Structure Of Real Materials Research Articles

Influence of steel fibre on bending behaviours of RPC based on X-ray CT and mesoscopic numerical simulation

The mechanical properties and fracture mechanism of quasi-brittle materials are considered to be largely reliant on the mechanical properties of meso-components and the structure formed when these components are combined. In recent numerical simulation practice, a trend has developed whereby the real structure of materials is considered in as accurate a way as possible. In this paper, the macroscopic and mesoscopic bending behaviours of steel fibre-reinforced reactive powder concrete (RPC) slabs with fibre volume content between 0.0% and 2.5% were investigated using both experimental tests and numerical simulations. Images obtained from X-ray computed tomography (CT) were input into a program named RFPA3D-CT to build a modified three-dimensional finite-element method model. The results indicate that the steel fibres can help to convert a brittle failure pattern of RPC into a ductile one. Compared with specimens without fibres, the crack tortuosity and bending strength of specimens with fibre content of 2.5% are increased by 20.25% and 308.80%, respectively. The bending performance according to numerical results obtained by the modified model are in good agreement with the experimental results. Moreover, the acoustic emission (AE) parameters and AE curves obtained in RFPA3D-CT can be used to reveal the initiation and propagation process of cracks in RPC.

Competing magnetic orders in a bilayer Hubbard model with ultracold atoms.

Fermionic atoms in optical lattices have served as a useful model system in which to study and emulate the physics of strongly correlated matter. Driven by the advances of high-resolution microscopy, the current research focus is on two-dimensional systems1-3, in which several quantum phases-such as antiferromagnetic Mott insulators for repulsive interactions4-7 and charge-density waves for attractive interactions8-have been observed. However, the lattice structure of real materials, such as bilayer graphene, is composed of coupled layers and is therefore not strictly two-dimensional, which must be taken into account in simulations. Here we realize a bilayer Fermi-Hubbard model using ultracold atoms in an optical lattice, and demonstrate that the interlayer coupling controls a crossover between a planar antiferromagnetically ordered Mott insulator and a band insulator of spin-singlets along the bonds between the layers. We probe the competition of the magnetic ordering by measuring spin-spin correlations both within and between the two-dimensional layers. Our work will enable the exploration of further properties of coupled-layer Hubbard models, such as theoretically predicted superconducting pairing mechanisms9,10.

Porous functions

A new approach to modeling the geometric structure of real materials based on a new type of functions–porous functions–is introduced. The general concept, basic operations, operations of calculus, visualization and interpretations, and potential applications of porous functions are outlined.

Continuous Time Quantum Monte Carlo in Combination with Machine Learning on the Hubbard Model

The acceleration of exact continuous time quantum Monte Carlo (CTQMC) approaches in multi-site or multi-orbital systems is extremely interesting work, because these approaches are very time-consuming in terms of numerical computation and might account for the nature of exotic behaviors such as high-temperature superconductivity and Mott insulator behavior observed in the strongly correlated materials. We extend the recently developed interaction-expansion CTQMC method in combination with a machine learning (CTQMC+ML) approach for the single-site and single-orbital systems to multi-site and multi-orbital ones. This method can be applied to explore the nonlocal correlation effects in lattice models and to study the electronic structure of real materials via an ab-initio density functional theory plus dynamical mean field theory approach. We find that our CTQMC+ML method for multi-site (and multi-orbital) systems accurately predicts the impurity Green’s function with less computational time than the CTQMC approaches, as in the case of the single-site and single-orbital version.

Impact model of WC/Co composite

WC/Co composite is a standard hard material used for the production of cutting tools. It has both very good thermo-mechanical and wear properties. During the cutting, process tools are subjected to impact loading and gradual degradation due to high-stress concentrations. This loading induced deterioration is complex process still not well investigated and explained.Up till now the dynamic response of the WC/Co composite was analysed under dynamic impulse compressive loading [1]. However, the behaviour of the above two-phase composite under impacts conditions was not investigated in details. In the presented micromechanical approach the real material structure geometry of the internal structure can be performed including spatial distribution of: (1) WC grains and their dimensions, (2) volume content of plastic Co binder with their thickness, (3) system of grain/binder interfaces and (4) cracks initiated and developed during impulse loading, (5) possible brittle grains rotation.The results reveal the dependence of the microcracking processes and the stress distribution on impact velocity and presence of discontinuities in the Co binder and the interface zone between the binders and the grains. The microcracks system was evaluated by the damage parameter according to Kachanov, 1986 [62].

