三维炭纤维预型体上烷烃气的催化炭沉积

研究了承载镍催化剂的三维(3D)炭纤维预型体,在含氢或无氢条件下,乙烷和合成天然气在其上面化学气相沉积形成的炭沉积率和碳纳米构结物.观察了炭沉积所获石墨纳米纤维、碳纳米管和石墨碳壳,得知沉积碳的纳米结构取决于沉积温度和碳源气组成.在650℃～800℃温间,总沉积时程直至12h,研究了烃类碳源气在氢气平衡下,从体积分数100%到20%变化情况.经Ni(NO3)2-6H2O甲醇溶液浸渍的3D炭纤维预型体在炭沉积前因溶液分解可还原出Ni.采用质量变化测定法、热谱-质谱分析仪、SEM、TEM和XRD对样品进行表征.结果表明:纤维状炭与囊包炭的比率随氢气含量的增加而增加,随反应温度的增高而减少.出乎意料的是,氢气的添加并不延长催化剂的寿命.该工艺过程对炭/炭复合材料的制备展示出良好的潜力.     

碳纤维增强聚苯硫醚复合材料激光加热原位成型过程中的温度场

连续纤维增强热塑性复合材料（Thermoplastic Composite,TPC）自动铺放（Automated Fiber Placement,AFP）可以实现铺层原位成型,因此在制造大型结构件、降低加工成本及提升生产效率方面潜力巨大。原位成型过程中铺层温度场分布对复合材料构件成型质量具有较大影响,且激光加热过程中又涉及激光能量场与预浸料吸收光能后产生的温度场之间相互耦联,机理复杂,因此结合传热模型,通过有限元模拟仿真研究激光辅助加热自动铺放成型连续碳纤维增强聚苯硫醚（CF/PPS）复合材料过程中铺层经历的温度历程。同时构建铺层温度场测量系统,对铺层经历的温度历程进行实时采集和存储。研究结果表明,铺放过程中黏合区域前方存在激光辐照阴影区,使压辊下方黏合区域的温度急剧下降;随着铺放速度的增加,黏合区域峰值温度逐渐降低,且成型速度越快,铺层间黏合区域峰值温度差越小,而热电偶测量结果与仿真结果相差越大;随着激光输出功率的增大,铺层峰值温度逐渐升高;为提高原位成型效率,当激光输出功率选择最大6kW时,最大铺放速度为0.75 m/s。通过对比,试验结果中的峰值温度与仿真模拟结果变化趋势相近,证明了有限元仿真模型的正确性。      

自动铺放工艺制备罐外固化复合材料的力学行为

针对碳纤维增强树脂基复合材料IM7/CYCOM5230-1罐外固化预浸料,研究了自动铺放（AFP）罐外固化（OOA）预浸料的制备过程并优化了铺放工艺参数,采用热分析手段研究了CYCOM5230-1树脂固化动力学及黏度特性,在此基础上开发了一种短时固化工艺,并评价了基于此工艺制备的OOA复合材料力学性能。结果表明,AFP铺放过程中预浸料间缝隙会影响OOA复合材料的成型质量,采用铺放压力为180 N、加热温度为50℃、铺放速率为0.20 m/s的铺放参数,可获得表面平整、成型质量优异的复合材料样件。热分析结果表明,罐外固化CYCOM5230-1树脂室温黏度大,满足OOA工艺中真空压实排气需求。短时固化工艺可达到与典型固化工艺相同水平固化度,提升了固化效率,且制备的复合材料可以达到59%的纤维体积分数及低于0.5%的孔隙率,其力学性能与典型固化工艺制备的复合材料相当,并且能够达到热压罐复合材料的水平。     

Stability of nickel coatings on carbon fiber preforms: A SEM investigation

Metal Matrix Composites (MMC's) reinforced with continuous fibers were generally fabricated by a foil-sandwich technique or by liquid metal infiltration. Liquid metal infiltration may be used to cast final shapes in molds containing fiber preforms. It is also used to make composite wire from which may be fabricated panels and shapes by hot-press diffusion bonding or pultrusion. The major drawback of this method is that the molten matrix must wet the fiber for successful infiltration to occur, requiring special fiber surface treatments or matrix additives, and that, molten metals generally dissolve or degrade the fibers, necessitating a barrier coating on the fibers. All these problems can be solved using carbon fibers coated with metallic layers, e.g. nickel. This work analyses an easy method to produce modified carbon fibers by electroplating and the process of its recristallization. The topography of the growth front of the deposit has been studied. At temperatures higher than about 300° C an annealing under vacuum is required, because of the high reactivity of metal coating, nevertheless the heat treatment of metal deposit produces always an embrittled material.     

