Micromechanical analysis on tensile properties prediction of discontinuous randomized zalacca fibre/high-density polyethylene composites under critical fibre length

In this research, the tensile properties' performance of compression moulded discontinuous randomized zalacca fibre/high-density polyethylene under critical fibre length was analysed by means of experimental method and micromechanical models. These investigations were used to verify the tensile properties models toward the effect of fibre length and volume fraction on the composites. The experimental results showed that the tensile properties of composites had significantly increased due to the enhancement of fibre length. On the contrary, a decline in the tensile properties was observed with the increase of volume fraction. A comparison was made between the available experimental results and the performances of Tsai-Pagano, Christensen and Cox-Krechel models in their prediction of composites elastic modulus. The results showed that the consideration of fibre's elastic anisotropy in the Cox-Krenchel model had yielded a good prediction of the composites modulus, nevertheless the models could not accurately predict the composites modulus for fibre length study.     

复合材料动态粘弹性能的细观研究

利用细观力学的Eshelby等效夹杂方法研究了颗粒增强复合材料的动态粘弹性力学性能,分析了材料复模量随夹杂体积分数、载荷频率之间的变化规律,给出了许多有意义的结论,为复合材料结构的优化设计及应用提供了理论基础。     

Relationship between single and bulk mechanical properties for zeolite ZSM5 spray-dried particles

In this work typical mechanical properties for a catalyst support material, ZSM5 （a spray-dried granular zeolite）, have been measured in order to relate the bulk behaviour of the powder material to the single particle mechanical properties. Particle shape and size distribution of the powders, determined by laser diffraction and scanning electron microscopy （SEM）, confirmed the spherical shape of the spray-dried particles. The excellent flowability of the material was assessed by typical methods such as the Hausner ratio and the Cart index, This was confirmed by bulk measurements of the particle-particle internal friction parameter and flow function using a Schulze shear cell, which also illustrated the low compressibility of the material. Single particle compression was used to characterize single particle mechanical properties such as reduced elastic modulus and strength from Hertz contact mechanics theory. Comparison with surface properties obtained from nanoindentation suggests heterogeneity, the surface being harder than the core. In order to evaluate the relationship between single particle mechanical properties and bulk compression behaviour, uniaxial confined compression was carried out. It was determined that the Adams model was suitable for describing the bulk compression and furthermore that the Adams model parameter, apparent strength of single particles, was in good agreement with the single particle strength determined from single particle compression test.     

金属基纳米复合材料等效弹性模量的均匀化方法数值模拟

均匀化理论利用位移场双尺度渐近展开建立有限元列式，本文将其与有限元通用程序相结合，应用于金属基复合材料的弹性本构数值模拟。通过对不同尺度增强相金属基复合材料等效模量的数值模拟，考察了均匀化方法的适用情况。数值计算结果表明，对常规尺度增强相金属基复合材料，均匀化方法可以较准确地预测其等效弹性模量；对纳米增强相金属基复合材料，该方法仍可给出较好的预测，但存在某种程度的系统偏差。通过对纳米尺度增强机理的分析讨论，认为纳米增强相与基体材料问的界面效应可能有别于连续介质假设，指出可以考虑采用离散原子-连续介质耦合模型改进数值模拟结果。     

Friction in unconforming grain contacts as a mechanism for tensorial stress-strain hysteresis

Materials composed of consolidated grains and/or containing internal contacts are widespread in everyday life (e.g. rocks, geomaterials, concretes, slates, ceramics, composites, etc.). For any simulation of the elastic behavior of this class of solids, be it in seismology, in NDT, or in the modeling of building constructions, the stress-strain constitutive equations are indispensable. Since the most common loading patterns in nature considerably deviate from simple uniaxial compression, the problem of tensorial stress-strain representation arises. In simple loading cases it may be sufficient to use a phenomenological constitutive model. However, in a more general case, phenomenological approaches encounter serious difficulties due to the high number of unknown parameters and the complexity of the model itself. Simplification of the phenomenology can help only partly, since it may require artificial assumptions. For instance, is it enough just to link the volumetric stress to the volumetric strain, or do we have to include shear components as well, and if yes, in what form? We therefore propose a physical tensorial stress-strain model, based on the consideration of plane cracks with friction. To do this, we combine known relations for normal displacements of crack faces given by contact mechanics, the classical Amonton's law of dry friction for lateral displacements, and the equations of elasticity theory for a collection of non-interacting cracks with given orientation. The major advantages of this model consist in the full tensorial representation, the realistic stress-strain curves for uniaxial stress compression and quantitative comparison with experimental data, and a profound account for hysteretic memory effects.     

