Image analysis and modeling of spherical and channel microstructures of fuel-cell materials

The development of novel fuel-cell materials demands accurate and flexible microstructure characterization techniques. Conventional electron microscopy-based microstructural morphology analysis is carried out through the conceptual interpretation of transmission electron microscope images. With this method, only qualitative information on material morphologies can usually be obtained. This paper presents a digital image analysis system that deals with the automatic measurement and quantitative characterization of the microstructural morphologies of polymer electrolyte membrane fuel-cell materials. In this approach, two types of essential microstructural morphologies (spheral particles and interconnected graft channels) are modeled based on statistical geometry theory, and the statistical analysis schemes of the microstructural morphologies are designed and applied to the characterization of the phase-separated microstructures in fuel-cell components such as solid electrolyte ionomers, catalyst layers, and gas diffusion layers. Experimental results on real fuel-cell materials specimens demonstrate the effectiveness of the method.     

A stochastic multiscale algorithm for modeling complex granular materials

Modeling of granular materials in which the grains have irregular shapes and surface is a long-standing problem that has been studied for decades. Almost all the current models either represent the grains as particles with geometrically-regular shapes or attempt to infer some low-order statistical properties of the materials in order to describe granular media. We use an approach to modeling of granular materials that utilizes a two- or three-dimensional image of the material’s morphology. It reconstructs realizations of the image based on a Markov process, and uses a multiscale approach and graph-theoretical concepts to refine the realizations and make them free of artefacts. The method is applied to several complex 2D and 3D examples of granular materials. Various morphological properties of the models are computed and are compared with those of the original images; very good agreement is found for all the cases. Furthermore, the computational cost of the method is very low and, therefore, the method can generate large-size models for complex granular materials.     

焊接接头组织模拟进展

介绍了焊接接头组织模拟的现状,阐述了组织模拟的主要方法:Monte Carlo方法、Cellular Automaton方法及相场法,分析了不同方法在模拟凝固及固态相变过程中的晶粒生长、组织形貌和各相分数等不同方面的特点和存在的不足.     

宏观结构性能约束下的材料微结构拓扑优化

为了设计周期性多孔钢或钢/铝复合材料优化微结构,基于独立连续映射法,建立了以结构总质量最小化为目标,节点位移为约束的拓扑优化模型。假设宏观结构由多孔材料或复合材料组成,其等效特性采用均匀化理论计算得到。定义了微观材料拓扑变量,节点位移约束采用一阶泰勒展开近似。各种材料设计要求作为约束条件纳入到优化模型中。推导了节点位移和总质量的敏度表达式。采用基于求解偏微分的过滤方法消除了数值不稳定性。在二维数值算例中获得了各种满足设计要求的优化材料微结构。结果表明:提出的方法在材料微结构拓扑优化设计中具有可行性和有效性。      

Computational modeling of crack propagation in real microstructures of steels and virtual testing of artificially designed materials

A computational approach to the optimization of service properties of two-phase materials (in this case, fracture resistance of tool steels) by varying their microstructure is developed. The main points of the optimization of steels are as follows: (1) numerical simulation of crack initiation and growth in real microstructures of materials with the use of the multiphase finite elements (MPFE) and the element elimination technique (EET), (2) simulation of crack growth in idealized quasi-real microstructures (net-like, band-like and random distributions of the primary carbides in the steels) and (3) the comparison of fracture resistances of different microstructures and (4) the development of recommendations to the improvement of the fracture toughness of steels. The fracture toughness and the fractal dimension of a fracture surface are determined numerically for each microstructure. It is shown that the fracture resistance of the steels with finer microstructures is sufficiently higher than that for coarse microstructures. Three main mechanisms of increasing fracture toughness of steels by varying the carbide distribution are identified: crack deflection by carbide layers perpendicular to the initial crack direction, crack growth along the network of carbides and crack branching caused by damage initiation at random sites.     

