeated collisions on energy transferred to the surface was found to be significant under conditions which involves high particle numbers or fluxes, and also high degrees of inelasticity. The overall trends for parametric variations were found to be in accordance with reported trends in the literature.

Low-intensity, unfocused, ultrasound-induced diathermy can produce undesired temperature increases at the interface of adjacent tissues within the body; particularly, at the interface of soft tissue and bone. This study provides a computational framework for predicting an upper bound on the temperature profile within a multiphase system composed of gel pad (water), tissue and bone from an input of acoustic energy, at frequencies and power levels consistent with applications of therapeutic hyperthermia. The model consists of solving a (one-dimensional) spatially discretized bioheat transfer equation via finite-difference method and updating the solution in time with a forward-Euler scheme. Simulations are then compared to experimental data to determine the energy-to-heat conversion factors within each constituent material using thermocouple-embedded, tissue-mimicking phantom material, with and without bone. Viscous heating artifacts from the presence of the thermocouples in the experimental phantom tissue are accounted for via additional experimental methods similar to those described by Morris et al. (Phys Med Biol 53:4759, 2008). Finally, an example application of the model is presented via prediction of the maximum temperature at the tissue–bone interface, as well as the peak temperatures in the composite structure at the end of a prescribed 2-min sonication, of blood-perfused, human soft-tissue at 1, 2 and 3 MHz frequencies and a spatial peak temporally averaged intensity of $$1.0 \ W/cm^{2}$$ . The results of this simulation are then related to comparable experimental studies in the literature.

Boxing and related sports activities have become a standard workout regime at many fitness studios worldwide. Oftentimes, people are interested in the calories expended during these workouts. This note focuses on determining the calories in a boxer's jab, using kinematic vector-loop relations and basic work–energy principles. Numerical simulations are undertaken to illustrate the basic model. Multi-limb extensions of the model are also discussed.

The final publication is available at Springer via http://dx.doi.org/10.1007/s40571-015-0089-9 A modular discrete element framework is presented for large-scale simulations of industrial grain-handling systems. Our framework enables us to simulate a markedly larger number of particles than previous studies, thereby allowing for efficient and more realistic process simulations. This is achieved by partitioning the particle dynamics into distinct regimes based on their contact interactions, and integrating them using different time-steps, while exchanging phase-space data between them. The framework is illustrated using numerical experiments based on fertilizer spreader applications. The model predictions show very good qualitative and quantitative agreement with available experimental data. Valuable insights are developed regarding the role of lift vs drag forces on the particle trajectories in-flight, and on the role of geometric discretization errors for surface meshing in governing the emergent behavior of a system of particles.

There exist a wide range of applications for lightweight ballistic fabric shields, such as the protection of critical structural components in transport systems and the human body. However, some deficiencies are (1) the susceptibility to being abruptly severed by sharp objects, which completely eliminates the fabric's ability to stretch and absorb incoming kinetic energy and (2) environmental degradation of the fabric due to moisture, heat and sunlight, which is of growing concern, since many new fabrics have multiple purposes, such as electrical and chemical sensing, in addition to being part of a protective system. Because of these issues, the coating of fabric can be advantageous, however, it adds weight to the shielding system. Experiments on this type of coated fabric system are extremely time-consuming. Accord-ingly, this paper seeks to develop a computational framework using a coated network model in order to capture the basic characteristics of such systems. One aspect of the model's usefulness is that it can provide qualitative information to guide and reduce costly, time-consuming experiments. Three-dimensional numerical examples are given to illustrate the computational model. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

SUMMARY In this paper, we propose a simple computational framework for the rapid simulation of the delivery of substances into cells. Our approach treats the substances and the cell membrane as a collection of particles forming a discrete dynamical system, which is described by Newtonian equations in a purely mechanistic way. Detailed aspects about the modeling of particle interactions are discussed and resolved. The main advantage of such an approach is that it can offer a good qualitative picture of the delivery mechanism without the need to resort to detailed descriptions of the complex intermolecular interactions that are observed at small scales of the cell membrane. A numerical time integration scheme is formulated for solution of the system dynamics, and examples of simulations are provided. Computational particle-based models render reliable and fast simulation tools. We believe they can be very useful to help advance the design of delivery systems. Copyright © 2014 John Wiley & Sons, Ltd.

