Effective elastoplastic properties of the periodic composites

The article presented deals with the homogenization of composite materials with elastoplastic constituents. The transformation field analysis (TFA) approach is presented and applied to compute the effective nonlinear behavior of multicomponent periodic composite structure. Computational implementation of the method consists in special utilization of the program ABAQUS, which makes it possible to homogenize n-component periodic composites with relatively general configuration of the periodicity cell. Numerical example of homogenization of a three-component periodic composite shows the comparison between the nonlinear behavior of a real composite and of a homogenized one in a specific boundary problem defined on its representative volume element (RVE).     

Influence of geometrical parameters on the elastic response of unidirectional composite materials

A numerical approach to predict the elastic properties of composite materials departing from the properties of the individual constituents is presented. Using a recently proposed algorithm which generates a random distribution of fibres emulating the real distribution in the transverse cross-section of composite materials, an estimate of the elastic properties is obtained by performing volumetric homogenisation of the results from micromechanical analyses. The influence of different geometrical parameters used in the generation of the random distribution of fibres is analysed, namely, the dimensions of the representative volume element, the fibre radius, and the interval between neighbouring fibres.     

The transverse elastic properties of chiral honeycombs

This work describes the out-of-plane linear elastic mechanical properties of trichiral, tetrachiral and hexachiral honeycomb configurations. Analytical models are developed to calculate the transverse Young’s modulus and the Voigt and Reuss bounds for the transverse shear stiffness. Finite Element models are developed to validate the analytical results, and to identify the dependence of the transverse shear stiffness vs. the gauge thickness of the honeycombs. The models are then validated with experimental results from flatwise compressive and simple shear tests on samples produced with rapid prototype (RP)-based techniques.     

Determination of sensitivity coefficients of the elastic effective properties for periodic fiber-reinforced composites using the response function method

Derivation and implementation of the homogenization method including determination of sensitivity gradients of the effective elasticity tensor using combined numerical-analytical approach are addressed in this paper. This is possible thanks to an application of the numerical response function together with the effective moduli method known from classical homogenization theory. Computational procedure is implemented using 4-noded quadrilateral plane strain finite elements (program MCCEFF) and the symbolic computations system MAPLE. The sensitivity coefficients are determined on the basis of partial derivatives of the homogenized elasticity tensor calculated using the response function method with respect to all composite components’ elastic characteristics. They are further separately subjected to normalization procedure for a final comparison with each other. Such an enriched homogenization procedure is tested on the periodic fiber-reinforced two component composites; the results of computational analysis are compared to the results of the central finite difference approach applied before. Computational methodology proposed here may be further successively applied not only in the context of homogenization method but also to extend various discrete computational techniques like boundary/finite element, finite difference and volumes together with various meshless methods.     

The mechanical properties of a transverse isotropic voided material

The mechanical properties of a transverse isotropic honeycomb porous material were investigated by the experiment and the finite element method. The relative density dependence of the macroscopic elastoplastic constants along the in-plane and out of plane directions (e.g., Young’s modulus, Poisson’s ratio, yielding stress, etc.) was systematically studied. Good agreement between the experimental and numerical results was observed. In addition, the three dimensional numerical results were compared with simplified foam models and self-consistent estimates, showing the necessity to improve available models for foams in order to achieve a better predictive capability. At the same time, the elastic–plastic failure mechanism was discussed in detail.     

Tight bounds for three-dimensional nonlinear incompressible elastic composites

Tighter variational bounds, in the whole range of inclusion volume fraction, that is to say, even near percolation, for the effective energy of nonlinear composites, in the special case of 3D two-phase incompressible elastic composites with isotropic constituents are presented. Following the methodology of Talbot, Willis and Ponte Castañeda, a linear comparison material with the same microgeometry as the nonlinear composite is employed. The asymptotic homogenization method (AHM) combined with a finite element analysis (FEM), is used to find the displacement field as well as the effective properties for the comparison material. An elastic composite with periodically distributed spherical inclusions in a cubic array is considered as an example. Various numerical examples are performed. Comparisons with others theories (i.e. variational bounds, self-consistent estimates, etc.) are shown. Coincidence of the AHM-FEM results with the universal bounds of Nemat-Nasser, Yu and Hori serves as a useful check to the numerical calculation.     

高孔率网状Cu+CeO_2/Cu抗菌材料的制备及其抗菌性能

以泡沫海绵为基体材料,通过电镀的方法制备了高孔率网状Cu+CeO2/Cu抗菌材料,研究了制备条件并测定了其孔隙率和比表面积,评价了其抗菌性能.结果表明,网状Cu+CeO2/Cu抗菌材料的孔隙率高达95%以上,其比表面积约为31.4m2/cm3,对于大肠埃希氏菌、金黄色葡萄球菌和白色念珠菌3种菌种均具有良好的抗菌性能.     