EFFECTIVE SPECIFIC HEATS OF SPHERICAL PARTICULATE COMPOSITES WITH INHOMOGENEOUS INTERPHASES

基于细观力学复合球模型研究了含非均匀界面相粒子填充复合材料的有效热弹性性质,重点讨论了界面相性质的径向分布对有效比热的影响. 首先,将非均匀界面相沿径向离散为多个同心球壳,每个球壳内的材料性质假设是均匀的. 基于上述离散模型,利用含界面相的复合球模型,推导了复合材料的有效体积模量、有效热膨胀系数及有效比热的数值求解表达式;进一步,假设界面相的性质沿径向连续变化,建立了一组微分方程,上述有效性质依赖于该微分方程组的解. 特别地,当界面相杨氏模量为幂次分布时,通过求解该微分方程组得到了有效比热等热弹性性质的解析解. 算例结果表明,应用此方法预测的有效热膨胀系数与实验结果吻合良好;界面相热膨胀系数的径向分布对有效比热和有效热膨胀系数均有显著的影响,而界面相弹性模量的径向分布对有效比热有显著的影响,对有效热膨胀系数的影响相对较小.     

含非均匀界面相粒子填充复合材料的有效比热

基于细观力学复合球模型研究了含非均匀界面相粒子填充复合材料的有效热弹性性质,重点讨论了界面相性质的径向分布对有效比热的影响. 首先,将非均匀界面相沿径向离散为多个同心球壳,每个球壳内的材料性质假设是均匀的. 基于上述离散模型,利用含界面相的复合球模型,推导了复合材料的有效体积模量、有效热膨胀系数及有效比热的数值求解表达式;进一步,假设界面相的性质沿径向连续变化,建立了一组微分方程,上述有效性质依赖于该微分方程组的解. 特别地,当界面相杨氏模量为幂次分布时,通过求解该微分方程组得到了有效比热等热弹性性质的解析解. 算例结果表明,应用此方法预测的有效热膨胀系数与实验结果吻合良好;界面相热膨胀系数的径向分布对有效比热和有效热膨胀系数均有显著的影响,而界面相弹性模量的径向分布对有效比热有显著的影响,对有效热膨胀系数的影响相对较小.      

319国道重庆段裸洞隧道灰岩力学特性试验研究

裸洞隧道在长期使用过程中,围岩遭受风化、地下水及区域地应力等不利因素的作用,导致岩体性质恶化,影响隧道的正常使用。在调研统计的基础上,选取319国道重庆至武隆段大溪河裸洞隧道为研究对象,对其表层及深层灰岩进行了室内单轴压缩、三轴压缩力学特性试验并对单轴压缩作用下的声发射特性进行了研究。结果表明:单轴压缩作用下两种岩样均出现脆性破坏特征,三轴压缩条件下,两种岩样呈压缩为主的延性扩容破坏,声发射可较好地表征岩石单轴压缩破坏过程;获得了两种岩样的基本力学参数,深层岩样强度为表层岩样强度的2.13倍,弹性模量、泊松比、内聚力、内摩擦角分别是表层岩样的1.04倍,0.63倍,1.45倍,1.37倍。研究结果可为该地区裸洞隧道的长期安全性与稳定性提供有益的理论支撑。     