Propagation of waves in nonlocal porous multi-phase nanocrystalline nanobeams under longitudinal magnetic field

ABSTRACTThis article investigates wave propagation behavior of a multi-phase nanocrystalline nanobeam subjected to a longitudinal magnetic field in the framework of nonlocal couple stress and surface elasticity theories. In this model, the essential measures to describe the real material structure of nanocrystalline nanobeams and the size effects were incorporated. This non-classical nanobeam model contains couple stress effect to capture grains micro-rotations. Moreover, the nonlocal elasticity theory is employed to study the nonlocal and long-range interactions between the particles. The present model can degenerate into the classical model if the nonlocal parameter, couple stress and surface effects are omitted. Hamilton’s principle is employed to derive the governing equations which are solved by applying an analytical method. The frequencies are compared with those of nonlocal and couple stress-based beams. It is showed that wave frequencies and phase velocities of a nanocrystalline nanobeam depend on the grain size, grain rotations, porosities, interface, magnetic field, surface effect and nonlocality.

Concrete Targets with Heterogeneities under Impact Loading

The main aim of the paper is to present an elegant procedure for introducing heterogeneity to simplified computational models. A computational model is considered here to be theoretically any numerical model that includes material with structural strength. Heterogeneity then means the randomness in material parameters, which allows a simplified model to behave like the comprehensively described structure of a material. The whole procedure is explained using the example of a material model of concrete in the form of a concrete block (target) which is exposed to high-speed impact loading. Even though the described procedure for the introduction of heterogeneity to computational models can be applied during the use of practically all numerical methods, the Smoothed Particle Hydrodynamics (SPH) method will be used due to the type of loading involved. The creation of the presented procedure was prompted by the constantly increasing number of input parameters used with material models. Elementary material models with a minimum of inputs can be used when applying the procedure. The end of the paper deals with the improvement in use that enables heterogeneity (i.e. randomness of material structure) to be created in such a way that a simplified model reflects as much as possible the real material structure, not only the variations of mathematical functions.

Axial magnetic field effects on dynamic characteristics of embedded multiphase nanocrystalline nanobeams

This article investigates vibrational behavior of a multi-phase nanocrystalline nanobeam resting on Winkler–Pasternak foundation and subjected to a longitudinal magnetic field in the framework of nonlocal couple stress and surface elasticity theories. In this model, the essential measures to describe the real material structure of nanocrystalline nanobeams and the size effects were incorporated. This non-classical nanobeam model contains couple stress effect to capture grains micro-rotations. Moreover, the nonlocal elasticity theory is employed to study the nonlocal and long-range interactions between the particles. The present model can degenerate into the classical model if the nonlocal parameter, couple stress and surface effects are omitted. Hamilton’s principle is employed to derive the governing equations and the related boundary conditions which are solved applying differential transform method. The frequencies are compared with those of nonlocal and couple stress based beams. It is showed that vibration frequencies of a nanocrystalline nanobeam depend on the grain size, grain rotations, porosities, interface, elastic foundation, magnetic field, surface effect, nonlocality and boundary conditions.

Stability analysis of porous multi-phase nanocrystalline nonlocal beams based on a general higher-order couple-stress beam model

This article investigates buckling behavior of a multi-phase nanocrystalline nanobeam resting on Winkler-Pasternak foundation in the framework of nonlocal couple stress elasticity and a higher order refined beam model. In this model, the essential measures to describe the real material structure of nanocrystalline nanobeams and the size effects were incorporated. This non-classical nanobeam model contains couple stress effect to capture grains micro-rotations. Moreover, the nonlocal elasticity theory is employed to study the nonlocal and long-range interactions between the particles. The present model can degenerate into the classical model if the nonlocal parameter, and couple stress effects are omitted. Hamilton\'s principle is employed to derive the governing equations and the related boundary conditions which are solved applying an analytical approach. The buckling loads are compared with those of nonlocal couple stress-based beams. It is showed that buckling loads of a nanocrystalline nanobeam depend on the grain size, grain rotations, porosities, interface, elastic foundation, shear deformation, surface effect, nonlocality and boundary conditions.