Atomistic to continuum modeling of solidification microstructures

We summarize recent advances in modeling of solidification microstructures using computational methods that bridge atomistic to continuum scales. We first discuss progress in atomistic modeling of equilibrium and non-equilibrium solid–liquid interface properties influencing microstructure formation, as well as interface coalescence phenomena influencing the late stages of solidification. The latter is relevant in the context of hot tearing reviewed in the article by M. Rappaz in this issue. We then discuss progress to model microstructures on a continuum scale using phase-field methods. We focus on selected examples in which modeling of 3D cellular and dendritic microstructures has been directly linked to experimental observations. Finally, we discuss a recently introduced coarse-grained dendritic needle network approach to simulate the formation of well-developed dendritic microstructures. This approach reliably bridges the well-separated scales traditionally simulated by phase-field and grain structure models, hence opening new avenues for quantitative modeling of complex intra- and inter-grain dynamical interactions on a grain scale.     

Multiscale continuum modeling of a crack in elastic media with microstructures

Cosserat type continuum theories have been employed by many authors to study cracks in elastic solids with microstructures. Depending on which theory was used, different crack tip stress singularities have been obtained. In this paper, a microstructure continuum theory is used to model a layered elastic medium containing a crack parallel to the layers. The crack problem is solved by means of the Fourier transform. The resulting integrodifferential equations are discretized using the Chebyshev polynomial expansion method for numerical solutions. By using the present theory, the explicit internal length effects upon the crack opening displacement and stress field can be observed. It is found that the stress field near the crack tip is not singular according to the microstructure continuum solution although the level of the opening stress shows an increasing trend until it gets very close to the crack tip. The rising portion of the near tip opening stress is used to project the stress intensity factor which agrees fairly well with that obtained using the FEM to perform stress analyses of the cracked layered medium with the exact geometry. The numerical solutions also indicate that treating the layered medium as an equivalent homogeneous classical elastic solid is not adequate if cracks are present and accurate stress intensity factors in the original layered medium is desired.     

自然材料微结构的几种特征

自然材料微结构是仿生机械结构设计的灵感来源.利用激光扫描共聚焦显微镜分析了鸭子下层绒毛、水稻叶毛、松针维管束鞘、水稻侧根和松针的微结构;用扫描电镜分析了黄瓜外表皮、仙人掌表皮组织、水稻叶脉、蚊子复眼、仙人掌组织、鹌鹑蛋蛋壳、水稻根和贝壳的微结构．结合前人对其它自然材料微结构的研究,总结了自然材料的结构特征,典型微结构特征有分形结构、分级结构、多尺度结构、多孔结构、梯度结构和整合结构;并且讨论了典型生物微结构原型在仿生结构设计方面的应用．分析了自然材料微结构特征的一般性特征,即对称性和自相似性．     

Generalized shape optimization of three-dimensional structures using materials with optimum microstructures

This paper deals with generalized shape optimization of linearly elastic, three-dimensional continuum structures, i.e. we consider the problem of determining the structural topology (or layout) such that the shape of external as well as internal boundaries and the number of inner holes are optimized simultaneously. For prescribed static loading and given boundary conditions, the optimum solution is sought from the condition of maximum integral stiffness (minimum elastic compliance) subject to a specified amount of structural material within a given three-dimensional design domain. This generalized shape optimization problem requires relaxation which leads to the introduction of microstructures. A class of optimum three-dimensional microstructures and explicit analytical expressions for their optimum effective stiffness properties have been developed by Gibiansky and Cherkaev (1987) [Gibiansky, L.V., Cherkaev, A.V., 1987. Microstructures of composites of extremal rigidity and exact estimates of provided energy density (in Russian). Report (1987) No. 1155. A.F. Ioffe Physical-Technical Institute, Academy of Sciences of the USSR, Leningrad. English translation in: Kohn, R.V., Cherkaev, A.V. (Eds.), Topics in the Mathematical Modelling of Composite Materials. Birkhaüser, New York. 1997]. The present paper gives a brief account of the results in Gibiansky and Cherkaev (1987) which will be utilized for our microlevel problem analysis. It is a characteristic feature that the use of optimum microstructures renders the global problem convex if an appropriate parametrization is applied. Hereby local optima can be avoided and we can construct a simple gradient based numerical method of mathematical programming for solution of the complete optimization problem. Illustrative examples of optimum layout and topology designs of three-dimensional structures are presented at the end of the paper.     

