基于自洽法的电化学沉积修复饱和混凝土细观描述

针对当前电化学沉积修复混凝土缺乏细观层次上的理论描述,首先,以饱和混凝土的细观结构和电化学沉积修复的主要机理为基础, 提出了含电化学沉积产物、水和混凝土基体在内的三相复合材料细观力学模型. 其次,为了从细观层次定量描述电化学沉积修复对混凝土宏观性能的影响,基于自洽法对上述细观力学模型进行多层次均匀化处理,获取电化学沉积修复饱和混凝土的有效性能,其中,第一层均匀化是采用广义自洽模型获取等效夹杂的有效性能,第二层均匀化是采用自洽方法和沃伊特(Voigt) 上限相结合的方式获取修复混凝土的有效性能. 最后,为了验证该文所提出模型和方法的有效性,对比了该文的预测结果、试验数据和已有模型,结果表明了该文模型和方法是合理的.      

Prediction of elastic modulus and Poisson’s ratio for unsaturated concrete

Many concrete structures are located in water environment, but the underwater concrete is usually unsaturated even though it has been soaked in the water for a very long time. Some experiments have proven that the mechanical properties of concrete are affected by the saturation degree of fluid and aspect ratio of pores. Several publications discussed the saturated concrete qualitatively, but few gave quantitative analysis especially for the unsaturated concrete. In terms of the microstructure of unsaturated concrete, equivalent medium and inclusion-based theory of composite materials, a model is proposed to explain the changes happened in the wet concrete and to predict the elastic parameters (including elastic modulus and Poisson’s ratio) of unsaturated concrete. The viscosity of water in pores, micro-cracks and the further hydration of cement are taken into account in this paper by means of the definition of saturation concept according to the effect of pore water on the modulus of concrete. In this model, both stiff effect of water and soft effect of cracks on the concrete are introduced to describe the bulk modulus, at the same time the effect of shear rate on the shear modulus is considered. The comparison between the theoretical models and experimental results in the extreme state indicates that the model proposed in this paper is valid to predict the elastic properties of unsaturated concrete.     

A micromechanical study of drying and carbonation effects in cement-based materials

This paper is devoted to a micromechanical study of mechanical properties of cement-based materials by taking into account effects of water saturation degree and carbonation process. To this end, the cement-based materials are considered as a composite material constituted with a cement matrix and aggregates (inclusions). Further, the cement matrix is seen as a porous medium with a solid phase (CSH) and pores. Using a two-step homogenization procedure, a closed-form micromechanical model is first formulated to describe the basic mechanical behavior of materials. This model is then extended to partially saturated materials in order to account for the effects of water saturation degree on the mechanical properties. Finally, considering the solid phase change and porosity variation related to the carbonation process, the micromechanical model is coupled with the chemical reaction and is able to describe the consequences of carbonation on the macroscopic mechanical properties of material. Some comparisons between numerical results and experimental data are presented.     

An elastoplastic damage model for metal matrix composites considering progressive imperfect interface under transverse loading

An elastoplastic damage model considering progressive imperfect interface is proposed to predict the effective elastoplastic behavior and multi-level damage progression in fiber-reinforced metal matrix composites (FRMMCs) under transverse loading. The modified Eshelby’s tensor for a cylindrical inclusion with slightly weakened interface is adopted to model fibers having mild or severe imperfect interfaces [Lee, H.K., Pyo, S.H., 2009. A 3D-damage model for fiber-reinforced brittle composites with microcracks and imperfect interfaces. J. Eng. Mech. ASCE. doi:10.1061/(ASCE)EM.1943-7889.0000039]. An elastoplastic model is derived micromechanically on the basis of the ensemble-volume averaging procedure and the first-order effects of eigenstrains. A multi-level damage model [Lee, H.K., Pyo, S.H., 2008a. Multi-level modeling of effective elastic behavior and progressive weakened interface in particulate composites. Compos. Sci. Technol. 68, 387–397] in accordance with the Weibull’s probabilistic function is then incorporated into the elastoplastic multi-level damage model to describe the sequential, progressive imperfect interface in the composites. Numerical examples corresponding to uniaxial and biaxial transverse tensile loadings are solved to illustrate the potential of the proposed micromechanical framework. A series of parametric analysis are carried out to investigate the influence of model parameters on the progression of imperfect interface in the composites. Furthermore, a comparison between the present prediction and experimental data in the literature is made to assess the capability of the proposed micromechanical framework.     

