二维编织 C/ SiC复合材料的热膨胀系数预测

根据二维编织 C/ SiC复合材料的细观结构及其制备工艺特点 , 提出了一种预测该材料面内热膨胀系数的单胞模型。模型充分考虑了编织结构复合材料中的纤维束弯曲和 CVI工艺制备陶瓷基复合材料产生的孔洞对热膨胀系数的影响。利用单胞模型预测了二维编织 C/ SiC的结构参数、 纤维体积含量、 孔洞含量对复合材料热膨胀系数的影响规律 , 结果表明 : 随着纤维束扭结处产生间隙与纱线宽度比值的增大 , 热膨胀系数增大 ; 当其它参数不变时 , 随着纤维体积含量的增大 , 热膨胀系数反而下降; 随着孔洞含量的增加 , 热膨胀系数也出现了下降的趋势。利用 DIL402C热膨胀仪测试了二维编织 C/ SiC复合材料纵向热膨胀系数 , 试验结果与模型预测结果吻合较好。     

复合材料热膨胀的一个理论模型

热膨胀是复合材料一个重要性能,从其组元的热膨胀性能来预测复合材料的热膨胀性能是材料设计等方面所必要的。在分析单向复合材料热膨胀性能的基础上,提出了一种计算混向短纤维复合材料线热膨胀系数的模型,并用碳纤维/钢复合材料的热膨胀系数的实验结果进行了验证。     

空心颗粒填充树脂复合泡沫材料热膨胀性能数值模拟

利用RSA(random sequential adsorption)方法生成了含不同体积分数和不同壁厚的空心玻璃微珠(HGMs)填充树脂(HGMs/树脂)复合材料代表体元模型, 用有限元软件ANSYS计算了该材料的热膨胀性能, 得到了材料热膨胀系数与HGMs体积分数和壁厚的变化关系以及温度变化产生的应力场和位移场分布情况。分别对HGMs/树脂热膨胀系数与HGMs体积分数和壁厚变化曲线进行线性拟合和指数函数拟合, 分析了HGMs体积分数和壁厚对HGMs/树脂热膨胀系数的灵敏度。结果发现, HGMs体积分数与壁厚的增大都会降低HGMs/树脂的热膨胀系数; 填充体积分数对材料热膨胀系数的灵敏度大于壁厚的灵敏度。     

空心颗粒填充树脂复合泡沫材料热膨胀性能数值模拟

利用RSA (random sequential adsorption)方法生成了含不同体积分数和不同壁厚的空心玻璃微珠(HGMs)填充树脂(HGMs/树脂)复合材料代表体元模型,用有限元软件ANSYS计算了该材料的热膨胀性能,得到了材料热膨胀系数与HGMs体积分数和壁厚的变化关系以及温度变化产生的应力场和位移场分布情况.分别对HGMs/树脂热膨胀系数与HGMs体积分数和壁厚变化曲线进行线性拟合和指数函数拟合,分析了HGMs体积分数和壁厚对HGMs/树脂热膨胀系数的灵敏度.结果发现,HGMs体积分数与壁厚的增大都会降低HGMs/树脂的热膨胀系数;填充体积分数对材料热膨胀系数的灵敏度大于壁厚的灵敏度.     

一种新的β-锂霞石增强铜基复合材料热性能

采用热压烧结工艺成功制备了一种新的β-锂霞石增强铜基复合材料.利用扫描电镜和透射电镜对复合材料的微观组织进行了分析,并对不同体积分数复合材料的致密性,热膨胀性能和热传导性能进行了测试.结果表明:β-锂霞石颗粒在铜基体中分布均匀,界面清晰,不发生界面反应;体积分数对复合材料致密性、热膨胀系数和热导率有明显影响,当β-锂霞石颗粒体积分数超过40%时,复合材料的致密性有明显下降,热膨胀系数在(9～15.4)×10-6/K,同时热导率在50～170W/m·K.     