A nonlinear compaction model for fibrous preforms

Based on the mechanics of porous media and physical insight gained from experimental observation, a model for predicting the nonlinear compaction of fibrous preforms in the resin transfer molding process is developed. A key physical constant — namely, preform bulk compressibility — is proposed to establish the relationship between the applied pressure and the preform bulk volume. The preform bulk compressibility is a function of fiber volume fraction and five parameters — the initial fiber volume fraction, the final (maximum attainable) fiber volume fraction, the initial pore volume compressibility, the fiber compressibility, and an empirical index. Results of compaction experiments on plain-woven fabric preforms and unidirectional non-woven materials support the validity of the model. Excellent agreement between theory and experiments has been obtained. The present model provides for fibrous preforms a nonlinear constitutive law whose coefficients can be physically interpreted.     

连续碳纤维/Mg复合材料异形薄板件预制体制备与成型

针对连续碳纤维增强镁基(Cf/Mg)复合材料异形薄板件预制体制备成本高、性能不稳定等问题,提出了"仿形编织-环向缠绕-铺放定位-缝合增强"的低成本组合制备工艺,设计开发了与之相配套的预制体制备装置。通过理论与实验研究,确定薄板与凸台预制体分体制备:薄板预制体采用径向编织、环向缠绕,仿形无纬布织物与环向纤维层针刺合成,其中纬纱引纬张力控制在1.7~2.3N;凸台预制体采用无纬布横向叠层,再与环状薄板预制体缠绕缝合而成。所制备预制体外形完整,形态良好,环向碳纤维分布均匀、与径向增强碳纤维呈规则排列。经液固挤压制备的Cf/Mg复合材料异形薄板件表明,凸台与薄板连接强度良好,为成形高质量Cf/Mg复合材料异形薄板件奠定了基础。     

Development of fibrous preforms for FRP pipe connections

This paper describes the experimental work carried on at the University of Minho concerning the design of weft-knitted fleecy fabrics for application in pipe connections produced with composite materials reinforced by 3D weft-knitted fabric preforms. The specifications of a T tube composite connexion have been established according to the information of the composites producer company involved in the project. The weft-knitted fleecy fabric has been optimized to perform a better mechanical performance, i.e., to increase the stiffness due to the use of straight fleece yarns. Special knitting techniques, developed by the authors, have been applied to produce 3D shaped performs to be impregnated by using RTM techniques. A special mould has been produced according to the required geometry. The results of the mechanical tests made on the final produced samples are presented, discussed and compared with those imposed in the initial specifications.     

热塑性复合材料自动铺放过程中红外加热技术研究

为保证热塑性预浸料树脂基体在自动铺放(automated fiber placement, AFP)过程中充分熔融,实现热塑性复合材料(thermoplastic composites,TPC)逐层"原位固结"成型,自动铺放成型过程中需精确控制预浸料的加热温度。针对自动铺放过程中铺放速率会在较大范围内变动的特点,本工作提出一种高速率响应的红外线辐射加热技术。通过对红外热源与铺层间能量传输过程的分析,提出红外加热过程中动态恒温控制方程,建立热源辐射强度与铺放速率之间的匹配关系。基于热塑性复合材料自动铺放实验平台,构建红外加热恒温控制系统,该系统采用前馈控制方式,根据动态恒温控制方程,制定相应控制策略,实现对预浸料加热过程中温度的精确控制。实验结果表明自动铺放过程中使用红外加热恒温控制系统满足变速工况下恒温加热要求,且铺放成型实验件的压缩强度及层间剪切强度均接近模压成型实验件。     

Fabrication of short carbon fiber preforms coated with pyrocarbon/SiC for liquid metal infiltration

A novel method of fabricating short carbon fiber preforms was proposed for liquid metal infiltration. The preforms were shaped by wet forming and strengthened by pyrocarbon (PyC). SiC layers were prepared on carbon fibers by the reaction of SiO and PyC at 1600 °C. X-ray Diffraction, Scanning Electron Microscopy, and Energy Dispersive X-ray Spectroscopy were applied in the characterization of the preforms. Gas pressure infiltration was done to demonstrate the feasibility of the preforms for the liquid metal infiltration. The microstructure analysis indicates that carbon fibers are uniformly distributed in the preforms, and fibers are coated with an inner layer of PyC and an outer layer of SiC. The infiltration experiment proves that the prepared preforms are feasible for liquid metal infiltration under low infiltration pressure and temperature.     

