Heterogeneous Material Systems Research Articles

Self-assembly of single “square” quantum rings in gold-free GaAs nanowires

Single nanostructures embedded within nanowires (NWs) represent one of the most promising technologies for applications in quantum photonics. However, fabrication imperfections and etching-induced defects are inevitable for top-down fabrications, whereas self-assembly bottom-up approaches cannot avoid the difficulties of its stochastic nature and are limited to restricted heterogeneous material systems. Here we demonstrate the versatile self-assembly of single "square" quantum rings (QR) on the sidewalls of gold-free GaAs NWs for the first time. By tuning the deposition temperature, As overpressure and amount of gallium-droplets, we were able to control the density and morphology of the structure, yielding novel single quantum dots, QR, coupled QRs, and nano-antidots. A proposed model based on a strain-driven, transport-dependent nucleation of gallium droplets at high temperature accounts for the formation mechanism of these structures. We achieved a single-QR-in-NW structure, of which the optical properties were analyzed using micro-photoluminescence at 10 K and a spatially resolved cathodoluminescence technique at 77 K. The spectra show sharp discrete peaks; of these peaks, the narrowest linewidth (separation) was 578 μeV (1-3 meV), reflecting the quantized nature of the ring-type electronic states.

Compression of Heterogeneous Material Systems Based on Wang Tilings

The present study is on the concept of modeling of heterogeneous materials by means of Wang tilings. The central idea is to store a microstructural information within a finite set of Wang Tiles, which allow for reconstructing heterogeneity patterns of random media in planar domains of arbitrary sizes. The particular objective of presented work is our automatic tile set designer in conjunction with stochastic tiling synthesis algorithm. The proposed methodology is demonstrated on different examples. The proximity of synthesized microstructures to reference media is explored by statistical descriptors and discussed in terms of parasitic spatial orientation orders that may occur.

Micro-RVE modeling of mechanistic response in porous intermetallics subject to weak and moderate impact loading

In this article we propose macroscopic (continuum) simulation schemes to predict response of porous heterogeneous material systems subjected to weak and moderate impact velocities. The proposed simulation model includes (1) an equation of state for porous solids that describes the evolution of porosity in the material as a function of shock pressure and, (2) a macroscopic rate dependent plasticity model for the porous composite that accounts for the deviatoric strength of the material at weak to moderate shock strengths. In addition, the numerical scheme employs cold-mixture theory to predict shock response of porous intermetallics. The material model is validated using gas-gun impact experiments on Ni/Al Intermolecular Reactive Composite (IRC) at 70% TMD. The proposed model is also used to understand the effect of microstructure on the material response predictions.

Blast Wave Loading Pathways in Heterogeneous Material Systems–Experimental and Numerical Approaches

Blast waves generated in the field explosions impinge on the head-brain complex and induce mechanical pressure pulses in the brain resulting in traumatic brain injury. Severity of the brain injury (mild to moderate to severe) is dependent upon the magnitude and duration of the pressure pulse, which in turn depends on the intensity and duration of the oncoming blast wave. A fluid-filled cylinder is idealized to represent the head-brain complex in its simplest form; the cylinder is experimentally subjected to an air blast of Friedlander type, and the temporal variations of cylinder surface pressures and strains and fluid pressures are measured. Based on these measured data and results from computational simulations, the mechanical loading pathways from the external blast to the pressure field in the fluid are identified; it is hypothesized that the net loading at a given material point in the fluid comprises direct transmissive loads and deflection-induced indirect loads. Parametric studies show that the acoustic impedance mismatches between the cylinder and the contained fluid as well as the flexural rigidity of the cylinder determine the shape/intensity of pressure pulses in the fluid.

Rational durability design of heterogeneous functional materials: Some first principles

The present paper proposes a rational approach to the design of constituents, their morphology, and their multiscale arrangement in heterogeneous functional material systems to control the evolution of their properties, morphology, and performance as they interact over specified histories of applied mechanical, electrical, chemical, and thermal fields. The principal thrust of the paper is the postulate of a “first law” for such an approach, which states that “only 100 percent efficient systems are 100 percent durable; the durability is controlled by non-conservative changes in the material state, which are uniquely reflected in changes in the material compliance to applied fields.” The application of the “first law of rational durability design” is discussed for mechanical and electrical applied fields acting upon primarily two-phase heterogeneous material systems. A rational design analysis is demonstrated, and the results are compared with experimental data for fiber-reinforced control materials. The application of the concepts to heterogeneous functional materials (HeteroFoaM) used in fuel cells, batteries, membranes, and electrolyzers is also discussed.