含锂霞石颗粒和硼酸铝晶须的铝基复合材料

为了制备具有低热膨胀系数和较高强度的复合材料，选用具有负体膨胀系数的β-锂霞石颗粒和高强度的硼酸铝晶须作为复合材料组分，用挤压铸造法制备了6061铝基复合材料，并对该复合材料的显微结构、拉伸性能和热膨胀性能进行了研究，结果表明，该复合材料能同时获得较低的热膨胀系数和较高的强度；通过改变复合材料中β-锂霞石和硼酸铝的体积分数，在较大的范围内可以对复合材料的强度和热膨胀系数进行设计和调节。     

增强体含量对Sip/LD11复合材料热物理性能影响

为研究增强体含量对电子封装用Si颗粒增强铝基复合材料热物理性能的影响,采用挤压铸造法制备了以高纯Si粉为增强体,LD11铝合金为基体,体积分数分别为55%、60%和65%的3种复合材料.利用金相显微镜、热膨胀分析仪、热导率测试仪等多种手段对复合材料的微观组织及热物理性能进行了研究,并对试验结果进行数值模拟.显微组织观察表明,复合材料的铸态组织均匀、致密.通过改变复合材料增强体的含量,复合材料的热膨胀系数介于(8.1～12)×10-6/℃之间可调,热导率大于87.7 W/m·℃,满足电子封装用材料的要求.Sip/LD11复合材料的热膨胀系数介于Rom模型和Turner模型之间,Kerner模型能够更好地预测Sip/LD11复合材料的热膨胀系数.热导率计算结果均大于测试值.     

编织结构复合材料热膨胀特性的实验研究

对二维和三维碳纤维/环氧树脂编织结构复合材料的热膨胀特性进行实验研究, 确定编织复合材料热膨胀系数的各向异性分布特征, 给出其热膨胀特性与纤维编织模式和纤维体积含量之间的相互依赖关系, 分析编织复合材料的热膨胀机理, 表明其热膨胀系数的可设计性, 为设计复合材料零膨胀结构提供实验依据和理论分析。     

考虑界面层和孔隙的SiCf/SiCm复合材料热膨胀性能研究

以三维四向编织SiCf/SiCm复合材料为对象,建立基于周期性边界条件、包含界面层和孔隙的复合材料单胞有限元模型,模型细观结构与工业CT扫描结果一致。计算了材料各个方向的热膨胀系数,发现界面层对纤维束热膨胀系数的影响不可忽略,基体孔隙位置的随机分布对热膨胀系数计算结果没有影响,孔隙率的增加会引起系数的显著减小,对胞元的热应力分析表明纤维束上的热应力水平大于基体。通过自由膨胀加温试验对材料纵向热膨胀系数进行了测定,在室温至1100℃区间内热膨胀性能稳定,试验结果与预测值符合较好。可为含界面层和基体孔隙的三维编织复合材料及其他多孔复合材料热膨胀性能研究提供理论基础。     

碳纤维随机填充橡胶复合材料导热性能的数值模拟

运用随机顺序添加算法RSA(Random sequential addition method),基于均匀化理论建立了碳纤维填充橡胶复合材料代表体积单元RVE(Representative volume element)模型,利用有限元方法数值模拟研究了碳纤维对橡胶复合材料导热性能的影响。结果表明:在相同的填充分数下,碳纤维根数对复合材料导热性能的影响较小;合理安排碳纤维空间分布及纤维取向能有效提高复合材料的导热性能;复合材料的导热性随着碳纤维填料含量及长径比的增加而增大;与理论模型相比,基于碳纤维填料随机分布模型所得模拟结果与实验值较接近,尤其在高填充分数时与实验值吻合较好,可以更好地预测纤维填料填充复合材料的导热性能,对制备具有高填充分数的高导热复合材料具有一定的指导意义。     