Stoichiography: New Ways Determining Chemical Composition and Real Structure of Materials

The principles of stoichiography and novel reference-free methods of molecular and phase analysis for complex unknown mixtures are considered. The stoichiography can be inferred from stoichiometry of mass transfer of unsteady homo- and heterophase processes and joins both operations: separation of mixture by means of chromatography, electromigration, dissolution or others and determination of stoichiometry of a substance flow with time. The stoichiography allows a chemical compound to be determined by its primary property, namely, by stoichiometry of elemental composition. Stoichiograms provided a basis for such type of information. They are time variances of molar ratio for mass transfer rates of chemical elements from multielement substances. Invariancy to concentration and temperature of solvents, hydrodynamic regime is a fundamental property of the stoichiograms in the case of individual compounds. Therefore the stoichiograms are kept constant and are equal to formula stoichiometric coefficients of the individual compound. Theory and methodology of new stoichographic methods, differential dissolution and ion-chromato-stoichiography are presented. New equipment, stoichiograph, and a new procedure of differential dissolution, stoichiographic titration, are discussed here in details. Applications of differential dissolution to analyze multielement and<br />polyphase crystalline and amorphous samples are given.

X-Ray Diffraction Analysis of Steel with Oxidised Surface Layer

The basic principle of the X-ray diffraction analysis is based on the determination of components of residual stresses. They are determined on the basis of the change in the distance between atomic planes. The method is limited by a relatively small depth in which the X-ray beam penetrates into the analysed materials. For determination of residual stresses in the surface layer the X-ray diffraction and electrolytic polishing has to be combined. The article is deals with the determination of residual stress and real material structure of a laser-welded steel sample with an oxide surface layer. This surface layer is created during the rolling and it prevents the material from its corrosion. Before the X-ray diffraction analysis can be performed, this surface layer has to be removed. This surface layer cannot be removed with the help of electrolytic polishing and, therefore, it has to be removed mechanically. This mechanical procedure creates “technological” residual stress in the surface layer. This additional residual stress is removed by the electrolytic polishing in the depth between 20 and 80 μm. Finally, the real structure and residual stresses can be determined by using the X-ray diffraction techniques.

Thermal transport in one-dimensional superlattice and quasicrystal chains: Fullerene phononic crystal

In order to investigate the thermal transport in one-dimensional (1D) superlattice and quasicrystal chains, the simple harmonic-oscillator model and the C60 chains model, are studied through non-equilibrium molecular dynamics simulation. In the simple harmonic-oscillator model, we construct a simple periodic harmonic-oscillator chain by using two different stiffness coefficients alternatively. It is found that its thermal conductivity is smaller than in non-periodic chains, no matter which stiffness coefficient is used in the non-periodic chains. In order to test if this is true in a real material structure, a superlattice C60 chain is constructed by connecting perfect and defective C60 alternatively. The calculation outcome of the C60 chains coincides with the results of the simple harmonic-oscillator model, that the thermal conductivity of the superlattice C60 chain is smaller than that of perfect and defective crystal C60 chains. Besides this, we also studied the thermal transport properties of the quasicrystal C60 chain, which consists of a random defective C60 molecular structure. It is found that the thermal conductivity of the quasicrystal structure is far less than that of the crystal and superlattice chains. The phonon spectra of the perfect, defective and superlattice C60 chain are provided to give corresponding supports. Our results also propose a controllable method for the thermal management in nanoscale materials.

Stereological analysis of the microstructure of pure iron with random nucleation

Microstructural characterization of metallic materials is of paramount importance in the qualification and quantification of their desired final properties. Metallic materials have often been characterized by means of optical and electronic microscopy. Results from these techniques are in most of the cases based on 2D analysis. Stereological methods are then employed to obtain 3D information. However, such methods are based on assumptions and approximations of the real material structure. Therefore, it is essential to know the limitations of these methods. A further complication arises when one wishes to compare real materials with computer simulation results and stereological analytical techniques. In this paper, methods normally applied to real microstructure are applied to microstructures simulated by cellular automata (CA) simulation. Experimental results from microstructural characterization of polycrystalline pure iron were used as the starting point for the simulation. Consequently, we could apply analytical formulae of stereology to experimental data from pure iron and to computer generated microstructures from cellular automata simulation. Comparison of analytical formulae, experimental results and computer simulation provided useful insights on limits of applicability and on the meaning of the stereological analysis.

OS1201 水素ポア機構によるアルミニウム合金延性破壊の3Dイメージベース有限要素解析

It has recently been clarified that ductile fracture in aluminum alloys occurs predominantly through coalescence of hydrogen micropores. This type of ductile fracture was investigated by the image-based finite element analysis using the 3D image taken by X-ray tomography. The studied FE model consists of Al_2Cu particles and pores. The analysis revealed the distribution of local stress, strain and the stress triaxiality developed within the real material's structure, providing important clues to anticipate the location that would undergo significant damage.