多相材料有效性能预测的高精度方法

有效介质方法是常用的细观力学方法之一.其可用于计算多相材料的有效性能,并建立材料微细观结构和宏观性能的定量关系;有助于指导新材料设计,减少试验工作量等.然而,当夹杂含量升高时,传统有效介质方法的计算精度下降.本文以两相材料为研究对象,提出一种新的参考介质,即:为更合理考虑不同夹杂颗粒间的相互作用,假定参考介质的应变是基体相平均应变和某一修正张量的双点积.在此基础上,推导了新参考介质下两相材料的有效模量表达式,并给出该修正张量的近似计算方法;通过反复更新参考介质,采用多层次均匀化思路,将本文方法进一步用于多相材料性能的预测.为验证方法的有效性,将预测结果与已有模型结果和试验数据进行对比.结果表明本文方法较已有方法更为合理、有效.当夹杂含量升高时,本文方法较传统有效介质方法的计算精度有所提升.     

Development and experimental validation of a continuum micromechanics model for the elasticity of wood

This contribution covers the development and validation of a microelastic model for wood, based on a four-step homogenization scheme. At a length scale of several tens of nanometers, hemicellulose, lignin, and water are intimately mixed, and build up a polymer (polycrystal-type) network. At a length scale of around one micron, fiberlike aggregates of crystalline and amorphous cellulose are embedded in an contiguous polymer matrix, constituting the so-called cell wall material. At a length scale of about one hundred microns, the material softwood is defined, comprising cylindrical pores (lumen) in the cell wall material of the preceding homogenization step. Finally, at a length scale of several millimeters, hardwood comprises larger cylindrical pores (vessels) embedded in the softwood-type material homogenized before. Model validation rests on statistically and physically independent experiments: The macroscopic stiffness values (of hardwood or softwood) predicted by the micromechanical model on the basis of tissue-independent (‘universal’) phase stiffness properties of hemicellulose, amorphous cellulose, crystalline cellulose, lignin, and water (experimental set I) for tissue-specific composition data (experimental set IIb) are compared to corresponding experimentally determined tissue-specific stiffness values (experimental set IIa).     

考虑孔隙影响的混凝土有效弹性模量预测分析研究

本文通过纳米压痕实验技术得到混凝土材料细观各相参数，基于渐进均匀化理论，采用蒙特卡洛方法和双向游走方法建立了含孔隙混凝土的胞元模型．分析了孔隙在冻融循环次数增加情况下对混凝土有效弹性模量的影响，同时与有限元模拟分析进行了比较．结果表明：随着冻融循环次数增加，孔隙体积分数增大，界面与砂浆压痕模量相对降低，但对骨料影响较小，导致混凝土宏观弹性模量随之降低；理论分析预测的混凝土有效弹性模量与有限元模拟结果吻合良好．应用含孔隙混凝土胞元模型能有效地预测混凝土宏观弹性模量，进而也为其在冻融作用下老化演变机理的研究评估提供了基础．     

A nonlinear homogenization procedure for periodic masonry

The present paper deals with the problem of the determination of the in-plane behavior of masonry material. The masonry is considered as a composite material composed by a regular distribution of blocks connected by horizontal and vertical mortar joints. The overall constitutive relationships of the regular masonry are derived by a rational micromechanical and homogenization procedure. Linear elastic constitutive relationship is considered for the blocks, while a new special nonlinear constitutive law is proposed for the mortar joints. In particular, a mortar constitutive law, which accounts for the coupling of the damage and friction phenomena occurring during the loading history, is proposed; the developed model is based on an original micromechanical analysis of the damage process of the mortar joint. Then, an effective nonlinear homogenization procedure, representing the main novelty of the paper, is proposed; it is based on the transformation field analysis, using the technique of the superposition of the effects and the finite element method. The presented methodology is implemented in a numerical code. Finally, numerical applications are performed in order to assess the performances of the proposed procedure in reproducing the mechanical behavior of masonry material.     

非饱和正冻土的三场耦合理论框架

在``饱和正冻土的理论构架'的基础上，假定了冻土体内的空气的流动是一 种Darcy流，进一步考虑了非饱和土中气相的影响，提议了一种综合考虑两种极端 情况：(1) 气体与外界完全相通. (2) 气体与外界完全不相通的相关水、热、力相 耦合的多孔多相介质理论构架------半连通、半封闭非饱和正冻土理论构架. 重点讨论了较简单的气体半连通半封闭的非饱和孔隙冻土体的三场耦合模型. 为 检验三场耦合分析模型及所开发的针对冻土工程三场耦合模型的软件系统3G2001 的合理性与正确性, 与214国道花石峡 试验路基实测的地温变化和路基路面变形 进行了对比验证, 对比结果显示: 路基中分析所得的温度场与实测值变化规律一 致, 量化相差大都在10\%$\sim$20\%以内; 分析所得耦合变形随时间的变化也与 实测值完全一致, 路中的分析变形与实测相差也在10%-20%以内     

An elastoplastic multi-level damage model for ductile matrix composites considering evolutionary weakened interface