The primary objective of this paper is to characterize the motion of small electromagnetically sensitive particles which are embedded in a flowing neutral fluid. There are a variety of industrial applications for electromagnetic particle-laden fluids, such as fluid-based actuators, coatings and functionalized inks, to name a few. This work compares the relative strengths of the forces induced by electromagnetic fields and fluid drag, and their composite effects on particle motion. Both analytical and numerical investigations are undertaken. After an analysis of a single isolated particle, a three-dimensional model problem comprised of a Representative Volume Element of flowing particle-laden fluid, under the action of external electromagnetic fields, is studied. A computational staggering scheme is developed to solve the coupled system utilizing a fully implicit Finite-Difference discretization of the Navier–Stokes equations for the fluid and a direct particle-dynamics description is used for the particles. For large numbers of embedded particles, because of the extreme computational difficulty of interface-conforming fine-mesh spatial discretizations for the fluid, simplifying assumptions for the coupling are made based on semi-analytical computation of drag-coefficients, allowing for the use of coarser meshes. Even after these simplifications, the particle-fluid system is strongly-coupled. The strongly-coupled system is implicitly solved, iteratively, within each time-step using a recursive staggering scheme, which employs temporal adaptivity to control the coupling error. The approach allows researchers to rapidly compute such systems with moderate laptop/desktop resources. Numerical examples are provided to illustrate the model and the numerical solution scheme, and limitations and extensions of the model are discussed.

Many additive manufacturing technologies involve the deposition of particles onto a surface followed by selective, targeted, laser heating. This paper develops a modular computational framework which combines the various steps within this overall process. Specifically, the framework synthesizes the following: Numerical examples are provided and extensions are also addressed for two advanced processing scenarios involving solid-liquid-gas phase transformations.

Recent efforts in the construction sector to develop panels made of agricultural waste and polymer mixtures are becoming more frequent. These panels are usually developed in pursuit of low embodied energy, biodegradability and energy efficiency. PMMA's transparency and structural advantages over other thermoplastics is making this material progressively more common in facades. While advances in PMMA's recyclability are under development, the process still presents multiple challenges regarding environmental efficiency and resulting structural integrity. It is important, therefore, to establish mixtures where PMMA's structural advantages are balanced with low carbon emission substrates, such as agricultural wastes. Typically, agricultural waste in architectural panels is used for structural reinforcement (i.e. fibers). However, agro-derived materials such as sucrose have additional and/or alternative potentials in construction applications. Sucrose's unique optical properties can be implemented for light and thermal control. Additionally, since the energy losses of buildings concentrate in the envelopes another major challenge of cladding substrates pertains to how the facade material can improve energy efficiency, i.e. acting as thermal mass while providing light transmission control avoiding conditions such as glare. Facade materials that can minimize environmental impact while supporting energy savings and appropriate natural light conditions bear strong potential for advancing sustainable building technologies. This analysis provides an expression for the overall reflectivity of combinations of sucrose and PMMA, as well as, estimates of the thermal heating rate, as a function of the relative volume fractions and material properties.

An important phenomenon in the field of thermoelectric conversion in certain materials is the Seebeck effect, which is characterized by an electrical field, E being produced by a temperature gradient, E = S ∇ θ , where S is known as the (isotropic) Seebeck number. The objective of this note is to develop bounds on the effective thermoelectric Seebeck property for heterogeneous mixtures of materials. Specifically, we develop bounds on 〈 E 〉 Ω = S ∗ 〈 ∇ θ 〉 Ω , where S ∗ is the effective Seebeck number for the mixture, where the averaging operator is defined as 〈 · 〉 Ω = def 1 | Ω | ∫ Ω ( · ) d Ω over a statistically representative volume element with domain Ω , using only the pointwise cross-correlation properties of the material and the average thermal fields. .

In particle finite element simulations, a continuous body is represented by a set of particles that carry all physical information of the body, such as the deformation. In order to form this body, the boundary of the particle set needs to be determined. This is accomplished by the α-shape method, where the crucial parameter α controls the level of detail of the detected shape. However, in solid mechanics, it can be observed that α has an influence on the structural integrity as well. In this paper, we study a single boundary segment of a body during a deformation and it is shown that α can be interpreted as the maximum stretch of this segment. On the continuum level, a relation between α and the eigenvalues of the right Cauchy–Green tensor is presented.

