The high-frequency response of random elastic composites

Summary A method is presented for analysis of the high-frequencies response of random elastic composites, which involves the joint application of asymptotic expansions for scattering cross section and the Kramers-Kronig relations. An example of a porous elastic medium is given.

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Résumé On donne méthode analytique pour l'investigation d'ondes en haute frequences dans les composites. La méthode se base sur une application des relations Kramers-Kronig, combiné avec des expansions asymptotiques pour leur attenuation. On donne un exemple d'un matériau poreux élastique.

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On leave from Holon Techn. Inst, P. O. B. 305, Holon 58102, Israel.     

An experimental methodology for response surface optimization methods

Response surface methods, and global optimization techniques in general, are typically evaluated using a small number of standard synthetic test problems, in the hope that these are a good surrogate for real-world problems. We introduce a new, more rigorous methodology for evaluating global optimization techniques that is based on generating thousands of test functions and then evaluating algorithm performance on each one. The test functions are generated by sampling from a Gaussian process, which allows us to create a set of test functions that are interesting and diverse. They will have different numbers of modes, different maxima, etc., and yet they will be similar to each other in overall structure and level of difficulty. This approach allows for a much richer empirical evaluation of methods that is capable of revealing insights that would not be gained using a small set of test functions. To facilitate the development of large empirical studies for evaluating response surface methods, we introduce a dimension-independent measure of average test problem difficulty, and we introduce acquisition criteria that are invariant to vertical shifting and scaling of the objective function. We also use our experimental methodology to conduct a large empirical study of response surface methods. We investigate the influence of three properties—parameter estimation, exploration level, and gradient information—on the performance of response surface methods.     

Nonlinear homogenization of metal-ceramic composites with lamellar microstructure

Using nonlinear homogenization methods for the computation of the material response of metal-ceramic composites with lamellar microstructure is a power approach to do computation less costly in comparison to finite elements modeling. A modified secant homogenization method is utilized in this study for simulation of inelastic behaviors of the composite micro-constituents. A nonlinear homogenization method is based on a linear homogenization scheme for multilayer composites. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigation of the three-dimensional micromechanical behavior of woven-fabric composites

A three-dimensional representative volume-element model is presented to study the micromechanical behavior of woven-fabric composites. The effects of the fiber undulation zone and the fiber braid angle on the elastic modulus of the composites are taken into account in the unit cell. Based on isostrain and isostress assumptions, a standard homogenization procedure is used to calculate the effective elastic properties of woven-fabric composites, and all the final stiffness components are expressed in an explicit form. The results obtained by the model considered agree with published experimental results very well. The relationship between the geometric parameters, such as fiber width, thickness, volume fraction, etc., and the macromechanical behavior of the composites can be obtained by this model. Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 42, No. 2, pp. 209–220, April–May, 2006.     

Inelastic behavior of the single domain of metal-ceramic composites with lamellar microstructure

Metal-ceramic composites are widely applied in the different brunches of industry. The composites are produced by squeeze-casting of the ceramic preform by molten aluminum alloy. The lamellar microstructure is obtained during freezing of ceramic suspension. The internal structure of the domains can be controlled via freeze-casting parameters. The material has high anisotropy level and its effective properties depend on lamella orientation. The aim of this study is numerical simulation of the inelastic behavior of the material and its verification by experiment. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Nonlinear micromechanical model of filament-wound composites considering fiber undulation

Flexible-matrix composites with highly anisotropic properties have successfully been used in numerous fields to improve the performance of conventional structures or to facilitate new innovations. Many of them are designed on the basis of tubes which are produced efficiently by the filament winding process. To predict the elastic behavior of filament-wound flexible-matrix composites, aspects of the nonlinear behavior of the flexible material have to be considered, as well as the features of the distinct fiber undulation geometry inherent to the filament winding process. The present study considers these characteristics in the micromechanical modeling of the elastic behavior by including a nonlinear material model to represent the strain-dependent moduli and manufacturing-dependent geometries. The structure is characterized by a unit cell and subcells, analyzed separately and combined based on different sets of isostress and isostrain assumptions that depend on the winding angle. On the basis of experimentally obtained nonlinear lamina properties, an iterative method of solution is chosen to calculate the axial stress–strain behavior of tubes with various winding parameters. The resulting predictions are validated by testing tubes in tension and compression. The model shows good agreement with the experiments. Predictions made using the model show a strong influence of filament winding parameters on the axial modulus of flexible-matrix composite tubes.     

Some optimization problems for elastic bodies

Two optimization problems for finite deformations of an elastic body are formulated. In the first problem, the motion that gives minimum value (maximum value) to a certain functional is chosen from a single family of controllable motions. The necessary conditions for optimality are stated for one specific problem of this type. In the second problem, the minimizing (maximizing) motion is chosen from the class of all possible motions connecting two configurations of the body. The necessary conditions for the optimality of singular solutions are obtained for one specific example of this type.This work was supported by the Pokrajinska Zajednica za Naucni Rad, Novi Sad, Yugoslavia.     

