Influence of particle elasticity in shear testers

Two dimensional simulations of non-cohesive granular matter in a biaxial shear tester are discussed. The effect of particle elasticity on the mechanical behavior is investigated using two complementary distinct element methods (DEM): Soft particle molecular dynamics simulations (Particle Flow Code, PFC) for elastic particles and contact dynamics simulations (CD) for the limit of perfectly rigid particles. As soon as the system dilates to form shear bands, it relaxes the elastic strains so that one finds the same stresses for rigid respectively elastic particles in steady state flow. The principal stresses in steady state flow are determined. They are proportional to each other, giving rise to an effective macroscopic friction coefficient which is about 10% smaller than the microscopic friction coefficient between the grains.     

Tailored elastic behavior of multilayers through controlled interface structure

Summary Atomistic simulations are reviewed that elucidate the causes of the anomalous elastic behavior of thin films and composition-modulated superlattice materials. The investigation of free-standing thin films and of superlattices, composed of grain boundaries, shows that the elastic anomalies are not an electronic but a structural interface effect that is intricately connected with the local atomic disorder at the interfaces. The consequent predictions that (i)coherent strained-layer superlattices should show the smallest elastic anomalies and (ii) making the interfaces incoherent should enhance all anomalies, are validated by simulations of dissimilar-material superlattices. Such simulations can be an effective aid in tailoring the elastic behavior of composite materials because, in contrast with experiments, they allow one to systematically investigate simple, but well-characterized model systems with increasing complexity. This unique capability of simulations has enabled us to elucidate the underlying driving forces and, in particular, (i) to deconvolute the distinct effects due to the inhomogeneous atomic disorder, localized at the interfaces from the consequent interface-stress-induced anisotropic lattice-parameter changes and (ii) to separate the homogeneous effects of thermal disordering from the inhomogeneous effects due to the interfaces.     

On the elastic modulus of metallic nanowires

Previous atomistic simulations and experiments have attributed size effects in the elastic modulus of Ag nanowires to surface energy effects inherent to metallic surfaces. However, differences in experimental and computational trends analyzed here imply that other factors are controlling experimentally observed modulus changes. This study utilizes atomistic simulations to determine how strongly nanowire geometry and surface structure influence nanowire elastic modulus. The results demonstrate that although these factors do influence the elastic modulus of Ag nanowires to some extent, they alone are insufficient to explain current experimental trends in nanowire modulus with decreasing dimensional scale. Future work needs to be done to determine whether other factors, such as surface contaminants or oxide layers, contribute to the experimentally observed elastic modulus increase.     

Fluid–structure interaction modeling of a patient-specific cerebral aneurysm: influence of structural modeling

Fluid–structure interaction (FSI) simulations of a cerebral aneurysm with the linearly elastic and hyper-elastic wall constitutive models are carried out to investigate the influence of the wall-structure model on patient-specific FSI simulations. The maximum displacement computed with the hyper-elastic model is 36% smaller compared to the linearly elastic material model, but the displacement patterns such as the site of local maxima are not sensitive to the wall models. The blood near the apex of an aneurysm is likely to be stagnant, which causes very low wall shear stress and is a factor in rupture by degrading the aneurysmal wall. In this study, however, relatively high flow velocities due to the interaction between the blood flow and aneurysmal wall are seen to be independent of the wall model. The present results indicate that both linearly elastic and hyper-elastic models can be useful to investigate aneurysm FSI.     

The influence of the degree of heterogeneity on the elastic properties of random sphere packings

The macroscopic mechanical properties of colloidal particle gels strongly depend on the local arrangement of the powder particles. Experiments have shown that more heterogeneous microstructures exhibit up to one order of magnitude higher elastic properties than their more homogeneous counterparts at equal volume fraction. In this paper, packings of spherical particles are used as model structures to computationally investigate the elastic properties of coagulated particle gels as a function of their degree of heterogeneity. The discrete element model comprises a linear elastic contact law, particle bonding and damping. The simulation parameters were calibrated using a homogeneous and a heterogeneous microstructure originating from earlier Brownian dynamics simulations. A systematic study of the elastic properties as a function of the degree of heterogeneity was performed using two sets of microstructures obtained from Brownian dynamics simulation and from the void expansion method. Both sets cover a broad and to a large extent overlapping range of degrees of heterogeneity. The simulations have shown that the elastic properties as a function of the degree of heterogeneity are independent of the structure generation algorithm and that the relation between the shear modulus and the degree of heterogeneity can be well described by a power law. This suggests the presence of a critical degree of heterogeneity and, therefore, a phase transition between a phase with finite and one with zero elastic properties.     