The objective of this paper is to develop a computational model and corresponding solution algorithm to enable rapid simulation of laser processing and subsequent targeted zonal heating of materials composed of packed, discrete, particles. Because of the complex microstructure, containing gaps and interfaces, this type of system is extremely difficult to simulate using continuum-based methods, such as the Finite Difference Time Domain Method or the Finite Element Method. The computationally-amenable model that is developed captures the primary physical events, such as reflection and absorption of optical energy, conversion into heat, thermal conduction through the microstructure and possible phase transformations. Specifically, the features of the computational model are (1) a discretization of a concentrated laser beam into rays, (2) a discrete element representation of the particulate material microstructure and (3) a discrete element transient heat transfer model that accounts for optical (laser) energy propagation (reflection and absorption), its conversion into heat, the subsequent conduction of heat and phase transformations involving possible melting and vaporization. A discrete ray-tracking algorithm is developed, along with an embedded, staggered, iterative solution scheme, which is needed to calculate the optical-to-thermal conversion, particle-to-particle conduction and phase-transformations, implicitly. Numerical examples are given, focusing on concentrated laser beams and the effects of surrounding material conductivity, which draws heat away from the laser contact zone, thus affecting the targeted material state.

As a method for bonding powdered materials, sintering has distinct advantages, such as the production of a near final-shape of the desired product, without the need for significant post-processing. However, sintering has certain deficiencies, such as incomplete or weak bonding. Research is ongoing to improve the process. One approach to improve sintering processes of powdered materials is via electrically enhanced bonding, whereby electricity is pumped through the material, while it is compressed in a press, in order to induce Joule-heating. This paper develops a computationally based model for the direct simulation of electrically enhanced sintering of powdered materials using particle-based methods. The overall approach is to construct three coupled sub-models which primarily involve: (a) particle-to-particle mechanical contact, (b) particle-to-particle thermal exchange and (c) particle-to-particle electrical current flow. These physical processes are strongly coupled, since the dynamics dictates which particles are in contact and the contacts determine the electrical flow. The flow of electricity controls the Joule-heating and the induced thermal fields, which soften the material, leading to enhanced particle binding. The strong multiphysics-coupled sub-models are solved iteratively within each time-step using a recursive staggering scheme, which employs temporal adaptivity to control the error. If the process does not converge (to within an error tolerance) within a preset number of iterations, the time-step is adapted (reduced) by utilizing an estimate of the spectral radius of the coupled system. The modular approach allows for easy replacement of submodels, if needed. Numerical examples are provided to illustrate the model and numerical solution scheme.

The design synthesis of compliant mechanisms yields optimized topologies that combine several stiff parts with highly elastic flexural hinges. The hinges are often represented in a finite element analysis by a single node (one-node hinge), which leaves the actual physical meaning of the hinge (to be fabricated) ambiguous. In order to circumvent this problem, in this work, one-noded hinges have the fatigue-life incorporated into them during the design synthesis by embedding analytical expressions accounting for stress concentration, surface finish, non-zero mean stresses and superposed multiple loading conditions into the formulation. Various flexural hinges with rectangular, circular and parabolic profile geometries are investigated. By incorporating the hinge geometry and fatigue behavior into the design process, unclear interpretation issues that would be encountered during any later manufacturing stage of a compliant mechanism design are removed. Examples are provided to illustrate the overall process.

article i nfo We report systematic experimental and computational studies to analyze evaporative self-assembly of micro particles in printed picoliter suspension droplets. Evaporative self-assembly of micro particles in picoliter droplets is enabled by a droplet-printing system for small-scale particle suspension droplets. Experiments were performed to study the regime where particle interactive forces become comparable to hydrodynamic, evaporative forces of an evaporating droplet. A particle-based computational method was developed to calculate the particle-to-particle clustering time. In this study, we verify that there is a time-scale competition between particle-to-particle clustering and evaporation of the liquid medium that determines the final morphology of micro particle assemblies.

This work develops a model for thermally induced damage from high current flow through biological tissue. Using the first law of thermodynamics, the balance of energy produced by the current and the energy absorbed by the tissue are investigated. The tissue damage is correlated with an evolution law that is activated upon exceeding a temperature threshold. As an example, the Fung material model is used. For certain parameter choices, the Fung material law has the ability to absorb relatively significant amounts of energy, due to its inherent exponential response character, thus, to some extent, mitigating possible tissue damage. Numerical examples are provided to illustrate the model's behavior.

This work is concerned with the identification of microscale properties of additives for base lubricants in order to reduce heat generation. An application of specific interest is the thin film lubrication of bearings. In order to isolate the thermal effects in the fluid film, we assume that the bearing and housing are insulated. A relation for the temperature rise in the fluid film between the bearing and housing is developed as a function of the rotation speed, the viscosity of the base lubricant and properties of the additives, namely (1) their viscosities, (2) their mass density, (3) their heat capacity and (4) volume fraction, which are free design parameters. Nondimensionalization of the developed relations allows for the construction of a design parameter space which can identify desirable parameter combinations that deliver a target value of heat generation reduction and simultaneously deliver the appropriate overall viscosity of the modified lubricant mixture.