An elastoplastic multi-level damage model considering evolutionary weakened interface is developed in this work to predict the effective elastoplastic behavior and multi-level damage evolution in particle reinforced ductile matrix composites (PRDMCs). The elastoplastic multi-level damage model is micromechanically derived on the basis of the ensemble-volume averaging procedure and the first-order effects of eigenstrains. The Eshelby’s tensor for an ellipsoidal inclusion with slightly weakened interface [Qu, J., 1993a. Eshelby tensor for an elastic inclusion with slightly weakened interfaces. Journal of Applied Mechanics 60 (4), 1048–1050; Qu, J., 1993b. The effect of slightly weakened interfaces on the overall elastic properties of composite materials. Mechanics of Materials, 14, 269–281] is adopted to model particles having mildly or severely weakened interface, and a multi-level damage model [Lee, H.K., Pyo, S.H., in press. Multi-level modeling of effective elastic behavior and progressive weakened interface in particulate composites. Composites Science and Technology] in accordance with the Weibull’s probabilistic function is employed to describe the sequential, progressive weakened interface in the composites. Numerical examples corresponding to uniaxial, biaxial and triaxial tension loadings are solved to illustrate the potential of the proposed micromechanical framework. A series of parametric analysis are carried out to investigate the influence of model parameters on the progression of weakened interface in the composites. Furthermore, the present prediction is compared with available experimental data in the literature to verify the proposed elastoplastic multi-level damage model.     

Application of the asymptotic homogenization method to find the expansion coefficient of a water-saturated porous medium during freezing processes

An approach is proposed to determine the effective relative expansion coefficient of a porous medium filled with water during its freezing. This approach is based on an asymptotic homogenization method. An explicit formula is derived to find the expansion coefficient in the case of open pores. In the case of closed pores, the expansion coefficient is a second-rank tensor. Its determination requires to solve the so-called local problems in a representative volume element. The proposed approach can be used to determine the effective expansion coefficient during the freezing of water in the soil. Its efficiency is confirmed for model and realistic geological structures.     

A stochastic homogenization approach to estimate bone elastic properties

The mechanical properties of bone tissue depend on its hierarchical structure spanning many length scales, from the organ down to the nanoscale. Multiscale models allow estimating bone mechanical properties at the macroscale based on information on bone organization and composition at the lower scales. However, the reliability of these estimates can be questioned in view of the many uncertainties affecting the information which they are based on. In this paper, a new methodology is proposed, coupling probabilistic modeling and micromechanical homogenization to estimate the material properties of bone while taking into account the uncertainties on the bone micro- and nanostructure. Elastic coefficients of bone solid matrix are computed using a three-scale micromechanical homogenization method. A probabilistic model of the uncertain parameters allows propagating the uncertainties affecting their actual values into the estimated material properties of bone. The probability density functions of the random variables are constructed using the Maximum Entropy principle. Numerical simulations are used to show the relevance of this approach.     

Numerical modeling of carbon/carbon composites with nanotextured matrix and 3D pores of irregular shapes

A multiscale modeling approach is utilized to evaluate the contribution of irregularly shaped three-dimensional pores to the overall elastic properties of carbon/carbon composites. The degree of anisotropy of a carbon matrix depends on nanotexture, which is defined by manufacturing conditions. Elastic properties of the matrix are predicted assuming a Fisher distribution of orientations of graphene planes with respect to the pyrolytic carbon deposition direction. X-ray computed microtomography is employed to identify pores in a sample of carbon/carbon composite. The pores have highly irregular shapes so that micromechanical modeling based on the analytical solutions of elasticity becomes inapplicable. Thus, the cavity compliance tensor of an individual pore is found numerically by finite element method, and then used in a micromechanical modeling procedure. Examples of pores in isotropic and transversely isotropic pyrolytic carbon matrices are considered. The accuracy of pore approximation by ellipsoidal shapes is evaluated.     

Saturated and unsaturated behaviour modelling of Meuse–Haute/Marne argillite

Poromechanical behaviour modelling of the Callovo–Oxfordian argillite under saturated and partially saturated conditions is proposed using the equivalent stress concept. In comparison with the previous works on this rock, the particular form of the yield criterion and the plastic flow potential proposed here help to better describe the rock behaviour in tension–stress paths. The evolution of the poroelastic parameters due to the induced cracks is also considered in a simple way. Due to its physical nature, different from classical soils, the partially saturated behaviour of this rock could not be correctly described by any of the pre-existing theories usually used for the partially saturated porous media (soils). Based upon experimental results on this rock and developing some ideas proposed by other authors, an extension of the saturated elastic–plastic model in unsaturated field is proposed. The key hypothesis of this extension is the evolution of the Biot’s coefficient as a function of the suction, justified by laboratory results and micromechanical analyses. The predictions of the model in saturated and partially saturated conditions are compared with laboratory results and a good general agreement is found.     