Multiphysics Design and Development of Heterogeneous Functional Materials for Renewable Energy Devices: The HeteroFoaM Story

The electrochemical science that makes many energy conversion and storage technologies work rests on our knowledge and understanding of heterogeneous materials and material systems. The function and functionality of those systems share many common features across a wide range of technologies including fuel cells, batteries, capacitors, and membranes. The science that controls that functionality for these complex material systems is typically summoned in fragments to design a specific device. The present paper discusses an attempt to create a codified multiphysics approach to that general subject, across multiple scales in space and time, for heterogeneous functional materials, or “HeteroFoaM” as we call it. The scope of the paper will be necessarily limited to a general definition of the problem focused on a few specific examples of the progress made for directions that support technologies such as conversion of chemical energy to electricity, membranes for selective transport, and charge storage devices. The principal motivation for this approach is to establish the science that controls emergent properties in heterogeneous functional materials as a foundation for design of functional material systems with performance not bounded by constituent properties.

ChemInform Abstract: Electric Field Control of Magnetism in Multiferroic Heterostructures

AbstractReview: 632 refs.

Multiscale modeling of organic-inorganic interface: From molecular dynamics simulation to finite element modeling

ABSTRACTBi-layer material systems are found in various engineering applications ranging from nanoscale components, such as thin films in circuit boards, to macroscale structures, such as adhesive bonding in aerospace and civil infrastructure. They are also found in many natural and biological materials such as nacre or bone. The structural integrity of a bi-layer system depends on properties of both the interface and the constitutive materials. In particular, interfacial delamination has been observed as a major integrity issue. Here we present a multiscale model, which can predict the macroscale structural behavior at the interface between organic and inorganic materials, based on a molecular dynamics (MD) simulation approach combined with the metadynamics method used to reconstruct the free energy surface (FES) between attached and detached states of the bonded system. We apply this technique to model an epoxy-silica system that primarily features non-bonded and non-directional van der Waals and Coulombic interactions. The reconstructed FES of the epoxy-silica system derived from the molecular level is used to quantify the traction-separation relation at epoxy-silica interface. In this paper, two different approaches in deriving the traction-separation relation based on the reconstructed FES are described. With the derived traction-separation relation, a finite element approach using cohesive zone model (CZM) can be implemented such that the structural behavior of epoxy-silica interface at the macroscopic length scale can be predicted. The prediction from our multiscale model shows a good agreement with experimental data of the interfacial fracture toughness. The method used here provides a powerful new approach to link nano to macro for complex heterogeneous material systems.

Homogenization estimates for multi-scale nonlinear composites

Abstract This work is concerned with the generalization of the “variational linear comparison” method of Ponte Castaneda (J. Mech. Phys. Solids 39 (1991) 45)) to multi-scale, random, heterogeneous material systems with nonlinear isotropic constituents. This method has the distinguishing feature of allowing the conversion of bounds or estimates that might be available for linear systems into corresponding bounds or estimates for the nonlinear composites of interest. Furthermore, the method is fairly simple to implement and quite general. General estimates are developed for two-scale systems and applied to various model composites with “particulate” and “granular” micro- and meso-structures, and compared with the corresponding results for their single-scale counterparts. It is found that the way that the material heterogeneity is distributed at the two separate scales can in most cases have a significant effect on the macroscopic behavior of the composite system.

A nonintrusive stochastic multiscale solver

AbstractIn this paper, we describe a practical nonintrusive multiscale solver that permits consideration of uncertainties in heterogeneous materials without exhausting the available computational resources. The computational complexity of analyzing heterogeneous material systems is governed by the physical and probability spaces at multiple scales. To deal with these large spaces, we employ reduced order homogenization approach in combination with the Karhunen–Loeve expansion and stochastic collocation method based on sparse grid. The resulting nonintrusive multiscale solver, which is aimed at providing practical solutions for complex multiscale stochastic problems, has been verified against the Latin Hypercube Monte–Carlo method. Copyright © 2011 John Wiley & Sons, Ltd.

Plastic deformation in multilayered thin films during indentation unloading: a modeling analysis incorporating viscoplastic response

The indentation behavior of metal/ceramic multilayered thin films is studied numerically using the finite element method. The axisymmetric model consists of alternating aluminum (Al) and silicon carbide (SiC) layers above a silicon (Si) substrate, with the rate-dependent viscoplastic response of Al accounted for. Different unloading rates, with and without a hold period at the peak indentation load, are considered. Attention is devoted to plastic deformation in the Al layers during the unloading phase of indentation. It is found that the hold period stabilizes the deformation so the unloading response becomes insensitive to the unloading rate. However, significant parts of the Al layers under indentation still experience plastic deformation during unloading, due to the mechanical constraint imposed by the hard SiC layers. Consequently unloading is no longer an elastic event in this heterogeneous material system. The unloading induced plasticity is further analyzed by tracking the stress and strain histories inside the material throughout the course of indentation.