Printed electronics are becoming widespread in modern industrial devices. In many of the manufacturing processes of printed electronics, one step is the deposition of initially molten (or liquid), microscale, “print-lines” of material onto a flexible substrate. As the deposited molten print-line solidifies, the bonded print-line and substrate may have the tendency to curl (attain a finite curvature), due to the differences in the thermal expansion coefficients, elastic properties, etc. The quality and durability of the solidified print-line (which is mechanically-weak) is adversely affected by residual tensile stress states. Tensile stress states have a tendency to induce damage in the form of cavities or cracks in the deposited material, which would hinder the printed electronics operation. Therefore, ideally, one would like the solidified print-line to be in a state of compression. Inducing a compressive stress-state in the print-line is particularly important in the vicinity of the substrate interface, since damage in that location may also initiate delamination of the deposited material and, eventually, a malfunction of the intended printed electronics application. In this work, employing an elementary thermo-mechanical model, a mathematical expression is derived for the combination of system parameters needed to ensure that the print-line material at remains in a compressive state at the bimaterial interface.

This work develops a rigorous variational upper bound for the difference between thermal fields generated in uniform media and thermal fields generated in heterogeneous media, for the same external loading. The bound can be calculated in a simple manner, with knowledge only of the heterogeneous material properties and a relatively easy-to-compute thermal field associated with a uniform medium. In order to evaluate the bound, the difficult-to-compute thermal field, associated with the heterogeneous material, does not need to be calculated. Three-dimensional numerical examples are provided to illustrate the results.

This paper presents the synthesis of large-displacement compliant mechanisms (CM) with specific output motion paths. For this purpose, a robust and efficient staggered topology optimization scheme, based on the optimality criteria (OC) method and the globally convergent method of moving asymptotes (GCMMA), is proposed, described and implemented to maximize the output motion of the design CM for a user-specified motion path. Nonlinear geometric effects are taken into account to ensure proper modelling of large displacements occurring in CM. Different setups of a vector-based formulation are described and benchmarked. Numerical results for a gripping mechanism model problem show that the presented approach generates path-following compliant mechanisms in an efficient and systematic manner. (© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)

This work investigates the deformation of electrified textiles in the presence of an externally supplied magnetic field ( B ext ). The electrification is delivered by running current ( J ) through the fibers from an external power source. Of primary interest is to ascertain the resulting electromagnetic forces imposed on the fabric, and the subsequent deformation, due to the terms J × B ext and P E , where P is the charge density, E is the electric field and the current given by J = σ ( E + v × B ext ) , where σ is the fabric conductivity, and v is the fabric velocity. As the fabric deforms, the current changes direction and magnitude, due to the fact that it flows through the fabric. The charge density is dictated by Gauss’ law, ∇ · D = P , where D = ϵ E , ϵ is the electrical permittivity and D is the electric field flux. In order to simulate such a system, one must solve a set of coupled equations governing the charge distribution, current flow and system dynamics. The deformation of the fabric, as well as the charge distribution and current flow, are dictated by solving the coupled system of differential equations for the motion of lumped masses, which are coupled through the fiber-segments under the action of electromagnetically-induced forces acting on a reduced order network model. In the work, reduced order models are developed for (a) Gauss’ law ( ∇ · D = P ), (b) the conservation of current/charge, ∇ · J + ∂ P ∂ t = 0 , and (c) the system dynamics, ∇ · T + f = ρ d v dt , where T is the Cauchy stress and f represents the induced body forces, which are proportional to P E + J × B ext . A temporally-adaptive, recursive, staggering scheme is developed to solve this strongly coupled system of equations. We also consider the effects of progressive fiber damage/rupture during the deformation process, which leads to changes (reduction) in the electrical conductivity and permittivity throughout the network. Numerical examples are given, as well as extensions to thermal effects, which are induced by the current-induced Joule-heating.

