Multiscale characterization of carbonated wollastonite paste and application of homogenization schemes to predict its effective elastic modulus

This paper describes the multiscale characterization of the carbonated wollastonite paste using X-ray diffraction (XRD), scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS), and statistical nanoindentation (SNI, also known as ‘grid indentation’) methods as well as micromechanical homogenization models. Wollastonite (CaSiO₃) fibers are commonly used as filler in ceramics or plastics. However, wollastonite can also be regarded as non-hydraulic binder material since upon carbonation it forms a heterogeneous matrix with mechanical properties similar to those of the conventional hydrated cement pastes. Carbonation reaction of wollastonite results in the formation of two main products: calcium carbonate (CaCO₃) and amorphous silica gel (SiO₂). The SEM/EDS microanalysis performed on this system revealed that the average calcium to silica (Ca/Si) atomic ratio of the silica gel phase was around 0.40. Three individual carbonated wollastonite paste samples, each representing a different degree of carbonation were selected for nanoindentation tests. The obtained elastic moduli for silica gel, calcium carbonate, and unreacted wollastonite grains were, respectively, 41.7 GPa, 67.3 GPa, and 134.7 GPa. The micromechanical homogenization models were then utilized to predict the effective (also referred to as ‘homogenized’) elastic moduli of the carbonated wollastonite paste. The predicted values of the effective elastic moduli of carbonated wollastonite pastes were found to be in the range of corresponding values for hydrated high to ultra-high performance cement pastes. Additionally, the values of the effective elastic moduli of the carbonated wollastonite pastes were observed to increase with the increase in the degree of carbonation.     

Computational up-scaling of anisotropic swelling and mechanical behavior of hierarchical cellular materials

The hygro-mechanical behavior of a hierarchical cellular material, i.e. growth rings of softwood is investigated using a two-scale micro-mechanics model based on a computational homogenization technique. The lower scale considers the individual wood cells of varying geometry and dimensions. Honeycomb unit cells with periodic boundary conditions are utilized to calculate the mechanical properties and swelling coefficients of wood cells. Using the cellular scale results, the anisotropy in mechanical and swelling behavior of a growth ring in transverse directions is investigated. Predicted results are found to be comparable to experimental data. It is found that the orthotropic swelling properties of the cell wall in thin-walled earlywood cells produce anisotropic swelling behavior while, in thick latewood cells, this anisotropy vanishes. The proposed approach provides the ability to consider the complex microstructure when predicting the effective mechanical and swelling properties of softwood.     

Microstructure–strength models for heat treatment of Al–Si–Mg casting alloys I: microstructure evolution and precipitation kinetics

A comprehensive microstructure–strength mathematical model for the heat treatment of Al–Si–Mg casting alloys is presented. As part of the model development, the evolution of microstructure and mechanical properties during heat treatment of an industrially cast A356 aluminium alloy was studied in an extensive experimental investigation. For the solution treatment process, the changes in dendritic composition and eutectic morphology in the temperature range 773–833 K (500–560°C) were quantified using microprobe and image analysis techniques. For natural and artificial ageing, the kinetics of precipitation/clustering was determined using an isothermal calorimetry technique in conjunction with hardness and mechanical property measurements. Two other Al–Si–Mg model alloy compositions were used to study the effects of alloy chemistry on microstructure response during heat treatment. The overall aim of the experimental work presented here is to facilitate the development of a comprehensive microstructure–strength model for the heat treatment of Al–Si–Mg casting alloys that will be presented in part II of this paper.

On présente un modèle mathématique détaillé de la microstructure-résistance du traitement thermique des alliages de moulage d'Al–Si–Mg. Faisant partie du développement du modèle, on a étudié l'évolution de la microstructure et des propriétés mécaniques lors du traitement thermique d'un alliage d'aluminium A356 moulé industriellement lors d'un examen expérimental de grande envergure. Pour le traitement de mise en solution, on a quantifié les changements de la composition dendritique et la morphologie de l'eutectique dans la gamme de température de 773 à 833 K (500 à 560 °C) en utilisant les techniques de la microsonde et de l'analyse d'image. Pour le vieillissement naturel et artificiel, on a déterminé la cinétique de précipitation/agrégation en utilisant une technique de calorimétrie isotherme en conjonction avec les mesures de dureté et de propriétés mécaniques. On a utilisé deux autres compositions de l'alliage modèle d'Al–Si–Mg pour étudier les effets de la chimie de l'alliage sur la réponse de la microstructure lors du traitement thermique. Le but global du travail expérimental présenté ici est de faciliter le développement d'un modèle détaillé de microstructure-résistance du traitement thermique des alliages de moulage d'Al–Si–Mg qui sera présenté dans la seconde partie de cet article.     

