Fatigue Behaviour of Al‐Matrix Composites

The cyclic deformation behaviour and fatigue lives of different Al‐matrix composites reinforced by alumina reinforcements were investigated under total strain control mode at room temperature. The composites differ in either matrix strength (peak aged vs. overaged AA6061 alloy and soft Al99.85, respectively) and/or type of reinforcement (particles, Saffil short fibres and Almax continuous fibres). The damage evolution was characterized by compliance experiments within individual stress‐strain hysteresis loops. The differences of the material behaviour are discussed with respect to matrix strength and reinforcement geometry.     

Interface stress relaxation in magnesium matrix composites studied by mechanical spectroscopy

Abstract

Mechanical spectroscopy has been used to study magnesium matrix composites reinforced either by long SiC fibres or randomly distributed Al₂O₃ (Saffil) short fibres. It is well known that, in metal matrix composites, thermal stresses can be built up at the interface due to the mismatch between the thermal expansion coefficients of matrix and reinforcements. In magnesium matrix composites, these thermal stresses are relaxed by dislocation motion in the matrix. This mechanism of thermal stress relaxation yields an extra transient component in the mechanical loss spectrum, which depends on the heating/cooling rate and disappears in isothermal condition in the behaviour of the shear modulus G with temperature has been observed during thermal cycling between 100 K and 500 K. The intensity of this phenomenondepends on the spatial distribution of the reinforcements in the matrix. In particular, composites reinforced with long fibres exhibit a more pronounced anomaly. This is interpreted by the modification of the interface strength when temperature is changed.     

Hierarchical composites: Analysis of damage evolution based on fiber bundle model

A computational model of multiscale composites is developed on the basis of the fiber bundle model with the hierarchical load sharing rule, and employed to study the effect of the microstructures of hierarchical composites on their damage resistance. Two types of hierarchical materials were considered: “hierarchical tree” (bundles-of-bundles of fibers) and self-similar particle and fiber reinforced composite (in which reinforcements at each scale level represents composites in turn consisting of lower level reinforcements and matrix). For the case of the hierarchical tree (“bundle-of-bundles” material), it was observed that the increase in the amount of hierarchy levels leads to the lower strength of material. In the self-similar fiber reinforced matrix materials, as differed from the hierarchical trees, the damage resistance of the hierarchical materials increases with increasing the amount of hierarchy levels. The effect of mixed fiber and particle reinforcement on the damage resistance of the hierarchical composites is investigated as well.     

不同TiB2颗粒粒径的TiB2/Cu复合材料耐电弧侵蚀行为

采用放电等离子烧结法（SPS）制备了不同TiB₂颗粒粒径的3wt% TiB₂/Cu复合材料,研究了3wt% TiB₂/Cu复合材料致密度、导电率、硬度和耐电弧侵蚀性能随TiB₂颗粒粒径的变化规律,重点分析了不同TiB₂颗粒粒径的3wt% TiB₂/Cu复合材料耐电弧侵蚀行为。结果表明:3wt% TiB₂/Cu复合材料致密度和硬度随TiB₂颗粒粒径的增大而略有降低;TiB₂颗粒粒径越小,TiB₂/Cu复合材料的综合性能越好。随着TiB₂颗粒粒径的增大,3wt% TiB₂/Cu复合材料耐蚀稳定性降低,3wt% TiB₂/Cu阴极材料的损耗量明显增加;当TiB₂颗粒粒径为10 μm时,3wt% TiB₂/Cu复合材料的耐电弧侵蚀性能最佳。电弧蚀形貌观察表明:不同TiB₂颗粒粒径的3wt% TiB₂/Cu复合材料经电弧侵蚀后,3wt% TiB₂/Cu复合材料均由阴极向阳极发生转移;随着TiB₂颗粒粒径的增大,阴极质量损耗逐渐增加,触头表面电弧侵蚀面积增加;而在Cu基体中引入较小的TiB₂颗粒,有利于减弱电接触实验过程中TiB₂/Cu复合材料的喷溅现象。      

真空压渗铸造铝基电子封装复合材料研究

叙述了真空反压渗透铸造法制铝基电子封装复合材料的过程：测试了ＳｉＣｐ，镀铜碳短纤维，Ｐ１３０石墨短纤维，Ａｌ２Ｏ３短纤维，石墨磷片，石墨颗粒增强铝硅铸造合金复合材料的密度，孔积率和增强体的体积分数；给出了每种复合材料的金相照片，分析了不同增强体，预制件制造方法和混杂增强对铝基复合材料的体积分数，孔积率等方面的影响。     