磁性多孔碳材料的研究进展

磁性多孔碳材料同时具有磁性和多孔性质,其拥有丰富的孔道结构、高的比表面积、高孔容、良好的活性位点和磁性可分离等优异的性能,可以很好的解决多孔碳材料在应用过程中难分离回收等问题,因此,磁性多孔碳材料已经在吸附领域得到广泛的应用.按照孔径大小、磁性强弱以及组合方式的不同将磁性多孔碳材料进行了分类,并综述了近年来磁性多孔碳材...     

Space-holder effect on designing pore structure and determining mechanical properties in porous titanium

The present work studied the effect of space holders on pore structure and mechanical properties in porous titanium. Four types of space holders (sodium chloride, starch, and urea with different size and morphology) were utilized to fabricate porous titanium. The space holders played a key role in the pore structure as confirmed by microstructural observations and three-dimensional computed tomography technique. Mechanical properties of each sample were investigated and discussed on the basis of the tomography results and finite element method. It is concluded that the porosity determines the elastic modulus regardless of the type and morphology of space holders, whereas both porosity and type of space holder affected strength.     

Pressure and temperature dependence of elastic properties of lanthanum gallogermanate glasses

The hydrostatic and the uniaxial pressure, and the temperature dependence of elastic properties of a series of lanthanum gallogermanate glasses were determined by ultrasonic pulse-echo techniques. The experimental results are used to obtain the complete set of second- and third-order elastic constants of these glasses. The normal behavior of negative temperature dependence and positive pressure dependence of the ultrasonic velocities were observed in these glasses. The pressure derivative of shear and bulk moduli, and the longitudinal and shear acoustic mode Gruneisen parameters are all positive for these glasses.     

Structural,elastic, electronic and optical properties of new layered semiconductor BaGa2P2

We report the results of a detailed first-principles based density functional theory study of the structural, elastic, electronic and optical properties of a recently synthesized layered semiconductor BaGa₂P₂. The optimized structural parameters are in excellent agreement with the experimental structural findings, which validates the used theoretical method. The single crystal and polycrystalline elastic constants are numerically estimated using the strain–stress method and Voigt–Reuss–Hill approximations. Predicted values of the elastic constants suggest that the considered material is mechanically stable, brittle and very soft material. The three-dimensional surface and its planar projections of Young’s modulus are visualized to illustrate the elastic anisotropy. It is found that Young’s modulus of BaGa₂P₂ show strong dependence on the crystallographic directions. Band structure calculation reveals that BaGa₂P₂ is a direct energy band gap semiconductor. The effective masses of electrons and holes at the minimum of the conduction band and maximum of the valence band are numerically estimated. The density of state, charge density distribution and charge transfers are calculated and analyzed to determine the chemical bonding nature. Dielectric function, refractive index, extinction coefficient, absorption coefficient, reflectivity and electron-loss energy function spectra are computed for a wide photon energy range up to 20 eV. Calculated optical spectra exhibit a noticeable anisotropy.     

多孔材料合成制备进展

综述了多孔材料在合成制备方面的进展 ,详细介绍了溶胶 凝胶法和水热合成法及模板技术在这些方法中的最新应用 ,指出了多孔材料的发展趋势     

多孔材料孔结构分形维数的研究现状

分形表征是多孔材料孔结构表征中的一种新兴的方法。通过构造分形几何模型，阐述了多孔材料孔结构分形表征中不同类型的分形维数。分析了现阶段研究中存在的问题，探讨了分形表征在多孔材料孔结构表征中未来的发展趋势。     

A numerical investigation of the connection between state of dispersion and percolation and its effect on the elastic properties of 2D random microstructures

The present work is dedicated to a numerical investigation of the connection between state of dispersion and percolation and its effect on the elastic properties of 2D random microstructures. The main objective consists in checking out the link between percolation and mechanical response in the context of a heterogeneous medium the reinforcements of which are not homogeneously dispersed. Besides, the influence of the stiffness of inclusions is also investigated since this could impact on the percolation effects. For these purposes, large samples of volume elements are generated according to the Monte Carlo method. We consider the low cost framework of 2D random grids which enables large and in-depth investigations. Besides, the spatial distribution of heterogeneities is simulated with the help of the 2-scale Boolean scheme of disks which is a powerful tool for modelling and studying several states of dispersion. The numerical results highlight beneficial mechanical reinforcements for a heterogeneous dispersion when the percolation phenomenon is enhanced. This improvement is highly sensitive to the stiffness of heterogeneities.     