The transverse coefficient of thermal expansion of a unidirectional composite

Various analytical models of the effective thermal expansion coefficients of unidirectional fibre-reinforced composite materials predict for certain fibre-matrix combinations an increase in the transverse coefficient of thermal expansion over that of its constituents at low fibre volume content. This effect is especially noticeable if the composite is fabricated with fibres of high modulus and low thermal expansion coefficient in matrices of low modulus and high thermal expansion coefficient. An experimental investigation was therefore conducted to study this behaviour in Textron fibre (SCS-6)-reinforced Hercules 3501 -6 epoxy matrix. Numerical calculations for this material system have shown that increases of the order of 20% over the matrix expansion coefficient is possible for fibre volume fraction in the range 3%–4%. Experimental measurements of the effective thermal expansion coefficients are seen to be in favourable agreement with the theoretical predictions. A parametric study is also undertaken to examine the influence of constituent properties on the effective composite behaviour. It is shown that the axial restraint of the fibre is responsible for a peak in the behaviour of the transverse expansion coefficient.     

Study of variation of thermal expansion coefficients in carbon/epoxy laminated composite plates

Thermal expansion is an important property to define the end use application, and to model residual stresses and related problems in a composite part. Thus its accurate determination is essential. In laminated composites, this parameter depends largely on the orientation of fibres, fibre fraction, type of resin and reinforcement, etc. In this article, numerical results on the coefficients of thermal expansion (CTE) at different angles in the composite plates (having different stacking sequences of plies) found using finite element analysis are presented. Experimental values of these coefficients for [0/90] are found in satisfactory agreement with the simulations. Finally, a mathematical model is proposed for modelling these coefficients.     

A combined experimental–numerical approach for generating statistically equivalent fibre distributions for high strength laminated composite materials

A technique is presented where actual experimental distributions, measured from a high strength carbon fibre composite, are considered in the development of a novel method to generate statistically equivalent fibre distributions for high volume fraction composites. The approach uses an adjusted measure of nearest neighbour distribution functions to define inter-fibre distances. The statistical distributions, characterising the resulting fibre arrangements, were found to be equivalent to those in the actual microstructure. Finite element models were generated and used to determine the effective elastic properties of the composite and excellent agreement was obtained. The algorithm developed is simple, robust, highly efficient and capable of reproducing actual fibre distributions for high strength laminated composite materials. It does not require further heuristic steps, such as those seen in fibre stirring/shaking algorithms, in order to achieve high volume fraction microstructures and provides a useful alternative to both microstructure reproduction and random numerical models.     

Thermal expansivities in fibrous composites incorporating hybrid interphase regions

The aim of this paper is the prediction of coefficients of thermal expansion in unidirectional fiber-reinforced composites. The representative volume element is a three phase composite structure subjected to a uniform temperature change. The advanced hybrid interphase concept is introduced, in which the interphase thickness depends on the property under consideration. This model involves also imperfect adhesion by immediate softening of material properties. Equations for the prediction of coefficients of thermal expansion are presented. Results are illustrated and discussed in terms of fiber volume fraction and adhesion coefficient. To validate the accuracy of these results finite element analysis has also been utilized. Predictions of coefficients of thermal expansion are in good agreement with experimental, finite element analysis and previous published results. The coefficients of thermal expansion of the considered polymer matrix composites are affected significantly by the parameters characterizing interphase.     

Film stacking impregnation model for a novel net shape thermoplastic composite preforming process

An impregnation model has been developed to evaluate the infiltration phenomena that occur during a novel near net shape preforming method. This process comprises automated deposition of thermoplastic resin and unidirectional (UD) carbon fibres to a pre-programmed stacking sequence, thereby forming tailored preforms for subsequent stamp-forming. Infiltration kinetics have been simulated to study the effect of different stacking scenarios, materials, and pre-consolidation routes on the novel preforming process. Isothermal infiltration of a liquid thermoplastic polymer into a compressed UD fibre bed has been examined and the experimental results have been used to validate an infiltration model based on local fluid flow in compressible porous media. This enables simulation of infiltration in alternating matrix film and fibre layers, relating pressure, time, and temperature with the local fibre volume fraction, pressure, and liquid and solid velocities in the stacked material. For a given set of processing conditions, the model fibre volume fraction distribution prediction enables the optimum matrix stacking layer thicknesses to be determined. It was shown that infiltration is inhibited above a limiting pressure which leads to increased fibre bed compaction and hence decreased permeability.     