空心材料导热性能预测研究

本文建立了一种预测空心材料导热性的方法。研究了空心材料的导热性。用柱形空心材料分析了体分比和孔洞的排列方式对整体材料导热性的影响，用圆柱形、方柱形空心材料和含裂纹材料，分析了空心形状对材料导热性的影响。同他人的实验结果和某些现有的理论模型比较表明，本文方法是有效的。本文的结果能够很好地解释实验结果。     

Piezoresistive fiber-reinforced composites: A coupled nonlinear micromechanical–microelectrical modeling approach

Piezoresistive composites are materials that exhibit spatial and effective electrical resistivity changes as a result of local mechanical deformations in their constituents. These materials have a wide array of applications from non-destructive evaluation to sensor technology. We propose a new coupled nonlinear micromechanical-microelectrical modeling framework for periodic heterogeneous media. These proposed micro-models enable the prediction of the effective piezoresistive properties along with the corresponding spatial distributions of local mechanical–electrical fields, such as stress, strain, current densities, and electrical potentials. To this end, the high fidelity generalized method of cells (HFGMC), originally developed for micromechanical analysis of composites, is extended for the micro-electrical modeling in order to predict their spatial field distributions and effective electrical properties. In both cases, the local displacement vector and electrical potential are expanded using quadratic polynomials in each subvolume (subcell). The equilibrium and charge conservations are satisfied in an average volumetric fashion. In addition, the continuity and periodicity of the displacements, tractions, electrical potential, and current are satisfied at the subcell interfaces on an average basis. Next, a one way coupling is established between the nonlinear mechanical and electrical effects, whereby the mechanical deformations affect the electrical conductivity in the fiber and/or matrix constituents. Incremental and total formulations are used to arrive at the proper nonlinear solution of the governing equations. The micro-electrical HFGMC is first verified by comparing the stand-alone electrical solution predictions with the finite element method for different doubly periodic composites. Next, the coupled HFGMC is calibrated and experimentally verified in order to examine the effective piezoresistivity of different composites. These include conductive polymeric matrices doped with carbon nano-tubes or particles. One advantage of the proposed nonlinear coupled micro-models is its ability to predict the local and effective electro-mechanical behaviors of multi-phase periodic composites with different conductive phases.     

非饱和结构性黄土本构模型的研究

本文在饱和土扰动概念基础上,提出了可以考虑外力和增湿两个扰动因素对非饱和土结构性影响的耦合扰动变量。根据复合材料均匀化理论的思想建立了适用于非饱和结构性黄土的本构模型,并给出了耦合扰动变量的演化方程。通过将BBM模型和本文模型的计算结果分别与已有的实验结果进行对比说明,本文模型能够更好地反映非饱和原状黄土的力学特性。通过对结构性参数β、α进行分析说明,所给出的耦合扰动变量的演化方程能够较好地描述土结构性随变形劣化的规律。     

非饱和颗粒材料的多孔连续体有效压力与有效广义Biot应力

多孔连续体理论框架下的非饱和多孔介质广义有效压力定义和Bishop参数的定量表达式长期以来存在争议,这也影响了对与其直接相关联的非饱和多孔介质广义Biot有效应力的正确预测.基于随时间演变的离散固体颗粒-双联液桥-液膜体系描述的Voronoi胞元模型,利用由模型获得的非饱和颗粒材料表征元中水力-力学介观结构和响应信息,文章定义了低饱和度多孔介质局部材料点的有效内状态变量:非饱和多孔连续体的广义Biot有效应力和有效压力,导出了其表达式.所导出的有效压力公式表明,非饱和多孔连续体的有效压力张量为各向异性,它不仅对非饱和多孔连续体广义Biot有效应力张量的静水应力分量的影响呈各向异性,同时也对其剪切应力分量有影响.文章表明,非饱和多孔连续体中提出的广义Biot理论和双变量理论的基本缺陷在于它们均假定反映非混和两相孔隙流体对固相骨架水力-力学效应的有效压力张量为各向同性.此外,为定义各向同性有效压力张量和作为加权系数而引入的Bishop参数并不包含对非饱和多孔连续体中局部材料点水力-力学响应具有十分重要效应的基质吸力.所导出的非饱和多孔介质广义Biot有效应力和有效压力公式(包括反映有效压力...     

A homogenized microstructural approach to creep fracture

A two-dimensional nonlocal continuum model is proposed in this paper for creep damage in polycrystalline materials. Starting from previous micromechanical modeling, a heuristic homogenization approach is adopted to derive a theory for the macroscopic response. The model accounts for the main damage mechanisms (grain boundary sliding, nucleation, growth and coalescence of cavities along the grain boundaries) responsible for the creep fracture process. The resulting constitutive law takes into account the nonlocalities expressed through the gradients of the stresses and the damage variables.