© 2016 Elsevier B.V. An electromagnetic beam model is developed for the simulation of actuated electronic textiles. The beam is solved using a nonlinear director-based kinematic description with additional temperature and electric potential fields along its length. The three fields are fully coupled by mutual dependences on the deformation, Lorenz force, back electromotive force, temperature dependent constitutive responses, and the Seebeck effect. Instead of solving Maxwell's equations in full detail, a quasistatic approximation is used to solve the electric potential in the presence of a moving material medium. The current-carrying beam approximation is used to further simplify the solution space for the potential. While this formulation alleviates the spatial and temporal discretization restrictions, the coupled problem is an index-1 semi-explicit Differential Algebraic Equation requiring special treatment. The time dependent problem is solved using different Runge-Kutta methods. Diagonally implicit Runge-Kutta methods and explicit Runge-Kutta methods using implicit solution of the electric potential problem are explored. The finite element model is implemented using the open source package FEniCS, which is able to automatically generate the linearizations of the multiphysics equations required for the implicit solutions. A model problem is constructed with which to test and analyze the physical formulation and numerical solution techniques. The time stepping methods are verified using the convergence orders of the higher-order Runge-Kutta methods. Runtime comparisons show that the explicit methods are generally more computationally efficient than the implicit schemes used for this problem. For the implicit schemes, a staggered solution is significantly faster than a monolithic solution at most time step sizes. However, at very large time steps, such as those that would be used for dynamic relaxation, the monolithic solution can be more efficient than the staggered solution.

Selective laser sintering (SLS) is an additive manufacturing technology whereby one can 3D print parts out of a powdered material. However, in order to produce defect free parts of sufficient strength, the process parameters (laser power, scan speed, powder layer thickness, etc.) must be carefully optimized depending on material, part geometry, and desired final part characteristics. Computational methods are very useful in the quick optimization of such parameters without the need to run numerous costly experiments. Most published models of this process involve continuum-based techniques, which require the homogenization of the powder bed and thus do not capture the stochastic nature of this process. Thus, the aim of this research is to produce a reduced order computational model of the SLS process which combines the essential physics with fast computation times. In this work the authors propose a coupled discrete element-finite difference model of this process. The powder particles are modeled as discrete, thermally and mechanically interacting spheres. The solid, underneath substrate is modeled via the finite difference method. The model is validated against experimental results in the literature and three-dimensional simulations are presented.

For composite materials, two quantities that are useful for characterizing the contribution of inhomogeneities in a matrix material to the overall properties are (1) the individual H-tensor, Hi, which describes the contribution of a single inhomogeneity and (2) the overall strain concentration tensor, which describes the relationship between the overall volumetric strain to the average strain of all of the inhomogeneities. In this paper, we develop a relationship expressing the overall H-tensor, \({\mathcal{H}}\) , in terms of the overall strain concentration tensor. An important feature of the derivation is that it allows for rigorous upper and lower bounds on the overall H-tensor. In the special case that the inhomogeneities are all the same, with the same orientation, then \({\mathcal{H} = {\bf H}_i}\) , and the results derived for \({\mathcal{H}}\) also hold for Hi.

In a variety of emerging energy applications such as photovoltaic conversion, thermo-electric conversion, electrochromic actuation, artificial photosynthesis, etc., the capture and trapping of light on a surface is a critical first step in a multistage process. The functionalization of a surface by adding small-scale features, such as fine-scale rods, to trap incoming light, is one possible approach. In this paper, a model that is amenable to large-scale computation is developed. The approach provides a computational tool that allows analysts to quickly study a wide variety of rod-like microstructures. Both analytical and large-scale computational results are presented.

Rapid, energy-efficient sintering of materials comprised heterogeneous powders is of critical importance in emerging technologies where traditional manufacturing processes may be difficult to apply. In particular, electrically aided sintering, which uses the material's inherent resistance to flowing current—resulting in Joule heating to bond the powder components—has great promise because it produces desired materials without much post-processing. Furthermore, it has advantages over other methods, such as high purity of processed materials, in particular, because there are few steps during the approach. In order to electrically process the material properly, one must ascertain the externally applied field to properly Joule heat the various material components in the powder mixture. The Joule-heating field is mathematically expressed by the inner product ( J ⋅ E ) of the current ( J ) and electric ( E ) fields throughout the system. This study develops estimates for the Joule-heating fields carried by each phase in a powder mixture, using knowledge of only the externally applied current, and the material properties of the components comprising the mixture. These estimates are useful in guiding and reducing time-consuming material synthesis involving laboratory experiments and/or large-scale numerical simulation.