Statistical analysis and stochastic modelling of fibre composites

The fibre component of many fibre reinforced composites can be modelled by a system of non-overlapping straight cylinders. In this paper we discuss a model based on a random sequential adsorption (RSA) process. Geometric characteristics of the fibre system such as the fibre volume fraction or the fibre direction distribution are estimated from tomographic images of composite samples. Using this information, we fit RSA models to samples of a glass fibre reinforced polymer and a fibre reinforced ultra high performance concrete.     

Fracture mechanisms and failure analysis of carbon fibre/toughened epoxy composites subjected to compressive loading

This study investigates the failure mechanisms of unidirectional (UD) HTS40/977-2 toughened resin composites subjected to longitudinal compressive loading. A possible sequence of failure initiation and propagation was proposed based on SEM and optical microscopy observations of failed specimens. The micrographs revealed that the misaligned fibres failed in two points upon reaching maximum micro-bending deformation and two planes of fracture were created to form a kink band. Therefore, fibre microbuckling and fibre kinking models were implemented to predict the compressive strength of UD HTS40/977-2 composite laminate. The analysis identified several parameters that were responsible for the microbuckling and kinking failure mechanisms. The effects of these parameters on the compressive strength of the UD HTS40/977-2 composite systems were discussed. The predicted compressive strength using a newly developed combined modes model showed a very good agreement to the measured value.     

Effects of the piezoelectric phase’s geometric properties on effective coefficients of 1–3 piezoelectric composites

The present paper reports the electromechanical coupling coefficients of piezoelectric composite material (PCM) are affected by different geometric properties of piezoelectric phase for 1–3 periodic composites that is made of piezoceramic fibers embedded in a soft non-piezoelectric matrix. Three-dimensional finite element model has been developed to study the three types of geometric models of piezoelectric phase with different volume fraction. Geometric models with circular cylinder, square column and circular cylinder alternated with square column are used to predict the coefficients of the validity via asymptotic homogenization method (AHM) and the numerical approach the finite element method (FEM). Three types of geometric model are built via the finite element software ABAQUS, and the elastic, piezoelectric and dielectric coefficients are evaluated via AHM both FEM. The results indicate that the validity parameters of PCM have the direct relationship with the volume fraction, and geometric shape is essential factor for distribution of Von-Misses when device working. The present work may improve application of 1–3 type PCM and offer useful guidelines to the design of PCM devices.     

Aspects of residual thermal stresses in continuous-fibre-reinforced ceramic matrix composites

An analytical model is presented that predicts the thermal stresses which arise from mismatch in coefficients of thermal expansion between a fibre and the surrounding matrix in a continuous fibre composite. The model consists of two coaxial isotropic cylinders. Stress transfer between the fibre and the matrix near an unstressed free surface has been modelled by means of a shear-lag analysis. Away from the free surface the theoretical approach satisfies exactly the conditions for equilibrium and continuity of stress at the fibre-matrix interface. Application of the model to a composite consisting of a glass-ceramic calcium alumino-silicate (CAS) matrix containing unidirectional Nicalon fibres points to a strong dependence of stress on fibre volume fraction. Surface effects are significant for depths of the order of one fibre diameter. Near-surface shear stresses resulting from cooling from the stress-free temperature are sufficiently high to suggest that a portion of fibre close to the surface is debonded at room temperature. Experimental results acquired with a scanning electron microscope (SEM) equipped with a heating stage are consistent with this prediction. Consequently, the model has been modified in a simple way to incorporate frictional slip at the interface, according to the Coulomb friction law. Although detailed measurements are limited by the resolution of the technique, experimental evidence suggests that the transfer length is within an order of magnitude of the model prediction.