界面特性对短纤维金属基复合材料蠕变行为的影响

基于短纤维增强金属基复合材料(MMC)的单纤维三维模型(三相)，利用粘弹性有限元分析方法对影响金属基复合材料的蠕变行为的因素进行了较为系统的分析。研究中主要讨论了界面特性和纤维取向角对金属基复合材料的蠕变性能的影响。研究结果发现，界面特性诸如厚度、模量和应力指数都对纤维最大轴应力和稳定蠕变率产生影响：稳态蠕变率随界面模量的增大而逐渐减小，当高于基体模量时基本保持不变；纤维轴应力的变化与蠕变率正好相反。稳态蠕变率随界面厚度、应力指数的增加而增大；而轴应力则随之减小。同时不同的纤维取向也影响金属基复合材料蠕变时的轴应力分布和稳态蠕变率。     

混杂短纤维增强复合材料弹性模量预测

在现代工程结构中,纤维增强复合材料具有较高的刚度重量比、优异的耐久性和设计灵活性等优点,因此得到了广泛应用．本文结合细观力学中的Mori-Tanaka方法和Halpin-Tsai方法推导了混杂碳纤维和玻璃纤维增强复合材料有效弹性模量的解析表达式．通过引入参数λ,提出了计算随机方向混合纤维增强复合材料弹性模量的新模型,分析了纤维长径比和体积分数对复合材料弹性模量的影响．结果表明,复合材料的弹性性能对纤维长径比和体积分数非常敏感．根据提出的理论,混杂纤维增强复合材料的弹性模量处于单一纤维（纯碳纤维或纯玻璃纤维）增强复合材料弹性模量之间．对于单一纤维增强复合材料,采用Halpin-Tsai方法计算的复合材料弹性模量高于Mori-Tanaka方法计算结果．     

基于近场动力学的单晶冰弹性各向异性数值模拟方法

天然冰材料在变形与破坏行为上的各向异性特征是冰与结构相互作用中产生复杂载荷过程的关键诱因, 而天然冰各向异性的根源则在于单晶冰的各向异性. 目前, 学术界针对单晶冰各向异性的数值模拟方法研究仍较为缺乏. 为了准确再现天然冰材料的特殊力学性质, 本文基于近场动力学理论, 提出了一种单晶冰弹性各向异性的数值模拟方法. 该方法的核心思想是将单晶冰杨氏模量沿不同加载方向的变化规律引入到近场动力学力密度向量的影响函数中. 以前人实验测试得到的杨氏模量值为参考, 通过开展与C轴呈0°, 45°和90°三个加载方向的单晶冰单轴压缩数值模拟实验, 提出了针对该影响函数的修正和辅助参数标定方法, 最终在15°, 30°, 60°和75°等其他四个加载方向进行了验证. 结果表明: 本文提出的针对影响函数的修正与参数标定方法, 能够较为便捷地找到数值模型杨氏模量与参考杨氏模量相一致的影响函数最优解, 即本文提出的基于影响函数的近场动力学数值模拟方法, 能够合理、准确地模拟单晶冰的弹性各向异性行为. 本文研究成果可为后续多晶冰各向异性数值模拟方法的建立提供基础性参考.      

不同拉压特性的双层厚壁圆筒极限承载力解答

采用统一强度理论并考虑材料拉伸与压缩弹性模量的差异性,建立均匀内压作用下双层厚壁圆筒的应力表达式,获得了其内压相应的弹性极限解答、塑性极限解答,并分析拉压强度比、拉压模量系数、统一强度理论参数、半径比及分层半径对弹性、塑性极限内压的影响规律．研究结果表明：弹性、塑性极限内压随拉压强度比的增加而减小,但随统一强度理论参数、半径比的增加而增大;弹性极限内压随分层半径的增加呈现先增大后减小变化,随拉压模量系数的增加而一直减小;塑性极限内压与拉压模量系数、分层半径无关．应用于实际工程时,可根据所得结果选择合理的壁厚及分层半径,再根据材料特性确定其他参数,以便更加准确地计算结构的受力状况．     