Prediction of the elastic properties of syntactic perforated hollow sphere structures

This work investigated the elastic properties of a new type of hollow sphere structures. For this new type, the sphere shell is perforated by several holes in order to open the inner sphere volume for a matrix material. The effective elastic properties of syntactic (i.e. spheres completely embedded in a matrix) perforated hollow sphere structures in a primitive cubic (PC) arrangement of unit cell models are numerically evaluated for different hole diameters, matrix volume fractions and different base materials. The results are compared to typical configurations without perforation. In the scope of this paper, three-dimensional finite element analysis is used in order to investigate these unit cell models.     

Distributions of electric and elastic fields at domain boundaries

In this paper we describe the application of the finite element method (FEM) in modelling spatial distributions of electric and elastic fields in a ferroelectric crystals with two domains separated by a 90° domain wall. The domain boundary is idealized as a two-dimensional defect in an electro-elastic continuum. It represents the source of inhomogenity and internal distortion in both elastic and electric fields. The main results are distributions of electric field, strain and mechanical force along the domain boundary.     

Identification of the elastic and damping properties in sandwich structures with a low core-to-skin stiffness ratio

A mixed numerical–experimental identification procedure for estimating the storage and loss properties in sandwich structures with a soft core is developed. The proposed method uses at the experimental level a precise measurement setup with an electro-dynamic shaker and a scanning laser interferometer, and at the computational level an original structurally damped shell finite element model derived from the higher-order shear deformation theory with piecewise linear functions for the through-the-thickness displacement. The parameter estimation is derived from adequate objective functions measuring the discrepancy between the experimental and numerical modal data. Through a sensitivity analysis it is shown that for sandwich structures with a soft core only one specimen is required for characterizing the dominant properties of both the core and the skins. The procedure is then applied to two test cases for which all the influent elastic properties and the major damping parameters could be estimated with a fairly good precision.     

Elastic and plastic properties of metal-matrix composites: geometrical effects of particles

Three-dimensional finite element simulations on the base of cell models and micromechanics have been made in this investigation to predict effective elastic moduli and elastoplastic stress-strain behavior of particulate-reinforced metal-matrix composites. Numerical results are given for the influence of particle volume fraction, shape and orientation to the overall behavior of the composite.     

Mechanics of extended continua: modeling and simulation of elastic microstretch materials

The investigation of microstretch and micromorphic continua (which are prominent examples of so-called extended continua) dates back to Eringens pioneering works in the mid 1960, cf. (Eringen in Mechanics of micromorphic materials. Springer, Berlin Heidelberg New York, pp 131–138, 1966; Eringen in Int J Eng Sci 8:819–828; Eringen in Microcontinuum field theories. Springer, Berlin Heidelberg New York, 1999). Here, we re-derive the governing equations of microstretch continua in a variational setting, providing a natural framework within which numerical implementations of the model equations by means of the finite element method can be obtained straightforwardly. In the application of Dirichlets principle, the postulation of an appropriate form of the Helmholtz free energy turns out to be crucial to the derivation of the balance laws and constitutive relations for microstretch continua. At present, the material parameters involved in the free energy have been assigned fixed values throughout all numerical simulations—this simplification is addressed in detail as the influence of those parameters must not be underestimated. Since only few numerical results demonstrating elastic microstretch material behavior in engineering applications are available, the focus is here on the presentation of numerical results for simple twodimensional test specimens subjected to a plane strain condition and uniaxial tension. Confidence in the simulations for microstretch materials is gained by showing that they exhibit a “downward-compatibility” to Cosserat continuum formulation: by switching off all stretch-related effects, the governing set of equations reduces to the one used for polar materials. Further, certain material parameters can be chosen to act as penalty parameters, forcing stretch-related contributions to an almost negligible range in a full microstretch model so that numerical results obtained for a polar model can be obtained as a limiting case from the full microstretch model.     

The in-plane linear elastic constants and out-of-plane bending of 3-coordinated ligament and cylinder-ligament honeycombs

Four novel cylinder-ligament honeycombs are described, where each cylinder has 3 tangentially-attached ligaments to form either a hexagonal or re-entrant hexagonal cellular network. The re-entrant cylinder-ligament honeycombs are reported for the first time. The in-plane linear elastic constants and out-of-plane bending response of these honeycombs are predicted using finite element (FE) modelling and comparison made with hexagonal and re-entrant hexagonal honeycombs without cylinders. A laser-crafted re-entrant cylinder-ligament honeycomb is manufactured and characterized to verify the FE model. The re-entrant honeycombs display negative Poisson’s ratios and synclastic curvature upon out-of-plane bending. The hexagonal and ‘trichiral’ honeycombs possess positive Poisson’s ratios and anticlastic curvature. The ‘anti-trichiral’ honeycomb (short ligament limit) displays negative Poisson’s ratios when loaded in the plane of the honeycomb, but positive Poisson’s ratio behaviour (anticlastic curvature) under out-of-plane bending. These responses are understood qualitatively through considering deformation occurs via direct ligament flexure and cylinder rotation-induced ligament flexure.