碳纤维丝束结构对碳纤维/酚醛复合材料烧蚀性能的影响EI北大核心CSCD

以酚醛树脂为基体,以平纹碳布和短切碳纤维两种结构形式的碳纤维为增强剂,制备碳纤维增强的碳/酚醛复合材料。采用氧/乙炔烧蚀实验对复合材料的耐烧蚀性能进行了对比性研究,采用电子拉力试验机对复合材料的弯曲性能进行表征,采用扫描电镜对复合材料烧蚀形面进行观察,并通过固体火箭发动机对复合材料的烧蚀性能进行考核验证。研究结果表明:以这两种结构形式的碳纤维为增强剂制备的碳/酚醛复合材料,其氧乙炔质量烧蚀率的大小与碳纤维丝束的大小具有正相关的特性,碳纤维丝束越小碳纤维质量烧蚀率越低,当碳纤维增强剂处于单丝状态时,复合材料的氧乙炔质量烧蚀率达到最低为0.046 g/s,并且碳纤维的型号规格对复合材料氧乙炔质量烧蚀率的影响变小。固体火箭发动机实验表明,单丝状态下的碳纤维/酚醛复合材料的抗烧蚀冲刷性能明显优于束状碳纤维/酚醛复合材料。     

Effect of fiber arrangement on mechanical properties of short fiber reinforced composites

The present paper developed a three-dimensional (3D) “tension–shear chain” theoretical model to predict the mechanical properties of unidirectional short fiber reinforced composites, and especially to investigate the distribution effect of short fibers. The accuracy of its predictions on effective modulus, strength, failure strain and energy storage capacity of composites with different distributions of fibers are validated by simulations of finite element method (FEM). It is found that besides the volume fraction, shape, and orientation of the reinforcements, the distribution of fibers also plays a significant role in the mechanical properties of unidirectional composites. Two stiffness distribution factors and two strength distribution factors are identified to completely characterize this influence. It is also noted that stairwise staggering (including regular staggering), which is adopted by the nature, could achieve overall excellent performance. The proposed 3D tension–shear chain model may provide guidance to the design of short fiber reinforced composites.     

Influence of Thermal Strain on Thermal Properties of Composites

The temperature dependence of the thermal expansion characteristics of the AS21 magnesium alloy, AS21–25 vol% Saffil fiber composite and the Mg-10 vol% Saffil fiber composite has been measured in the temperature range from 20 to 380°C. The expansion characteristics were influenced by the residual and thermal strains. The residual strain was removed within the first thermal cycle. The relative elongation of the tensile pre-deformed AS21–25 vol% Saffil fiber composite is lower than that for the sample without deformation and for the composite pre-deformed in compression. The anisotropy of the thermal expansion is demonstrated on the Mg-10 vol% Saffil fiber composite. The coefficient of thermal expansion (CTE) in the transverse direction (TD) (the planes of the short Saffil fibers are laid perpendicular to the axis of the sample) is higher than the CTE in the longitudinal direction (LD) (the planes of the short Saffil fibers are laid parallel to the axis of the sample). The anisotropy and the thermal strain influence the thermal diffusivity for AS21 composites investigated at temperatures between 20 and 380°C.Paper presented at the Seventeenth European Conference on Thermophysical Properties, September 5-8, 2005, Bratislava, Slovak Republic.     

铸态(TiB+TiC)/Ti复合材料组织和性能的研究

利用钛与碳化硼及石墨之间的自蔓燃高温合成反应,经非自耗电弧熔炼工艺制备了TiB晶须和TiC粒子混杂增强的钛基复合材料。借助扫描电镜和透射电镜观察了复合材料的微观结构,结果表明:原位合成增强体均匀地分布在基体合金中,TiB增强体以晶须状生长,而TiC增强体以树枝状、等轴状生长。原位合成增强体与基体合金的界面非常干净,不存在界面反应产物,但TiC粒子周围的基体合金中存在高密度的位错。原位合成增强体的加入提高了复合材料的力学性能,合金化元素铝的加入不仅固溶强化了钛基体合金,同时使增强体长得更为细小,也有利于改善复合材料的性能。      

Tensile behaviour and ductility of 10 vol.-% Saffil short fibre reinforced aluminium

Abstract

The tensile behaviour of Saffil short fibre reinforced aluminium composites has been investigated in terms of Young's modulus, yield strength, ultimate tensile strength, and strain to failure. A rationale is given for the unusually high strain to failure found in this material by applying a failure criterion based on damage accumulation, recently developed for particulate reinforced MMCs.     