We report a fast, high-throughput method to create size-tunable micro/nanoparticle clusters via evaporative assembly in picoliter-scale droplets of particle suspension. Mediated by gravity force and surface tension force of a contacting surface, picoliter-scale droplets of the suspension are generated from a nanofabricated printing head. Rapid evaporative self-assembly of the particles on a hydrophobic surface leads to fast clustering of micro/nanoparticles and forms particle clusters of tunable sizes and controlled spacing. The evaporating behavior of the droplet is observed in real-time, and the clustering characteristics of the particles are understood based on the physics of evaporative-assembly. With this method, multiplex printing of various particle clusters with accurate positioning and alignment are demonstrated. Also, size-unifomity of the cluster arrays is thoroughly analyzed by examining the metallic nanoparticle cluster-arrays based on surface-enhanced Raman spectroscopy (SERS).

Due to increasing demands for faster and faster manufacturing of new complex materials, such as casting of particulate composites, the determination of pumping pressures needed for particle-laden fluids through channels is critical. In particular, the increase in viscosity as a function of the particle volume fraction can lead to system malfunction, due to an inability to deliver necessary pressures to pump the more viscous fluid through the system. This paper studies the pressure gradient needed to maintain a given flow rate, explicitly as a function of the volume fraction of particles present in the fluid. It is also crucial to control voids in the casted products, which can be traced to air-entrainment, spurious internal reactions, dewetting, etc., which can be traced to high Reynolds numbers. Accordingly, an expression for the resulting Reynolds number as a function of the particle volume fraction and flow rate is also developed. Numerical examples are provided to illustrate the practical use of the derived relations to characterize the necessary pumping pressures for process-driven, particle-laden fluid flows.

The microstructure of chain-mail (CM) armor consists of a network of small links that are connected together to form a sheet. A network-type model, amenable to straightforward numerical simulation, is formulated, where the links are modeled as supporting only axial (tensile) loading, and where the interconnections are idealized as three-dimensional frictionless pin-joints. Because of its use as a ballistic shield, the strain-rate dependent thermo-mechanical (viscoplastic) response is important, due to thermal softening. The philosophy behind the proposed direct modeling approach is to harness the dramatic increases in readily available scientific computing to simulate realistic responses of structural CM, by starting directly at the microscale, where relatively simple description of the material is possible. By employing enough of these simple structural elements, one can build an entire macroscale sheet of CM. The deformation of the CM is dictated by solving a (“link-coupled”) system of differential equations for the motion of the interconnected masses. Large-scale simulations, illustrating the thermomechanical response of chain-mail material armor, undergoing impact with a rigid indenter, are presented to illustrate the potential of the approach in delivering realistic responses, involving dynamic rupture and penetration of structural CM.

Doping dielectric materials with particulates for use in electronic device applications is wide-spread, particularly for energy-storage devices such as ultracapacitors and batteries. This work investigates the resulting distortion of the electrical fields in multiphase (particle and matrix) material systems. Of particular interest is to ascertain safe overall electrical loading conditions in order to avoid current overload in heterogeneous media. Specifically, it is important to determine the phase-wise Joule-type heating field, formed by the inner product of the current and electric fields. General estimates are developed, and two asymptotic cases are studied: (1) high-conductivity (“superconducting”) particles added to a lower relative-conductivity matrix and (2) low-conductivity (“insulator”) particles added to a higher relative-conductivity matrix. The expressions developed provide a relatively easy guide for the selection of dopants in dielectric material design.

This work addresses the modeling and simulation of strongly coupled electromagnetic and thermodynamic fields that arise in particulate-doped dielectrics using an adaptive staggered adaptive FDTD (finite difference time domain) method. Of particular interest is to provide a straightforward modular approach to finding the effective dielectric (electromagnetic) response of a material, incorporating thermal effects, arising from Joule heating, which alter the pointwise dielectric properties such as the electric permittivity, magnetic permeability, and electric conductivity. This is important for “thermal (damage) management” of materials used in electromagnetic applications. Because multiple field coupling is present, a staggered, temporally-adaptive scheme is developed to resolve the internal microstructural electric, magnetic and thermal fields, accounting for the simultaneous pointwise changes in the material properties. Numerical examples are provided to illustrate the approach. Extensions to coupled chemical and mechanical fields are also provided.