SGBEM modeling of fatigue crack growth in particulate composites

The symmetric-Galerkin boundary element method (SGBEM) has previously been employed to model 2-D crack growth in particulate composites under quasi-static loading conditions. In this paper, an initial attempt is made in extending the simulation technique to analyze the interaction between a growing crack and clusters of perfectly bonded particles in a brittle matrix under cyclic loading conditions. To this end, linear elastic fracture mechanics and no hysteresis are assumed. Of particular interest is the role clusters of inclusions play on the fatigue life of particulate composites. The simulations employ a fatigue crack growth prediction tool based upon the SGBEM for multiregions, a modified quarter-point crack-tip element, the displacement correlation technique for evaluating stress intensity factors, a Paris law for fatigue crack growth rates, and the maximum principal stress criterion for crack-growth direction. The numerical results suggest that this fatigue crack growth prediction tool is as robust as the quasi-static crack growth prediction tool previously developed. The simulations also show a complex interplay between a propagating crack and an inclusion cluster of different densities when it comes to predicting the fatigue life of particulate composites with various volume fractions.     

Extension of a multi-surface plasticity model to two-phase materials

The work reported in this paper is part of the ongoing research on the development of suitable elastic–plastic constitutive models for multiphase materials. This paper is concerned with the application of an elastic–plastic constitutive model based on the Mróz-multi-surface kinematic hardening rule to particulate metal matrix composites (PMMCs). Details of the Mróz-based elastic–plastic constitutive model for PMMCs and its explicit implementation are presented to enhance the applicability of the model for a stress controlled simulation. Comparison between numerical predictions and experimental results is also presented for uniaxial loading and biaxial proportional and non-proportional loading paths. For the load paths tested, reasonable agreement is observed between the numerical and the experimental results.     

聚氨酯(PU)在力学性能测试中的摩擦效应与变形均匀性研究

作为防弹玻璃夹层材料,PU的动态力学性能一直受到学者们的关注。为准确表征其动态力学性能,本文采用ABAQUS有限元软件对不同摩擦系数下的单轴压缩试验进行数值仿真,分析试样加载端面的摩擦效应和几何尺寸对单轴压缩试验结果的影响;结合高速摄影技术(HSP)与数字图像相关技术(DIC)观测到试样在拉伸试验中的动态变形场和应变场,探讨标距段的应力均衡性;同时对PU材料在不同应变率下的单轴压缩、拉伸力学性能进行测试。结果表明:压缩试样的端面摩擦效应限制横向变形,影响了试样内部的受力分布,使得测量得到的应力值偏大;试样长径比越小,端面摩擦效应的影响越大;在单轴动态拉伸试验中,板状拉伸试样的标距段选取应当考虑两端倒角尺寸。通过测试PU的拉、压力学性能,发现材料具有显著的应变率敏感性。     

Elastic and Dielectric Properties of Active Ag/BaTiO<Subscript>3</Subscript> Composites

This study examines the elastic and dielectric properties of active composites consisting of barium titanate (BaTiO₃) and silver (Ag) constituents using experimental and numerical approaches. The elastic constants including Young’s modulus, shear modulus and Poisson’s ratio were measured by resonant ultrasound spectroscopy (RUS), a nondestructive dynamic technique, while a dielectric (impedance) spectroscopy was used to measure the relative permittivity and dielectric loss at different frequencies. The dielectric tests were also conducted at temperature ranges from ?50 to 200 °C where the two phase transformations of barium titanate at around 0 °C and 120 °C were examined. The experimental results in this study were compared to data available in the literature. In addition to the experimental work, a numerical method is also considered in order to study the effects of blending silver into barium titanate on the effective elastic and dielectric properties of the composite and the local field fluctuations. For this purpose, two micromechanics models describing the detailed composite microstructures were constructed. The first model is based on two dimensional (2D) images of realistic microstructures obtained by the scanning electronic microscopy (SEM), while the second model is based on randomly generated three-dimensional (3D) microstructures with spherical particles. The effects of loading direction, porosity, particle shape and dispersion were examined using the micromechanics models. Numerical predictions of the effective elastic and dielectric constants were compared to the experiment results.     