Polycrystalline behavior analysis of pure magnesium by the homogenization method

In this study, mechanical behaviors of pure magnesium polycrystals are numerically investigated. The homogenization method, which combines the crystal and macroscopic scales, is introduced to include the effect of crystalline scale behaviors. The polycrystal plasticity model modified for pure magnesium, in which twinning is considered as asymmetric slip-like deformation, is utilized as a constitutive equation. Within this framework, numerical convergence analyses are conducted, and a representative volume element to present realistic deformation of pure magnesium is investigated. Second, polycrystalline behaviors of pure magnesium are investigated. The present approach is shown to reproduce the typical phenomena induced by crystalline scale structure in pure magnesium: nonuniform strain distribution, asymmetric crystal lattice orientation, strength differential effect, and strongly anisotropic initial and subsequent yield surfaces.     

A method of predicting macroscopic yield strength of polycrystalline metals subjected to plastic forming by micro-macro de-coupling scheme

We introduce a method of predicting the macroscopic yield strength of polycrystalline metals subjected to plastic forming, which hinges on the reliability of the micro-macro decoupling scheme for solving the corresponding two-scale boundary value problem. A polycrystalline aggregate composed of several crystal grains is adopted as a periodic microstructure, namely unit cell, and is regarded as a numerical specimen for numerical material tests (NMTs) based on the homogenization theory. The NMTs are conducted to identify the material parameters of the assumed approximate macroscopic constitutive model and followed by the de-coupled macro-scale analysis to simulate the macro-scale forming process. We then conduct the de-coupled micro-scale analysis by applying the resulting macroscopic deformation histories to the microstructures associated with the macroscopic points of interest in order to obtain the numerical specimens after plastic forming. Finally, the NMTs are conducted on the prepared specimens to evaluate the macroscopic post-forming yield strengths. We validate the proposed method by taking the three-step forming process as an example of macroscopic plastic forming. The validation of the method is made in comparison with the results with those obtained by the equivalent two-scale analysis with the coupling scheme. To demonstrate the capability and promise of the proposed method for practical applications, we also present an numerical example of the rolling process that causes the texture development in crystal grains.     

聚烯烃重力掺混料仓及其评价

介绍了聚烯烃掺混料仓的常用形式和工作原理,并对5种常用掺混料仓的特点和性能进行了综合评价,提出了掺混料仓选用的原则。     

高Zn含量Al-Zn-Mg-Cu合金均匀化过程中显微结构演变研究

利用显微镜（OM）、差热分析（DSC）、扫描电镜（SEM）和X衍射（XRD）研究一种高Zn含量Al-Zn-Mg-Cu合金在均匀化处理过程中的显微结构演变,使用扩散动力学模型推导均匀化动力学方程,用于确认最佳均匀化参数。结果表明：合金的铸态组织中存在严重偏析,非平衡共晶结构包含α(Al)、Mg(Zn,Cu,Al)₂、 S(Al₂CuMg)、θ(Al₂Cu)和富Fe相。当前研究表明均匀化过程中没有发生Mg(Zn,Cu,Al)₂相向S(Al₂CuMg)相的转变,Mg(Zn,Cu,Al)₂相直接回溶。随着均匀化的进行,θ(Al₂Cu)相溶入基体。均匀化后富Fe相仍残留,但随着保温时间的延长,富Fe相中的Zn、Mg元素铸件减少或消失。最佳均匀化参数为440 oC×12 h 468 oC×24 h,这与均匀化动力学的分析相一致。     

波形碳纳米管增强复合材料的有效刚度和局部应力的宏微观均质化数值模拟

采用分子动力学方法简化的碳纳米管等效纤维模型,利用具有精确周期性边界条件的均质化理论和宏微观均质化法分析正弦波形非连续碳纳米管的有效刚度和局部应力分布规律.结果表明,纳米增强复合材料的有效刚度和局部应力对碳纳米管的波形非常敏感,碳纳米管稍有弯曲就会导致复合材料有效刚度降低和应力传递能力的下降,为揭示复合材料中碳纳米管的增强机制和改善增强效果提供理论依据.     

Homogenization pressures applied to citrus juice manufacturing. Functional properties and application

Homogenization is a unit operation that can be incorporated in citrus juice manufacturing to improve chemical and physical characteristics relevant for use in subsequent processing operations. The aim of this study was to evaluate the effect of different homogenization pressures on suspended solids and antiradical activity in mandarin low pulp juice (LPJ) and to understand their influence on a posterior vacuum impregnation operation. We found the pressure treatments applied to LPJ do not have negative effects on antiradical activity or functional compounds in the juice. In the vacuum impregnation study we found that more LPJ was introduced into the structural matrix of apples when homogenized at higher pressures and therefore more functional compounds may be introduced due to pulp stability and particle size reduction.     

Neural networks as material models within a multiscale approach

This paper shows the application of neural networks in a multiscale analysis of a reinforced concrete beam. A mesoscale model is presented, which simulates the pullout test of a reinforcement bar in concrete. By applying a homogenization procedure, a macroscopic stress vs. crack opening response is obtained from the mesoscale simulations. The neural network is used to approximate this relation in a macroscale simulation and replaces the material formulation of the interface layer between concrete and reinforcement, thus avoiding the computationally expensive parallel simulation on different scales.