Erosive wear behaviour of aluminium based composites

Al-MMCs reinforced with short fibres or particles of ceramics such as alumina, titanium diboride and silicon carbide result in composites of high specific strength and stiffness, suitable for advanced engineering applications such as in the aerospace and automotive industries. This paper studies the erosion wear behaviour of Al-based composites reinforced with alumina-fibre and in-situ TiB₂ particles using a water/SiC particles slurry jet. From the results of our experiment, the erosion resistance of reinforced Al-MMCs depends on that of the Al-alloy and the reinforcing ceramics, as well as on the bonding strength between the matrix and ceramic fibres or particles. Some design strategies to enhance the erosion resistance of Al-MMCs reinforced with short fibres and particles have been discussed.     

Polymer derived ceramic matrix composites

Preceramic polymers offer a unique method to fabricate ceramic matrix composites (CMC). Relatively large and complex shapes were fabricated using a polysilazane polymer and silicon carbide based reinforcements of CG Nicalon™ and HI-nicalon™ fibers. This paper summarizes a raw material system and the fabrication process used to prepare two-dimensional cloth reinforced composites. Typical tensile, shear and compressive properties of CMCs prepared with the two types of reinforcements are presented. Although CG Nicalon reinforced composites exhibit good mechanical stability at moderate stress levels at 1100°C, HI-Nicalon reinforced composites show improved creep behavior at 1200°C.     

Discontinuously-reinforced aluminum matrix composites

This article reviews the literature relating to aluminum matrix composites reinforced by ceramic particles, short fibers or whiskers. The main reinforcements which have been used are presented with respect to their nature, morphology and mechanical behavior. The influence of matrix alloying elements on ceramic-metal compatibility is discussed. Most fabrication techniques which have been proposed are described and particular attention is paid to the methods suitable for large scale production. The mechanical characteristics of this kind of composite are reported and compared in relation to processing parameters. Finally, models giving a rough prediction of the mechanical behavior of the composites are discussed.     

Influence of coating on short steel fiber reinforcements on corrosion behavior of aluminium base short steel fiber reinforced composites

Steel fiber reinforced aluminium composites are attractive materials of high specific strength but exhibit poor resistance against electrochemical corrosion. The study discusses the electrochemical corrosion behavior of uncoated, copper and nickel coated short steel fiber reinforced aluminium and Al–2Mg matrix composites in 1 (N) NaCl solution. Galvanic corrosion between the steel fiber and aluminium governs the corrosion behavior of these composites. It has been observed that open circuit potential (OCP) is shifted to more negative side with copper coating on the fibers and to the more positive side on coating the fibers with nickel. Compared to the uncoated fiber higher corrosion current density indicates corrosion rate was observed for the copper coated fiber reinforced composites where as a lower current density was noted for the nickel coated fiber reinforced composites was observed. Addition of 2 wt% magnesium to aluminium alloy matrix increased the corrosion current density. The corrosion mechanism in these composites is dominated by galvanic cell formation that is evident from the dissolution of Al matrix near the peripheral region of steel fibers.     

碳纤维增强尼龙1010的力学性能及其对摩擦磨损的影响

用碳纤维填充尼龙1010制备了碳纤维增强尼龙复合材料,并对碳纤维增强尼龙复合材料的力学性能和摩擦学性能进行了实验研究。力学实验结果表明:碳纤维增强使尼龙复合材料的拉伸强度、表面硬度增大,碳纤维增强尼龙材料的拉伸强度在20%碳纤维含量时达到最大值;碳纤维表面处理对尼龙复合材料的拉伸强度有很大影响,碳纤维表面氧化处理提高了碳纤维增强尼龙复合材料的拉伸强度。摩擦磨损实验表明:碳纤维增强尼龙复合材料的摩擦系数和磨损率与其拉伸强度和硬度有密切关系。随着拉伸强度和硬度的提高,尼龙复合材料摩擦系数和磨损率降低;摩擦系数和磨损率与拉伸强度具有反比关系,与材料硬度具有二次方程关系,与碳纤维填充量之间存在负指数变化规律。      

