Microstructure evolution,mechanical properties and diffusion behaviour of Mg-6Zn-2Gd-0.5Zr alloy during homogenization

The microstructure evolution and mechanical properties of Mg-6Zn-2Gd-0.5Zr alloy during homogenization treatment were investigated. The as-cast alloy was found to be composed of dendritic primary α-Mg matrix, α-Mg + W (Mg₃Zn₃Gd₂) eutectic along grain boundaries, and icosahedral quasicrystalline I (Mg₃Zn₆Gd) phase within α-Mg matrix. During homogenization process, α-Mg + W (Mg₃Zn₃Gd₂) eutectic and I phase gradually dissolved into α-Mg matrix, while some rod-like rare earth hydrides (GdH₂) formed within α-Mg matrix. Both the tensile yield strength and the elongation showed a similar tendency as a function of homogenization temperature and holding time. The optimized homogenization parameter was determined to be 505 °C for 16 h according to the microstructure evolution. Furthermore, the diffusion kinetics equation of the solute elements derived from the Gauss model was established to predict the segregation ratio of Gd element as a function of holding time, which was proved to be effective to evaluate the homogenization effect of the experimental alloy.     

Long-term consolidation mechanisms based on micro–macro behavior and in situ XRD measurement of basal spacing of clay minerals

An important issue in the area of geological disposal of high-level radioactive waste (HLW) is to demonstrate the long-term mechanical stability of the buffer. In particular, it has to be clarified whether a waste package would continue to sink in the buffer over a long time period, resulting in a significant decrease in the buffer thickness. The candidate buffer material in Japan is a mixture of silica sand and bentonite. Consolidation tests have revealed that the bentonite shows secondary consolidation phenomena similar to clay in general. Therefore, it is important to investigate the mechanism of secondary consolidation behavior.Bentonite is a microinhomogeneous material consisting of clay minerals, macrograins (mainly quartz) and others. The unique combination of molecular dynamics (MD) and homogenization analysis (HA) procedures, termed the unified MD/HA method, has been proposed for estimating the micro to macro behavior of such an inhomogeneous material (Ichikawa, Y., Kawamura, K., Nakano, M., Kitayama, K., Kawamura, H., 1998. Unified molecular dynamics/homogenization analysis for water flow in bentonite. Proc. 1998 Int. High-Level Radioactive Waste Management Conf., Las Vegas. American Nuclear Society, La Grange Park, IL, pp. 422–428). In this study, the unified MD/HA method is applied to bentonite in order to understand its long-term consolidation mechanism. Thus, it was found that the permeability decreases significantly with a decrease in the void ratio due to the evolution of consolidation. It was therefore assumed that secondary consolidation is governed by drainage from the interlayer pores (micropores) with very low permeability, and that this is the reason why secondary consolidation is very slow.This paper also documents the result of an X-ray diffraction (XRD) experiment on bentonite under consolidation (in situ XRD), which was performed in order to validate the assumption mentioned above. It was observed that interlayer space starts to decrease after the latter half of primary consolidation. This finding strongly supports the long-term consolidation mechanism presumed above from a microscopic point of view.One-dimensional consolidation analyses of the bentonite, into which the relationship between the void ratio and the permeability determined using the unified MD/HA method was introduced, were performed for comparison with a long-term consolidation test. The good agreement between the analytical result and the test result including secondary consolidation behavior also supports the long-term consolidation mechanism presumed above.     

Effective mechanical behavior of hyperelastic honeycombs and two-dimensional model foams at finite strain

The aim of the present study is the analytical and numerical determination of the effective stress–strain behavior of solid foams made from hyperelastic materials in the finite strain regime. For the homogenization of the microstructure, a strain energy-based concept is proposed which assumes macroscopic mechanical equivalence of a representative volume element for the given microstructure with a similar homogeneous volume element if the strain energy of both volume elements is equivalent, provided that the volume average of the deformation gradient is equal for both volume elements. The concept is applied to an analysis of hyperelastic solid foams using a two-dimensional model. The effective stress–strain behavior is analyzed under uniaxial and biaxial loading conditions in the tensile and in the compressive range as well as under simple shear deformation. It is observed that the effective mechanical behavior of cellular solids at infinitesimal and finite deformation is essentially different on both, the quantitative and the qualitative level.     

Micromechanical modeling of dual phase steels

Dual phase (DP) steels having a microstructure consisting of a Ferrite matrix, in which particles of Martensite are dispersed, have received a great deal of attention due to their useful combination of high strength, high work hardening rate and ductility, all of which are favorable properties for forming processes. Experimental investigation into the effect of the harder phase volume fraction, morphology and phase distribution on mechanical properties of the dual phase steels is well established and comprehensive in the literature. In the present work, a micromechanical model is developed to capture the mechanical behavior of such materials, adopting the constitutive behavior of the constituents from the literature. Analytical approaches have been used in the past to model the DP steel material behavior, but theoretical treatments are based on the assumption of uniform deformation throughout the constituents, neglecting the local strain gradients. This assumption contradicts experimental observations, reduces the understanding of the mechanics and mechanism of deformation of such materials. Based on the micromechanical modeling of cells, several idealizations are investigated out of which the axisymmetric model is shown to display intrinsic ability to capture the expected material behavior in terms of the trend of the stress–strain curves with increasing volume fraction of the second phase and in terms of the deformation fields of the constituents.     