Shock compression behavior of a S2-glass fiber reinforced polymer composite

Synthetic heterogeneous material systems, e.g., layered composite materials with organic matrices reinforced by glass fibers (GRP), are attractive materials for a variety of lightweight armor applications. However, while the dynamic response of homogeneous materials, such as, metals and ceramics, has been well documented, the ballistic response of heterogeneous material systems is poorly understood. In the present study, in an attempt to better understand the shock-induced compression response of GRPs, a series of plate impact experiments were conducted on a S2-glas fiber reinforced polymer composite comprising S2-glass woven roving in a Cycom 4102 polyester resin matrix. The plate-impact experiments were conducted using an 82.5 mm bore single-stage gas-gun at the Case Western Reserve University. The history of the shock-induced free-surface particle velocity at the rear surface of the target plate was monitored using the multibeam VALYN™ VISAR system. The results of the experiments indicate the absence of an elastic front in the shock-induced free-surface particle velocity profile in the GRP. Moreover, in the low impact velocity range, relatively weak late-time oscillations are observed in the particle velocity profiles. Increasing the amplitude of the shock-induced compression resulted in a decrease in the rise-time of the shock wave front. The critical shock stress amplitude at which a clear shock-front is seen to develop during the shock loading was determined to be between 1.5 and 2.0 GPa. The results of the experiments are used to obtain the equation of state of the GRP in the stress range 0.04–20 GPa. Moreover, the Hugoniot curve (Hugoniot stress versus Hugoniot strain) was calculated using the Rankine–Hugoniot relationships; the departure of the Hugoniot stress versus the particle velocity curve from linearity allowed the estimation of the Hugoniot elastic limit of the GRP to be about 1.6 GPa.

Material State Changes in Heterogeneous Material Systems: Multiphysics Foundations for Design, Durability and Prognosis - Invited

not Available.

Gradient-based microstructure reconstructions from distributions using fast Fourier transforms

Reconstructions are an important step in analyzing the statistics of local state distributions in the internal structure of heterogeneous material systems and in estimating their effective properties using deterministic models. In this paper, we demonstrate the use of fast Fourier transforms (FFTs) and gradient-based algorithms for the reconstruction of microstructure realizations from 2-point statistics. The FFT method greatly improves the computational efficiency of the algorithm, facilitating use of the full set of 2-point statistics in the reconstruction. This approach introduces periodic boundary conditions naturally into the model. The reconstruction of several two-phase 2D structures is demonstrated, resulting in exact replicas of the original microstructures. The limitations of the technique, especially for more complex structures, are also discussed.

Micromechanical Modeling Of Random OrImperfect Composites

A class of heterogeneous material systems often regarded as random or imperfect composites is addressed in this paper. The literature now offers a number of contributions that open the way to the analysis of large material systems with complex microstructures while taking advantage of popular micromechanics based approaches building on periodicity and first order homogenization techniques. Until recently the attention has been mostly limited to rather classical material systems such as unidirectional fibrous composites and textiles with emphasis on various types of imperfections developed during fabrication process on both microscale (level of fiber bundles) and meso-scale (level of textile geometry). From the basic mechanics point of view, however, it appears logical to exploit the essential principles of the proposed procedures in bridging the gap between mechanical and civil engineering applications. In this regard, historical masonry structures classified as systems composed of more than one material component serve as a typical example of civil engineering applications, which may benefit from standard first order homogenization schemes extended to account for possibly irregular arrangement of individual stone blocks. In this contribution, both groups of material systems will be treated on the same footing demonstrating the applicability of basic homogenization techniques as well as similarities between various heterogeneous material systems when referred to as random or imperfect.

The effect of interfacial properties on the cohesion of highly filled composite materials

It has long been recognized that the cohesion of composite materials, in low confinement, is strongly affected by the properties of the interfacial transition zone (ITZ) between inclusions and matrix. While the effect of the ITZ on the elasticity properties of composites has been studied by many authors in the context of linear homogenization methods, the upscaling of the cohesion strength of highly filled composite materials has not been addressed. This is the focus of the non-linear homogenization procedure developed in this paper, which is based on the separation of the heterogeneous material system in phases of constant strength properties, a non-linear elastic representation of the limit stress state in each phase, and the definition of appropriate effective strain quantities that capture the morphological features of the microstructure. Applied to a three phase composite model composed of rigid inclusion, interface zone and matrix, the model provides a quantitative means of studying the effect of the interface cohesion and the interface volume fraction on the composite cohesion. In particular, we identify a critical interface-to-matrix cohesion ratio, below which the composite cohesion is smaller than the one of the matrix. Furthermore, the model lends itself readily to the study of the degradation of the interfacial properties in composite materials. This is shown for non-degraded and chemically softened cement-based materials, for which we provide conclusive evidence (1) that the interface strength properties of mortar are far more affected by chemical degradation than the one of the cement paste matrix; and (2) that chemical degradation does affect the mechanical strength performance of the cement paste not only through a change of volume proportions (i.e. increase of porosity), but as well through a pure chemical softening of the solid’s cohesion.