The rapid testing of red blood cell (RBC) volume fraction (“hematocrit”) is becoming increasingly important to identify blood disorders (“hemoglobinopathies”). Electromagnetic techniques provide an advantageous way to measure blood properties, primarily because they are inexpensive, quick and noninvasive. The goal of this paper is to develop estimates of the RBC volume fraction levels of whole blood from macroscopic electromagnetic (permittivity) measurements. The approach taken is to generate volume fraction estimates by inverting classical bounds on the effective permittivity of dielectric mixtures. The usefulness of the approach is that, given the permittivities of the plasma (known), cells (known) and whole mixture (measured), one can determine the cell volume fraction and compare it to the levels found in healthy blood. The deviation of the properties can be used to help characterize certain blood disorders. The expressions developed are not limited to RBC measurement, and are applicable to any cell-in-solution system. Through correlation of our laboratory measurements, the analytical expressions and direct large-scale numerical simulations, the results suggest that RBCs form high-permittivity cell-networks by making cell-to-cell contact, even at relatively low volume fraction.

SUMMARY The dynamics of rigid clusters of charged particulates is the subject of this work. The work ascertains what properties of the cluster control its dynamic response to an external electromagnetic field. A primary focus is on the role of the distribution of the charges within the cluster and the effects of the Lorentz force on the overall body’s linear and angular momentum. The presentation contains a derivation of the equations governing a charged cluster’s dynamics and development of corresponding numerical methods for the simulation. Numerical examples are presented, along with comparisons to qualitative analytical results, where possible. Copyright 2010 John Wiley & Sons, Ltd. Received 24 March 2010; Revised 2 June 2010; Accepted 30 June 2010

The trajectory of free atmospheric electrical currents, such as lightning and sparks, is strongly influenced by microscale events that occur at the current front. In particular, highly conductive pathways can occur at the free surface front due to dielectric breakdown. The specific directions of the local pathways are minutely perturbed, due to the gaseous, disordered, nature of the media at the small scale. This results in highly conductive, anisotropically perturbed, continuum-level properties at the electrical current front. In this work, a model is developed to investigate the role of the resulting anisotropically perturbed conductivity at the propagation front on the overall trajectory of free atmospheric electrical currents. The approach is to relate the electrical current velocity to the local anisotropic conductivity at the propagation front and the surrounding electric field. The conductive anisotropy is decomposed into an isotropic 'base state' and an anisotropic perturbation. The current trajectory is shown to be governed by a set of non-linear differential equations, for which a numerical solution scheme is developed. The difference between paths taken through anisotropically perturbed and isotropic media is analytically bounded and quantified numerically as a function of the magnitude of the anisotropic perturbation. The analysis and numerical experiments indicate that, in a statistical sense, the difference in the paths taken in anisotropically perturbed and isotropic media depends quasilinearly on the perturbation magnitude.

We develop a novel patterning technique to create 3D patterns of micro and nanoparticle assembly via evaporative self-assembly based on the coffee-ring effect of an evaporating suspension. The principle of the technique is analyzed theoretically by the scaling analysis of main parameters of the process and the scaling effect, the effect of the volume, the concentration of the suspension, and the effect of surface treatment on the patterning are studied. On the basis of the presented technique, we demonstrate that the patterns of 3D assembly of various sizes of microparticles (Silica), metal oxide nanoparticles (TiO(2), ZnO), and metallic nanoparticles (Ag) can be successfully generated by low-concentrated particle suspension (1.25-5 wt %) without additional sintering steps, and we also show the geometries of the patterns can be finely controlled by adjusting the parameters of the process.

In modern advanced manufacturing processes, such as three-dimensional printing of electronics, fine-scale particles are added to a base fluid yielding a modified fluid. For example, in three-dimensional printing, particle-functionalized inks are created by adding particles to freely flowing solvents forming a mixture, which is then deposited onto a surface, which upon curing yields desirable solid properties, such as thermal conductivity, electrical permittivity and magnetic permeability. However, wear at solid–fluid interfaces within the machinery walls that deliver such particle-laden fluids is typically attributed to the fluid-induced shear stresses, which increase with the volume fraction of added particles. The objective of this work is to develop a rigorous strict upper bound for the tolerable volume fraction of particles that can be added, while remaining below a given stress threshold at a fluid–solid interface. To illustrate the bound’s utility, the expression is applied to a series of classical flow regimes.

This work addresses the modeling and simulation of charged particulate jets in the presence of electromagnetic fields. The presentation is broken into two main parts: (1) the dynamics of charged streams of particles and their interaction with electromagnetic fields and (2) the coupled thermal fields that arise within the jet. An overall model is built by assembling submodels of the various coupled physical events to form a system that is solved iteratively. Specifically, an approach is developed whereby the dynamics of charged particles, accounting for their collisions, inter-particle near-fields, interaction with external electromagnetic fields and coupled thermal effects are all computed implicitly in an iterative, modular, manner. A staggered, temporally-adaptive scheme is developed to resolve the multiple fields involved and the drastic changes in the physical configuration of the stream, for example when impacting a solid wall or strong localized electromagnetic field. Qualitative analytical results are provided to describe the effects of the electromagnetic fields and quantitative numerical results are provided for complex cases.