Fatigue behavior of composite materials

While various expected and accepted interpretations of the term fatigue behavior exist for homogeneous materials, especially metals, fatigue behavior of composite materials is not well defined—or understood. Generally speaking, one can discuss that topic by addressing the change in residual strength, life, and stiffness during cyclic (or otherwise variable) load histories. The present discussion will begin by defining the problem of fatigue behavior of composite materials, after which a representative spectrum of physical observations will be cited and models of the behavior presented. Both unnotched and notched behavior will be discussed. A brief section will deal with interpretations of data and statistical data handling schemes. Frontiers of the field will be identified. While the paper is presented in the spirit of a discussion of physical behavior rather than a review of the literature, representative work of numerous investigators is cited.

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Résumé Bien qu'il existe des interprétations variées et largement acceptées du terme de comportement en fatigue dans le das des matériaux homogènes, notamment les métaux, le comportement en fatigue des matériaux composites n'est pas bien défini voire compris. D'une manière générale, on peut aborder ce sujet en prenant en considération le changement de résistance résiduelle, la vie, et la rigidité au cours de Mistoire des contraintes cycliques ou tout du moins variables. La présente discussion commence à définir le problème du comportement en fatigue des matériaux composites suivant lequel un spectre représentatif d'observations physiques peut être invoqué et des modèles de comportement présentés. On discute le comportement à l'état non entaillé et à l'état entaillé. Un chapître spécial est relatif à l'interprétation des données et les schémas de traitement des données statistiques. Bien que le mémoire soit présenté dans l'esprit d'une discussion sur un comportement physique plutôt que dans 1'esprit d'une revue de la littérature, les travaux de nombreux chercheurs sont cités sous référence.

     

Refractive-index profiling of embedded microstructures in optical materials

We describe use of a phase-sensitive low-coherence reflectometer to measure spatial variation of refractive index in optical materials. The described interferometric technique is demonstrated to be a valuable tool to profile the refractive index of optical elements such as integrated waveguides and photowritten optical microstructures. As an example, a refractive-index profile is mapped of a microstructure written in a microscope glass slide with an ultrashort-pulse laser.     

An approach for simulating microstructures of polycrystalline materials

In this paper, an approach is identified using concepts in molecular dynamics (MD) and discrete element method (DEM) to generate the microstructure of polycrystalline materials. Using the proposed methods, different types of particles with different grain size and volume fraction in the real material, can be easily generated. It is assumed that the particles can be randomly packed together into a simulation region, by defining artificial interaction forces among them. Such forces may be either adopted from Van der Waals potential energy, or Hooke pair and gravity forces. The proposed method has proved to be fast due to the fact that the algorithm has been implemented on graphical processing units (GPU). Utilizing the Voronoi tessellation method, the set of the generated discrete grains have been altered to space-filling, adjoining polyhedrons with respect to the real geometry. Moreover, as an advantage, the boundary and the interface region of the microstructures were modeled.     

Limit analysis of composite materials with anisotropic microstructures: A homogenization approach

A nonlinear mathematical programming approach together with the finite element method and homogenization technique is developed to implement kinematic limit analysis for a microstructure and the macroscopic strength of a composite with anisotropic constituents can be directly calculated. By means of the homogenization theory, the classical kinematic theorem of limit analysis is generalized to incorporate the microstructure - Representative Volume Element (RVE) chosen from a periodic composite/heterogeneous material. Then, using an associated plastic flow rule, a general yield function is directly introduced into limit analysis and a purely-kinematic formulation is obtained. Based on the mathematical programming technique, the finite element model of microstructure is finally formulated as a nonlinear programming problem subject to only one equality constraint, which is solved by a direct iterative algorithm. The calculation is entirely based on a purely-kinematical velocity field without calculation of stress fields. Meanwhile, only one equality constraint is introduced into the nonlinear programming problem. So the computational cost is very modest. Both anisotropy and pressure-dependence of material yielding behavior are considered in the general form of kinematic limit analysis. The developed method provides a direct approach for determining the macroscopic strength domain of anisotropic composites and can serve as a powerful tool for microstructure design of composites.     