Outline of Nguetseng's approach to non-periodic homogenization

Classical homogenization theory developed for modeling heterogeneous media with regular fine structure is based on the assumption that the structure is periodic. Since the structure of real materials is not perfectly periodic, several approaches were proposed. The approach proposed by Gabriel Nguetseng in 2003 seems to be the most general deterministic approach, it covers periodic, almostperiodic and other structures. It is based on the notion of Banach algebra spectrum. The aim of this survey paper is to give a brief self-contained outline of this rather complicated approach described in many-page papers.

On the mechanical properties of sintered metallic fibre structures

The present study investigates mechanical properties of a novel sintered metallic fibre structure with different relative densities (i.e. 0.19, 0.27, and 0.46). The compressive mechanical properties Young's modulus, Poisson's ratio and 0.2% offset yield stress are determined. For this purpose, state of the art simulations are performed based on the real material structure using micro-computed tomography images. Computed results are compared with experimental uni-axial compression tests and good agreement between both methods is observed. Numerical analysis allows the investigation of directional dependence and mechanical anisotropy is observed to be governed by the fibre orientation. In addition, Young's modulus and 0.2% offset yield stress increase with rising relative density.

Surface phonons, elastic response, and conformal invariance in twisted kagome lattices

Model lattices consisting of balls connected by central-force springs provide much of our understanding of mechanical response and phonon structure of real materials. Their stability depends critically on their coordination number z . d -dimensional lattices with z = 2 d are at the threshold of mechanical stability and are isostatic. Lattices with z < 2 d exhibit zero-frequency “floppy” modes that provide avenues for lattice collapse. The physics of systems as diverse as architectural structures, network glasses, randomly packed spheres, and biopolymer networks is strongly influenced by a nearby isostatic lattice. We explore elasticity and phonons of a special class of two-dimensional isostatic lattices constructed by distorting the kagome lattice. We show that the phonon structure of these lattices, characterized by vanishing bulk moduli and thus negative Poisson ratios (equivalently, auxetic elasticity), depends sensitively on boundary conditions and on the nature of the kagome distortions. We construct lattices that under free boundary conditions exhibit surface floppy modes only or a combination of both surface and bulk floppy modes; and we show that bulk floppy modes present under free boundary conditions are also present under periodic boundary conditions but that surface modes are not. In the long-wavelength limit, the elastic theory of all these lattices is a conformally invariant field theory with holographic properties (characteristics of the bulk are encoded on the sample boundary), and the surface waves are Rayleigh waves. We discuss our results in relation to recent work on jammed systems. Our results highlight the importance of network architecture in determining floppy-mode structure.

OOF3D: An image-based finite element solver for materials science

Recent advances in experimental techniques (micro-CT scans, automated serial sectioning, electron back-scatter diffraction, and synchrotron radiation X-rays) have made it possible to characterize the full, three-dimensional structure of real materials. Such new experimental techniques have created a need for software tools that can model the response of these materials under various kinds of loads. OOF (Object Oriented Finite Elements) is a desktop software application for studying the relationship between the microstructure of a material and its overall mechanical, dielectric, or thermal properties using finite element models based on real or simulated micrographs. OOF provides methods for segmenting images, creating meshes of complex geometries, solving PDE's using finite element models, and visualizing 3D results. We discuss the challenges involved in implementing OOF in 3D and create a finite element mesh of trabecular bone as an illustrative example.

Solution of the inverse problem for reconstructing the real structure of materials from the data of different X-ray methods

A software program has been developed for joint solution of the inverse problem on the basis of the X-ray diffractometry and reflectometry data. A system of basic objects supporting some functions of automatic data flow processing in the program and oriented to mathematical calculations has been created. It is shown by the example of experimental data for an InxGa1 − xAs-InyAl1 − yAs/InP(001) sample that joint fitting of X-ray diffractometry and reflectometry curves makes it possible to reconstruct the model parameters of a multilayer with smaller rms errors. The data were processed taking into account the angular dependences of the background intensity and the aperture factors stemming from the geometry of the experiment.

Computer-aided simulation of the AMg6 alloy rolling process with allowance for its anisotropy

Some data obtained in the course of computer-aided simulation of a process of plane AMg6 alloy specimen rolling in the CAE-system MSC.SuperForm are presented. The influence of a real material structure upon the stress-strain state and the flow pattern of layers throughout the specimen height is taken into account. Also determined are the values of total pressure of metal on the rolls and a moment of rolling with the use of the isotropic and anisotropic finite element models (FEM) of specimens.