Hybrid molecular dynamics–finite element simulations of the elastic behavior of polycrystalline graphene

In this paper, we develop an efficient multiscale molecular dynamics (MD)–finite element (FE) modeling scheme capable of determining the elastic and fracture properties of polycrystalline graphene. The local elastic properties of a grain boundary (GB) connecting two adjacent graphene grains, with different lattice orientations, were first determined using MD simulations. In a two-dimensional medium, randomly distributed grains connected with GBs were then created using the Voronoi tessellation method. The constructed Voronoi diagrams were used to create FE models of the polycrystalline graphene, where the GBs were represented by interphase regions with their local properties determined using MD. The grains were modeled as pristine graphene and the accuracy of the polycrystalline FE model was validated with MD simulations of a geometrically identical polycrystalline graphene. The results reveal good agreement between MD and FE simulations. They further show that the elastic and fracture properties of polycrystalline graphene are greatly influenced by the grain size and the misorientation angle. They also indicate that the predicted elastic properties are in agreement with earlier reported experimental and MD results. We believe that this newly proposed multiscale scheme could be easily integrated into current design software to model graphene based nano- and micro-devices.     

Various elastic moduli of AA6016 and their application on accurate prediction of springback

First of all, the elastic modulus variation of 6016-T4 aluminum alloy has been investigated by uniaxial tensile experiments. Secondly, a new model to describe the change of elastic modulus has been proposed based on our experimental data. Then Finite Element (FE) simulations with various elastic moduli have been conducted to predict the springback of V bending. The results are shown as follows: the elastic modulus decreases, obviously, with increase of equivalent plastic strain; the proposed model is more suitable to explain our experimental data than the other models considered; and the springback prediction of V bending with various elastic moduli is more precise than with constant modulus.     

Numerical studies of polymer networks and gels

The elastic and relaxational properties of polymer networks are strongly influenced by quenched disorder, introduced by the crosslinks in the systems. By extensive computer simulations of appropriately tailored model systems these effects are systematically investigated.     

Use of Nanoindentation,Finite Element Simulations,and a Combined Experimental/Numerical Approach to Characterize Elastic Moduli of Individual Porous Silica Particles

This article describes the use of a combination of experimental nanoindentation and finite element numerical simulations to indirectly determine the elastic modulus of individual porous, micron-sized silica (SiO₂) particles. Two independent nanoindentation experiments on individual silica particles were employed, one with a Berkovich pyramidal nanoindenter tip, the other with a flat punch nanoindenter tip. In both cases, 3D finite element simulations were used to generate nanoindenter load–displacement curves for comparison with the corresponding experimental data, using the elastic modulus of the particle as a curve-fitting parameter. The resulting indirectly determined modulus values from the two independent experiments were found to be in good agreement, and were considerably lower than the published values for bulk or particulate solid silica. The results are also consistent with previously reported modulus values for nanoindentation of porous thin film SiO₂. Based on a review of the literature, the authors believe that this is the first article to report on the use of nanoindentation and numerical simulations in a combined experimental/numerical approach to determine the elastic modulus of individual porous silica particles.     

Assessment of forming parameters influencing spring-back in multi-point forming process: A comprehensive experimental and numerical study

Like all sheet metal forming methods, one of the main characteristics of parts formed by multi-point forming is dimensional deviation caused by elastic recovery that is known as spring-back. In this paper the effects of material property, sheet thickness and anisotropy ratio along with process parameters such as elastic layer thickness, elastic layer hardness and number of punch elements on spring-back are studied utilizing finite element simulations and experimental tests. Experimental tests are carried out under various conditions by forming V-shaped and Sin-shaped geometries. Aluminum alloy 3105, stainless steel 304 and pure copper were used as sheet materials for experiments. Likewise, black rubber with shore A hardness of 50 and polyurethane with hardness of 65 and 85 were allocated as elastic layers. The Abaqus® commercial code is employed for finite element simulations. The definition of yield behavior of utilized sheet materials is fulfilled by using three yield criteria of Barlat-89, Hill-48 and Von-Mises. Since the Barlat-89 is not adopted in Abaqus, VUMAT and UMAT user defined subroutines are provided and integrated with explicit simulation of forming process and implicit simulation of spring-back phenomenon respectively. The results indicate that parameters such as material property, blank thickness and anisotropy affect spring-back in multi-point forming. Also the thickness and hardness of elastic layers are novel ideas that should be considered in order to minimize the spring-back. In general, using the elastic layer with minimum possible thickness and greater hardness beside the maximum number of pins leads to minimum spring-back.     

Shear circular dislocation loops investigated with elasticity theory and atomistic simulations

The elastic fields of displacements, strains, and stresses for a shear circular loop are obtained with the Burgers formula. In addition, interactions between two shear circular loops are obtained based on elasticity theory. A series of molecular dynamics (MD) simulations on a shear circular partial dislocation loop in copper have been performed to examine the elastic solutions. It is found that the results of the MD simulations are in good agreement with those of elasticity theory for a loop with radius 7.5 nm.     