Potential and challenges of metal-matrix-composites reinforced with carbon nanofibers and carbon nanotubes

With a continuous improvement of the production techniques for carbon nanofibers and carbon nanotubes along with an improvement of the available qualities of the materials, these reinforcements have been introduced into polymers, ceramics and metals. While in the field of polymers first success stories have been published on carbon nanofiller reinforcements, up to now metals containing these types of nanofillers are still a topic of intensive research. Basically a similar situation were found in those days, when micron sized carbon fibers came on the market. Today many applications of carbon fiber reinforced composites are existing, while metals reinforced with conventional carbon fibers are still only found in niche applications.     

Erosion wear response of ramie-epoxy composites using artificial neural network integrated with Taguchi technique

This paper aims on evaluating the erosion wear behavior of epoxy composites reinforced with ramie fibers. The possibility of reinforcing ramie fiber to improvise the wear resistance of epoxy is investigated in this study. Composites are fabricated by reinforcing multiple layers of woven ramie fiber mats into epoxy resin using conventional wet lay-up technique and erosion wear trials are conducted using solid particle erosion test setup. Taguchi analysis is done to assess the relative significance of each of the factors influencing the erosion rate using L₁₆ orthogonal array. The analysis reveals that the impact velocity followed by impingement angle are the most significant control factors affecting the erosion wear rate of ramie-epoxy composites. Steady state erosion analysis is done to ascertain the effect of each of the significant factors while keeping other factors fixed. Further, an analytical and predictive model based on the principle of neural computation is used to predict the rate of erosion wear of the composites and the obtained results are compared with the experimental outcomes. The worn morphologies of the eroded surfaces of the composites are studied and analyzed to identify possible mechanisms causing wear.     

Effect of TiB2 particle size on the material transfer behaviour of Cu–TiB2 composites

Cu-5.8vol.-%TiB₂ composites reinforced with different sized TiB₂ particles were prepared by spark plasma sintering (SPS). The arc erosion resistance was examined using a JF04C electrical contact instrument. The results indicate that the TiB₂ particle size and contact currents have important effects on the material transfer mode. Both lower mass loss and relative mass transfer was observed in the Cu-5.8vol.-%TiB₂ composites reinforced with fine TiB₂ particles (10?um, 30?um) compared to those with coarse TiB₂ particles (70?um, 110?um). The investigation and analysis of the microstructure and arc erosion mechanism show uniform distribution of fine TiB₂ particles in the copper matrix can significantly improve the viscosity of the molten pool and decrease the splashing of molten Cu.     

Continuous carbon nanotube reinforced composites

Carbon nanotubes are considered short fibers, and polymer composites with nanotube fillers are always analogues of random, short fiber composites. The real structural carbon fiber composites, on the other hand, always contain carbon fiber reinforcements where fibers run continuously through the composite matrix. With the recent optimization in aligned nanotube growth, samples of nanotubes in macroscopic lengths have become available, and this allows the creation of composites that are similar to the continuous fiber composites with individual nanotubes running continuously through the composite body. This allows the proper utilization of the extreme high modulus and strength predicted for nanotubes in structural composites. Here, we fabricate such continuous nanotube polymer composites with continuous nanotube reinforcements and report that under compressive loadings, the nanotube composites can generate more than an order of magnitude improvement in the longitudinal modulus (up to 3,300%) as well as damping capability (up to 2,100%). It is also observed that composites with a random distribution of nanotubes of same length and similar filler fraction provide three times less effective reinforcement in composites.     

短切炭纤维增强沥青基C/C复合材料的组织特征

利用新型、高效的模压半炭化成型工艺，在大气环境下制备出了短切炭纤维增强沥青基C／C复合材料制品，并借助光学显做镜和扫描电镜对其微观组织和断口形貌进行了观察。通过分析，解释了短切炭纤维增强沥青基C／C复合材料中炭纤维损伤的形成机制，提出了作为增强体相的短切炭纤维和焦炭颗粒与基体炭之间独特的界而结构模型。研究还表明：复合材料中明显存在着基体相和颗粒相一基体相的显微结构不仅呈层片状，而且层片状的结构好像数层桔子皮，将颗粒相包裹起来，这种“桔皮包裹”式的结构与炭纤维表面的POG结构基本相似。