Structure of weak shock waves in 2-D layered material systems

Shock waves in homogeneous materials in the absence of phase transitions are understood to have a one-wave structure. However, upon loading of a layered heterogeneous material system a two-wave structure is obtained––a leading shock front followed by a complex pattern that varies with time. This dual shock-wave pattern can be attributed to material architecture through which the shock wave propagates, i.e. the impedance (and geometric) mismatch present at various length scales, and nonlinearities arising from material inelasticity and failure. The objective of the present paper is to provide a better understanding of the role of material architecture in determining the structure of weak shock waves in 2-D layered material systems. Normal plate-impact experiments are conducted on 2-D layered material targets to obtain both the precursor decay and the late-time dispersion. The particle velocity at the free surface of the target plate is measured by using a multi-beam VALYN VISAR. In order to understand the effects of layer thickness and the distance of wave propagation on elastic precursor decay and late-time dispersion several different targets with various layer and target thicknesses are employed. Moreover, in order to understand the effects of material inelasticity both elastic–elastic and elastic–viscoelastic bilaminates are utilized. The results of these experiments are interpreted by using asymptotic techniques to analyze propagation of acceleration waves in 2-D layered material systems. The analysis makes use of the Laplace transform and Floquet theory for ODE’s with periodic coefficients [Asymptotic solutions for wave propagation in elastic and viscoelastic bilaminates. In: Developments in Mechanics, Proceedings of the 14th Mid-Eastern Mechanics Conference, vol. 26, no. 8, pp. 399–417]. Both wave-front and late-time solutions for step-pulse loading on layered half-space are compared with the experimental observations. The results of the study indicate that the structure of acceleration waves is strongly influenced by impedance mismatch of the layers constituting the laminates, density of interfaces, distance of wave propagation, and the material inelasticity.

Analytical solution to the plate impact problem of layered heterogeneous material systems

An analytical solution to the problem of one-dimensional high amplitude wave propagation in layered heterogeneous material systems has been developed, based on Floquet's theory of ODEs with periodic coefficients. The problem is formulated based on a conventional plate impact experimental configuration. In a plate impact test, the boundary condition at the plane of the impact varies with time as a result of multiple wave interactions at the interfaces of the layered target material. The approach of the solution is to convert the initial velocity boundary value problem to a time-dependent stress boundary value problem and then obtain the stress time history by means of superposition. By taking this approach, we explicitly consider multiple wave interactions at the heterogeneous interfaces. A characteristic steady-state stress σ mean for heterogenous material has been identified which is quite different from σ 0 the stress at the initial time of impact. It is shown that σ mean can be obtained by summing up the stress increments at the interfaces or by using mixture theory. The late-time (steady-state) solution procedures for the plate impact problem are presented for impact velocities corresponding to elastic as well as shock wave loading conditions. Results from the analytical model compare well with both numerical results obtained from a shock wave based finite element code and experimental data.

IR-THERMOGRAPHY FOR STRAIN ANALYSIS IN PULTRUDED FIBER REINFORCED PLASTICS

PULTRUDED FRP COMPOSITES Pultruded composites are thick heterogeneousmaterial systems thatmay combine different layers repeated through the thickness of themember, such as roving, continuous filamentmats (CFM),woven fabrics,and braidedpreforms. Commonpultrudedcomposites have roving layers as the primary reinforcement. The CFM layer consists of relatively long and swirled filaments that are randomly dispersed in the plane of the layer. The CFM is usually used for multi-directional secondary reinforcement.

Algorithms for automated meshing and unit cell analysis of periodic composites with hierarchical tri‐quadratic tetrahedral elements

AbstractUnit cell homogenization techniques together with the finite element method are very effective for computing equivalent mechanical properties of composites and heterogeneous materials systems. For systems with very complicated material arrangements, traditional, manual mesh generation can be a considerable obstacle to usage of these techniques. This problem is addressed here by developing automated meshing techniques that start from a hierarchical quad‐tree (in 2D) or oc‐tree (in 3D) mesh of pixel or voxel elements. From the pixel/voxel mesh, algorithms are presented for successive element splitting and nodal shifting to arrive at final meshes that accurately capture both material arrangements and constituent volume fractions, and the material‐scale stress and strain fields within the composite under different modalities of loading. The performance and associated convergence behaviour of the proposed techniques are demonstrated on both densely packed fibre and particulate composites, and on 3D textile‐reinforced composites. Copyright © 2003 John Wiley Sons, Ltd.