A computational study of the relations between material properties of long-rod penetrators and their ballistic performance

The paper summarizes a series of two-dimensional numerical simulations which were performed to study the effects of material properties on the terminal ballistics of long-rod penetrators. Our focus was on the properties of the rod material, unlike recent works which concentrated on a target’s properties. We varied almost all the relevant parameters within a large range of values in order to study the separate effects of each one. These parameters included: compressive and tensile strengths, elastic moduli, melting temperatures and the maximum equivalent plastic strain (failure strain) of the rod material. Most of the simulations were performed for an actual experiment with 300 mm tungsten-alloy long-rod, impacting a semi-infinite steel target. The simulations show that the mechanical and thermal softening mechanisms are the most dominant, as far as the depth of penetration is concerned. In contrast, the elastic moduli and spall strength have a negligible effect as far as penetration depth is concerned.     

On the nature of attractive dislocation crossed states

Attractive non-coplanar dislocations that cannot react to form junctions can, nevertheless, form crossed states, i.e., junctions of null length. Such configurations have recently been described by Wickham and co-workers as an output of numerical simulations. The physical origin of the crossed states is cleared out and their conditions of occurrence are calculated within a simplified elastic frame. The results are further discussed by comparison with mesoscopic simulations of intersecting dislocations in fcc and bcc crystals.     

Effects of thickness and fiber volume fraction variations on strain field inhomogeneity

In this study, variations in thickness and fiber volume fraction are investigated as causes of elastic strain inhomogeneity in composite laminates under an applied transverse load. Standard carbon/epoxy tensile specimens were fabricated from unidirectional pre-impregnated material using two different manufacturing techniques that produced two different levels of surface roughness. Fiber volume fraction variation was computed by analyzing optical micrographs of the samples. During loading and unloading of the samples two-dimensional surface strain fields were measured on the specimen using digital image correlation. It was shown that in both cases the strain in the specimen is not uniform, as is generally assumed. Using finite element simulations the effects of fiber volume fraction variation and thickness variation were modeled individually and in combination. The simulations agree well with the experimental results and suggest that thickness variations are the dominant mechanisms involved in this elastic strain inhomogeneity.     

Material property prediction of thermoset polymers by molecular dynamics simulations

Molecular dynamics simulations are conducted to predict thermal and mechanical properties of a family of thermoset polymers. We focus on the effect of cross-linkers on density, glass transition temperature, elastic constants, and strength. The polymers are composed of the epoxy resin DGEBA (EPON825) and a series of cross-linkers with different number of active sites and rigidity 33DDS, 44DDS, APB133, TREN, and TAPA. Our simulations quantify effects of cross-linkers on thermal and mechanical properties.     

Measuring the nonlinear elastic properties of tissue-like phantoms

A direct mechanical system simultaneously measuring external force and deformation of samples over a wide dynamic range is used to obtain force-displacement curves of tissue-like phantoms under plain strain deformation. These measurements, covering a wide deformation range, then are used to characterize the nonlinear elastic properties of the phantom materials. The model assumes incompressible media, in which several strain energy potentials are considered. Finite-element analysis is used to evaluate the performance of this material characterization procedure. The procedures developed allow calibration of nonlinear elastic phantoms for elasticity imaging experiments and finite-element simulations.     

Development of numerical scheme for phase field crystal deformation simulation

The phase field crystal (PFC) method is anticipated as a new multiscale method, because this method can reproduce physical phenomena depending on atomic structures in metallic materials on the diffusion time scale. Although the PFC method has been applied to some phenomena, there are few studies related to evaluations of mechanical behaviors of materials by appropriate PFC simulation. In a previous work using the PFC method, tensile deformation simulations have been performed under conditions where the volume increases during plastic deformation. In this study, we developed a new numerical technique for PFC deformation simulation that can maintain a constant volume during plastic deformation. To confirm that the PFC model with the proposed technique can reproduce appropriate elastic and plastic deformations, we performed a series of deformation simulations in one and two-dimensions. In one- and two-dimensional single-crystal simulations, linear elastic responses were confirmed in a wide strain rate range. In bicrystal simulations, we could observe typical plastic deformations due to the generation, annihilation and movement of dislocations, and the interaction between the grain boundary and dislocations. Moreover, the deformation behaviors of a nanopolycrystalline structure at high temperature were simulated and the intergranular deformations caused by grain rotation and grain boundary migration were reproduced.     

Molecular dynamics simulation of SWCNT–polymer nanocomposite and its constituents

Elastic and engineering properties of nanoparticle enhanced composites and their constituents (matrix, reinforcement and interface) are calculated. The nanocomposites considered in this study consist of a single-wall carbon nanotube (SWCNT) embedded in polyethylene matrix. Molecular dynamics simulations are used to estimate the elastic properties of SWCNT, interfacial bonding, polyethylene matrix and composites with aligned and randomly distributed SWCNTs. The elastic properties of bundles with 7, 9, and 19 SWCNTs are also compared using a similar approach. In all simulations, the average density of SWCNT–polymer nanocomposite was maintained in the vicinity of CNTs, to match the experimentally observed density of a similar nanocomposite. Results are found to be in good agreement with experimentally obtained values by other researchers. The interface is an important constituent of CNT–polymer composites, which has been modeled in the present research with reasonable success.