Fatigue crack closure and crack growth behaviour in a titanium alloy with different microstructures

Fatigue crack closure and crack growth behaviour in Ti–2.5 wt % Cu alloy with two equiaxed and two lamellar microstructures have been investigated by constant-load amplitudetests. Plasticity-induced crack closure and roughness-induced crack closure have been characterized separately by experimental methods. A change in closure mechanism from plasticity-induced crack closure at high K values (region of high stress intensity ranges)to roughness-induced crack closure at low K values occurs in a solution-annealed equiaxed microstructure, while plasticity-induced crack closure is the operative closure mechanism in an over-aged equiaxed microstructure over the whole range of K and roughness-induced crack closure occurs in two lamellar microstructures. The crack closing stress intensity factor for plasticity-induced crack closure increases continuously with increasing maximum stress intensity. The crack closing stress intensity factor for roughness-induced crack closure increases with increasing maximum stress intensity at low K, and remains constant at high K. Crack closure and crack path deflection have a significant influence on the crack growth rates. © 1998 Kluwer Academic Publishers     

Laser-beam and photon-assisted processed materials and their microstructures

Laser processing is a relatively new technique for modifying the near-surface region of materials without altering the in-bulk characteristics. A single laser can perform several functions by manipulating processing conditions such as laser power, beam diameter, and traverse speed. Lasers have shown attractive applications, such as cutting, welding, glazing, alloying, and cladding. A laser glazing process has demonstrated an improvement in the microstructure of vacuum plasma-coated copper-based alloys containing cavities, unmelted particles, and segregation. Laser glazing has also been shown to restore the degraded microstructure of components and make them equivalent to, or better than, the original wrought alloy. The laser cladding concept was used to develop nickel-based alloys for high-temperature applications that exhibited higher thermal stability than the nickel-based Rene-95 alloy. Rapid melting and quenching occurred during the laser glazing, alloying, and cladding processes resulting in a fine-grained microstructure, metastable phases and extended solid solubility of alloying additions in the matrix.Photon-assisted processing of material is a relatively new technique being explored to synthesize new materials from various substrates (solid, liquid, and gas). This process is successfully used to fabricate high-quality thin films for electronic industries. Thin films of multicomponents can be deposited with stoichiometric composition. Diamond thin films have been synthesized from liquid hydrocarbon (Benzene, C₆H₆) by laser-liquid hydrocarbon-substrate interaction. A laser-assisted physical vapour deposition process was found to be very successful in depositing stoichiometric compositions of multilayered thin films such as superconducting YBa₂Cu₃O₇, ferroelectric Pb_0.52Zr_0.48TiO₃ and other coatings such as TiN and CoSi₂. This review reports some of the major advances in the understanding and engineering of new materials for electronic industries and high-temperature applications in the auto, aerospace, and turbine industries.     

Rheological study of microstructures and properties for polymeric materials

Rheological measurement has been a preferred approach in the characterization of the formation and evolu-tion of microstructures for multi-component or multi-phase polymeric material systems owing to i...     

Bend stiffness of laminate microstructures containing three dissimilar materials

This article examines the effective flexural modulus of a multilayered micro-system evolving into alternative layered structures consisting of three dissimilar materials. A multiscale model of the bending stiffness is presented to capture the impact of changing the constituent materials, the layer architecture and the cross-section geometry. The results are plotted onto maps to show the existence of specific domains, within which fall the effective properties of all possible tri-material multilayered configurations. The potential to stiffen a bi-material system is demonstrated by integrating additional layers of a more flexible material for given constraints on the volume fraction. The proposed scheme is conducive to contrast structural alternatives in constrained and unconstrained design. A case study shows how the maps enable optimum selection among various design concepts, which may range from monolithic materials with alternative shape geometries to systems consisting of two and three materials arranged in dissimilar multiple layer architectures.