Asymptotic simulation of imperfect bonding in periodic fibre-reinforced composite materials under axial shear

An asymptotic approach for simulation of the imperfect interfacial bonding in composite materials is proposed. We introduce between the matrix and inclusions a flexible bond layer of a volume fraction c⁽³⁾ and of a non-dimensional rigidity λ⁽³⁾, derive a solution for such three-component structure, and then set c⁽³⁾→0, λ⁽³⁾→0. In the asymptotic limit depending on the ratio λ⁽³⁾/c⁽³⁾ different degrees of the interface's response can be simulated. A problem of the axial shear of elastic fibre-reinforced composites with square and hexagonal arrays of cylindrical inclusions is considered. The performed analysis is based on the asymptotic homogenization method, the cell problem is solved using the underlying principles of the boundary shape perturbation technique. As a result, we obtain approximate analytical solutions for the effective shear modulus and for the stress field on micro level depending on the degree of the interfacial debonding. Developed solutions are valid for all values of the components’ volume fractions and properties. In particular, they work well in cases of rapid oscillations of local stresses (e.g., in the case of densely packed perfectly rigid inclusions), while many of other commonly used methods may face computational difficulties.     

高速匀浆法提取辣蓼茎中总黄酮

以辣蓼茎为原料,对匀浆法提取辣蓼茎中总黄酮工艺进行了研究,确定了最优工艺参数:提取1次,提取时间2 min,60%的乙醇溶剂,转速为20 000 r/min,料液比为1∶25 g/m L。在最佳条件下,提取并测定了辣蓼茎中总黄酮含量,结果表明,黄酮提取率为4.75%。     

Numerical assessment of an anisotropic porous metal plasticity model

The objective of this paper is to perform numerical assessment of a micromechanical model of porous metal plasticity developed previously by the authors. First, upper bound estimates for the yield loci are computed using homogenization and limit analysis of a spheroidal representative volume element containing a confocal spheroidal void, neglecting elasticity. Unlike in the development of the analytical model, the computational limit analysis is performed without recourse to approximations so that the obtained yield loci are rigorous upper bounds for the true criterion. Next, the model’s macroscopic dilatancy at incipient plastic flow is compared against that of the numerical limit analysis approach. Finally, finite-element calculations, with elasticity included, are presented for transversely isotropic porous unit-cells loaded axisymmetrically. The effective stress–strain response as well as evolution of the unit-cell porosity and void aspect ratio are compared with theoretical predictions.     

Influence of homogenisation and the degradation of stabilizer on the stability of acidified milk drinks stabilized by carboxymethylcellulose

The present work deals with the influences of both homogenisation and the degradation of carboxymethylcellulose (CMC) on the stability of two kinds of acidified milk drinks (AMDs), directly acidified milk drinks and yoghurt drinks. The effect of homogenisation pressure for direct acidification process was investigated and evaluated. The experimental results showed that homogenisation was required to achieve a significantly small particle size (0.7 μm in the present work) and to prevent sedimentation and serum separation. However, homogenisation at too high pressures was not beneficial for the stability of the colloidal systems. The occurrence of degradation of CMC during homogenisation weakened the stabilisation effect of CMC. A qualified homogenisation pressure of 20 MPa should be chosen to achieve a good stability when a usually practical pressure range of 0–30 MPa was applied. In addition, the stability of directly acidified milk and yoghurt drinks prepared under the same homogenisation pressure was also investigated. While their stability increased with increasing CMC concentration, the degradation of CMC at low pH during storage gave rise to instability of the final products.     

An inverse procedure for determination of material constants of a periodic multilayer using Floquet wave homogenization

An inverse numerical method for periodic composite characterization is reported. The method utilizes the velocity data based on Floquet wave homogenization. The optimization procedure is performed on the basis of the gradient method. An efficient polynomial function is derived from Christoffel equation. The numerical procedure leads to an analytical form of the minimized function which is related to the whole Floquet data. The set of input data is collected from different azimuthal plane orientations inside the homogenization domain. The output results mainly include the effective elastic constants of the multidirectional composite and the reliability factor. The initialization of the elastic stiffness matrix is obtained by averaging the rigidity tensor corresponding to each layer orientation. This procedure is examined for [0/90] and [0/60/−60] composites; some of the obtained elastic constants are significantly dependent on the frequency. The agreement between the adopted Floquet velocities and the calculated ones is good; the reliability factor does not exceed 1%. Slight deviations are pointed out in the vicinity of the homogenization limits.     

Homogenized properties of elastic–viscoplastic composites with periodic internal structures

A homogenization theory for elastic–viscoplastic composites with periodic internal structures is developed in rate and incremental forms by considering unit cells subjected to macroscopically uniform stress and strain. The theory enables us to incrementally compute the macroscopic as well as the microscopic stress and strain states in nonlinear time-dependent periodic composites. The theory is effective for problems in which the history of either macro-strain or macro-stress, or a combination of them, is prescribed as a function of time. As applications of the theory, transverse and off-axial deformation problems of metal matrix composites reinforced unidirectionally with continuous fibers are analyzed to discuss the effects of fiber arrangement and orientation on the macroscopic deformation behavior.