This work deals with the dynamic contact of a rigid body with a deformable electromagnetically sensitive fabric structure, represented by a network model. Of particular interest are the electromagnetically induced forces generated on the fabric, which are proportional to the external electric field (E EXT ) and the velocity crossed with the external magnetic field (v × B EXT ). These forces transmit reactions to the rigid contacting object, which can induce rotational motion. Modeling and simulation of this effect can be useful in ballistic shielding applications, because the rotation of an incoming, ogival, projectile allows it to be more easily impeded. A modular formulation for the deformation of impacted fabric structures, represented by a network model, is developed in this paper, characterized by (1) stretching of interconnected yarn networks, described by simple constitutive relations, including yarn damage, (2) interaction with impacting objects, incorporating contact with friction and (3) electromagnetic sensitivity and actuation, demonstrating how the Lorentz force can be harnessed to break symmetric deformation patterns in order to induce spin onto an incoming object, whether that object is electromagnetically sensitive or not.

The primary objective of this communication is to qualitatively describe charged wall growth in channel flow. The reduction of channel flow cross-section is often attributed to microscale suspensions and dissolved minerals which adhere to the flow boundaries (walls). In this analysis, the wall is comprised of an electrically conductive material, capable of carrying a charge. The flowing fluid, containing ions, is also assumed to be capable of carrying a charge. The resulting electrical field can be determined using Gauss’ law, which allows the associated electrical force to be computed at the solid–fluid interface. This force can impede or enhance the attachment of suspensions to the wall. Thus, there are two effects that play a role at the charged solid–fluid interface/wall: (1) mechanical shear due to the fluid flow and (2) an electrical force. It is the goal of this communication to develop a relationship that characterizes both contributions and to ascertain how this relationship scales with wall growth.

We developed an ultrafast microfluidic approach to self-assemble microparticles in three dimensions by taking advantage of simple photolithography and capillary action of microparticle-dispersed suspensions. The theoretical principles of high-speed assembly have been explained, and the experimental verifications of the assembly of various sizes of silica microspheres and silica gel microspheres within thin and long open microchannels by using this approach have been demonstrated. We anticipate that the presented technique will be widely used in the semiconductor and Bio-MEMS (microelectromechanical systems) fields because it offers a fast way to control 3D microscale particle assemblies and also has superb compatibility with photolithography, which can lead to an easy integration of particle assembly with existing CMOS (complementary metal oxide-semiconductor) and MEMS fabrication processes.

In many materials, strong electrical fields can cause highly conductive pathways to occur due to dielectric breakdown, which can cause the material to “jump” to a higher permittivity state. This effect is often undesirable and can lead to electrically-induced failure of a device, for example due to overheating. The overall goal of this work is to estimate the volume fraction and properties of the particulate additives needed to reduce the electrical load carried by a bulk material, in order to avoid dielectric breakdown.

A central issue in the use of ballistic fabric shielding is the mode of attachment to the structure that it is intended to protect. In order to investigate this issue, a discrete multi-scale yarn-network model is developed for structural fabric undergoing ballistic impact, based on work found in Zohdi and Powell [Zohdi TI, Powell D. Multiscale construction and large-scale simulation of structural fabric undergoing ballistic impact. Comput Meth Appl Mech Eng 2006;195:94–109] and Zohdi [Zohdi TI. Modeling/simulation of progressive penetration of multilayered ballistic fabric shielding. Comput Mech 2002;29:61–7]. The model is comprised of a network of yarn with stochastic properties determined by smaller-scale fibrils, which are randomly misaligned. The effects of stochasticity on the overall response are explored, and the model is compared against macro-scale experiments. The key feature of the model is the fact that it does not depend on phenomenological parameters, and can be calibrated by simply measuring the properties of an individual, smallest-scale, fibril. The properties of a fibril are easily ascertained from a simple tension test. The response of the overall fabric model and ballistic experiments are in excellent agreement. The model indicates that fabric which is attached by being pinned at the corners generally absorbs more energy, relative to fabric clamped along the sides. The basis for this result is discussed at length in the body of this work. Furthermore, it is observed that a uniform-yarn model, one which ignores the stochastic nature of the yarn, over-estimates the amount of energy absorbed.