Thermoelastic homogenization of periodic composites using an eigenstrain-based micromechanical model

This paper presents a study on effective thermoelastic properties of composite materials with periodic microstructures. The overall elastic moduli and coefficients of thermal expansion of such materials are evaluated by a micromechanical model based on the Eshelby equivalent inclusion approach. The model employs Fourier series in the representation of the periodic strain and displacement fields involved in the homogenization procedures and uses the Levin's formula for determining the effective coefficients of thermal expansion. Two main objectives can be highlighted in the work. The first of them is the implementation and application of an efficient strategy for computation of the average eigenstrain vector which represents a crucial task required by the thermoelastic homogenization model. The second objective consists in a detailed investigation on the behavior of the model, considering the convergence of results and efficiency of the strategy used to obtain the approximate solution of the elastic homogenization problem. Analyses on the complexity of the eigenstrain fields in function of the inclusion volume fractions and contrasts between the elastic moduli of the constituent phases are also included in the investigation. Comparisons with results provided by other micromechanical methods and experimental data demonstrate the very good performance of the presented model.     

Application of optimization methods for finding equilibrium states of two-dimensional crystals

A two-dimensional model of a multilayer material and a procedure for simulating its properties based on global optimization methods are proposed. This model is applied for the case of a two-dimensional crystal. Global minima of the interaction energy of the material’s atoms are found, and geometric characteristics of its corresponding equilibrium states are described. The resulting lattices, in particular graphene’s lattices, agree with experimental data, which confirms the validity of the proposed approach. This approach can be extended to a wider class of layered structures, and it can be used for determining the mechanical properties of materials.     

Homogenization methods for two-phase composites

Temperature, magnetic, and electric fields in composite materials are of interest in many engineering applications. In order to use them effectively in modern constructions, it is necessary to predict their effective homogenized properties. The aim of the present paper is to describe, both theoretically and numerically, the effective properties of two-phase composites and to compare the results obtained. The analysis is conducted for 2D and 3D cases.     

Application of the Galerkin and Ritz methods to nonconservative problems of elastic stability

Zusammenfassung In dieser Arbeit werden nichtkonservative Probleme der elastischen Stabilität mittels des Galerkin- und des Ritz-Verfahrens behandelt.Zuerst wird bewiesen, dass die Resultate vonLeipholz bezüglich der Konvergenz des Galerkinschen Verfahrens für solche Probleme auf eine grössere Klasse zulässiger Versuchsfunktionen erweitert werden können. Danach wird das Hamiltonsche Prinzip für nichtkonservative mechanische Systeme so formuliert, dass die Ritzsche Methode auf nichtkonservative Probleme der elastischen Stabilität angewendet werden kann, im Gegensatz zur Ansicht, dass eine solche Anwendung nicht möglich ist. Zum Schluss werden einige angenäherte Lösungen des Beckschen Problems angeführt und verglichen, um die Begriffe zu erklären, die am Anfang dieser Arbeit dargestellt wurden.     

Multi-step quasi-Newton methods for optimization

Quasi-Newton methods update, at each iteration, the existing Hessian approximation (or its inverse) by means of data deriving from the step just completed. We show how “multi-step” methods (employing, in addition, data from previous iterations) may be constructed by means of interpolating polynomials, leading to a generalization of the “secant” (or “quasi-Newton”) equation. The issue of positive-definiteness in the Hessian approximation is addressed and shown to depend on a generalized version of the condition which is required to hold in the original “single-step” methods. The results of extensive numerical experimentation indicate strongly that computational advantages can accrue from such an approach (by comparison with “single-step” methods), particularly as the dimension of the problem increases.     

Objective-derivative-free methods for constrained optimization

We propose feasible descent methods for constrained minimization that do not make explicit use of the derivative of the objective function. The methods iteratively sample the objective function value along a finite set of feasible search arcs and decrease the sampling stepsize if an improved objective function value is not sampled. The search arcs are obtained by projecting search direction rays onto the feasible set and the search directions are chosen such that a subset approximately generates the cone of first-order feasible variations at the current iterate. We show that these methods have desirable convergence properties under certain regularity assumptions on the constraints. In the case of linear constraints, the projections are redundant and the regularity assumptions hold automatically. Numerical experience with the methods in the linearly constrained case is reported. Received: November 12, 1999 / Accepted: April 6, 2001?Published online October 26, 2001     

Multi-start methods for combinatorial optimization

Multi-start methods strategically sample the solution space of an optimization problem. The most successful of these methods have two phases that are alternated for a certain number of global iterations. The first phase generates a solution and the second seeks to improve the outcome. Each global iteration produces a solution that is typically a local optimum, and the best overall solution is the output of the algorithm. The interaction between the two phases creates a balance between search diversification (structural variation) and search intensification (improvement), to yield an effective means for generating high-quality solutions. This survey briefly sketches historical developments that have motivated the field, and then focuses on modern contributions that define the current state-of-the-art. We consider two categories of multi-start methods: memory-based and memoryless procedures. The former are based on identifying and recording specific types of information (attributes) to exploit in future constructions. The latter are based on order statistics of sampling and generate unconnected solutions. An interplay between the features of these two categories provides an inviting area for future exploration.     

Application of topological methods to estimate the number of longitudinal elastic waves in crystals

In the problem of estimating the number of longitudinal normals of elastic waves in crystals, several approaches based on the methods of modern topology and associated with the use of topological invariants are proposed. It is shown that for any symmetry class of the crystals there exists not less than three directions along which purely longitudinal waves can propagate. Under the Morse condition on the fundamental function, connections are obtained between the numbers of directions of longitudinal normals corresponding to different types of critical points of this function. Further prospects are discussed.Voronezh Polytechnic Institute. Translated from Teoreticheskaya i Matematicheskaya Fizika, Vol. 94, No. 1, pp. 146–152. January, 1993.