玻璃微珠增强超高分子量聚乙烯的耐磨耐热性能研究

采用热压成型工艺制备了偶联玻璃微珠填充的超高分子量聚乙烯(UHMWPE)复合材料,通过万能材料试验机、洛氏硬度计考察了玻璃微珠含量对复合材料力学性能、硬度的影响,对复合材料进行了X射线衍射分析(XRD)、差示扫描量热分析(DSC)和静态热机械分析(TMA),通过热变形、维卡温度测定仪考察了玻璃微珠含量对复合材料维卡软化温度的影响,通过高速环块磨损试验机和自制砂浆磨损水浴试验装置考察了玻璃微珠含量对复合材料耐摩擦磨损性能的影响,通过扫描电子显微镜观察复合材料磨损表面形貌并分析了其磨损机理.结果表明:偶联玻璃微珠在UHMWPE体系中起填充增强作用,能有效提高复合材料的硬度、维卡软化温度、熔点和玻璃化转变温度,从而降低材料的摩擦系数与磨耗.维卡软化温度最多能提高12.5%,而磨损质量最多能降低62%,玻璃微珠质量分数在5%~20%之间效果明显,材料的力学性能随玻璃微珠的含量增加而有所下降.     

A numerical approach to indentation technique to evaluate material properties of film-on-substrate systems

Spherical indentation approach (Lee et al., 2005, Lee et al., 2010) for the evaluation of bulk material properties is extended to that for elastic–plastic properties of film-on-substrate systems. Our interest focuses on single isotropic, metallic, and elastic–plastic film on a substrate, and we do not consider the size effects in plasticity behavior. We first determine the optimal data acquisition location, where the strain gradient is the least and the effect of friction is negligible. Dimensional analysis affords the mapping parameters as functions of normalized indentation variables. An efficient way is further introduced to reduce both the number of analyses and the regression order of mapping functions. The new numerical approach to the film indentation technique is then proposed by examining the finite element solutions at the optimal point. With the new approach, the values of elastic modulus, yield strength, and strain-hardening exponent of film materials are successfully obtained from the spherical indentation tests. We have shown that the effective property ranges such as indenter properties, substrate modulus, and E/E_s ratio can be extended without additional simulations and even loss of accuracy. For other ranges of variables or other properties, which are not dealt with in this study, this methodology is applicable through resetting FEA variables and finding proper normalized parameters.     

Finite and transversely isotropic elastic cylinders under compression with end constraint induced by friction

This paper derives an exact solution for the non-uniform stress and displacement fields within a finite, transversely isotropic, and linear elastic cylinder under compression with a kind of radial constraint induced by friction between the end surfaces of the cylinder and the loading platens. The main feature of the present work is the introduction of a general solution form for Lekhnitskii’s stress function such that the governing equation and all end and curved boundary conditions of the cylinder are satisfied exactly. Two different solutions were obtained corresponding to the real or complex characteristic roots of the governing equation, depending on the combination of the elastic material constants. The solution by Watanabe [Watanabe, S., 1996. Elastic analysis of axi-symmetric finite cylinder constrained radial displacement on the loading end. Structural Engineering/Earthquake Engineering JSCE 13, 175s–185s] for isotropic cylinders under compression test can be recovered as a special case. Our numerical results show that both the non-uniform stress distribution and the difference between the apparent and the true Young’s moduli of the cylinder are very sensitive to the anisotropy of Young’s moduli, Poisson’s ratios and shear moduli. A more distinct bulging shape of the cylinder is expected when anisotropy in shear modulus is strong, the cylinder is relatively short, and the end constraint is large. The bulging shape, however, does not depend strongly on anisotropy of either Poisson’s ratio or Young’s modulus.     

反平面剪切下电磁弹性纤维增强复合材料界面相效应研究

研究具有界面相电磁弹性纤维增强复合材料的反平面剪切问题,利用复变函数方法,获得了无穷域中带界面相纤维问题在远场力、电、磁多场作用下的闭合解,得到了复合材料内部各区域电磁弹性物理量的精确表达式.利用所得结果,考虑纤维和基体间的界面相效应,研究了界面相厚度及弹性模量对复合材料内部应力场、电场强度和磁场强度的影响,数值结果给出了复合材料电磁弹性物理量随界面相参数变化的规律,为该类复合材料的设计与计算提供了有价值的参考.