Radiation-induced absorption in high-purity silica fiber preforms

The effect of the preform fabrication procedure on the radiation-induced absorption in KU-1 and KS-4V high-purity silica glasses was investigated (these glasses are used in fiber preform fabrication via outside fluorosilicate glass deposition on substrate rods and in the rod-in-tube process). The results demonstrate that the deposition of a reflective cladding onto KU-1 rods drastically increases the radiation-induced UV absorption of the rods. In particular, the concentrations of radiation-induced nonbridging oxygen hole centers and E centers in gamma-irradiated KU-1 rods with a fluorosilicate cladding are an order of magnitude higher than those in rods that were not heat-treated in plasma torch flames. At the same time, the deposition of a reflective cladding on KS-4V rods has little effect on the concentrations of these radiation-induced color centers. The radiation-induced absorption in KU-1 rods depends on the deposition temperature and duration but is insensitive to the composition of the gas mixture in the torch (in particular, to the presence of fluorination agents) and the presence of a fluorosilicate cladding. It is shown that, in preforms fabricated by the rod-in-tube process, the net concentration of radiation-induced color centers in the substrate rod is lower compared to the outside plasma deposition of fluorosilicate glass on the rod.Translated from Neorganicheskie Materialy, Vol. 41, No. 3, 2005, pp. 377–384.Original Russian Text Copyright © 2005 by Zabezhailov, Tomashuk, Nikolin, Plotnichenko.This revised version was published online in April 2005 with a corrected cover date.This revised version was published online in April 2005 with a corrected cover date.     

热塑性复合材料自动铺放工艺参数分析与优化

热塑性复合材料自动铺放原位成型（AFP）技术是高效低成本制造大型复合材料构件的关键,而自动铺放过程中工艺参数的选取及控制精度对成型构件的性能有较大影响,因此为保证原位成型后成型构件的性能,需分析自动铺放工艺参数对成型构件性能的影响并对其进行优化。本文基于热塑性复合材料自动铺放平台,以连续玻璃纤维增强聚丙烯（GF/PP）预浸纱为原料制备复合材料层合板,以层合板的力学性能为优化目标,根据响应曲面法原理设计试验,分析热气温度、热压辊压力及冷压辊压力各工艺参数及其耦合作用对层合板力学性能的影响,建立各工艺参数与层合板力学性能的二次多项式回归方程预测模型,通过预测值与实际值对比等检验分析,验证回归模型的有效性和可靠性,进而获得热塑性复合材料AFP最优工艺参数组合为热气温度为385℃、热压辊压力为0.3 MPa、冷压辊压力为1.1 MPa。     

复合材料自动铺丝工艺参数对应力波传播特征的作用机制

利用应力波特征能够有效识别材料结构中内部缺陷及其分布形式,为了将工艺载荷作为应力波激励源对预成型体内部缺陷进行识别,应首先研究铺丝过程中工艺参数对应力波传播特征的影响,以确定应力传感器的放置位置。本文采用有限元方法构建了铺丝过程热力耦合模型,分析了应力波特征参数与工艺参数之间的关系,并进行了实验验证。为了从微观能量角度解释工艺参数对应力波特征的影响规律,采用分子动力学方法,建立了预浸丝界面的分子模型,并计算出总能量、纤维表面能、基体内能等能量参数,识别和评价了不同工艺参数作用下的应力波驱动能量及其贡献比,以揭示工艺参数对应力波特征的能量作用机制。结果表明,以基体内能作为驱动能量的侧向应力波与工艺参数的关系明显,在不同工艺参数施载下,内部缺陷的形成对该波形的作用易于识别,该结论可作为应力传感器放置位置的参考依据。      

Evaluation of eddy current testing for quality assurance and process monitoring of automated fiber placement

Standards in energy and cost efficiency are higher the ever especially in the aerospace industry. While structures made from carbon-fiber reinforced plastics (CFRP) show significant advantages in regards to specific strength and lightweight design, further improvements in their production processes are essential in order for CFRP to be competitive in the future. The authors present eddy current (EC) testing as a means for quality assurance (QA) and process monitoring for CFRP parts produced by automatic fiber placement (AFP), which is one the most prevalent production methods in aerospace industry. Eddy current testing shows the potential for highly automated process monitoring that can reduce error correction and cycle time in AFP.     

Resistive heating of multidirectional and unidirectional dry carbon fibre preforms

The electrical conductivity (EC) of continuous carbon fibre (CF) layers is highly anisotropic and is expressed by a second order tensor. In the present work, using continuity equation for anisotropic media, the electrical conductivity of a dry CF multilayer preform can be predicted. Hence, the electrical conductivity tensor of the CF preform can be calculated for any stacking sequence. By means of the calculated electrical conductivity tensor of the multilayer preform, the elliptical form of the governing equation can be solved numerically. Based on this, the generated heat (Joule effect) can be determined. Introducing the generated heat into the heat transfer equation, the temperature field over the CF preform can be predicted. For the experimental verification, a thermal camera was used to record the temperature field developed on a CF multilayer preform under given electric potential field. The experimental results were compared to the respective numerical calculations of the temperature field, where the electrical conductivity tensor was calculated analytically based on the proposed methodology. In all the tested cases the calculated electrical conductivity tensor leads to a numerical model which is in excellent agreement with the experimental results.     

Mechanical performance of carbon fibre-reinforced composites based on stitched preforms

A comparative assessment of the influence of pure assembly seams based on a thin (11 tex) polyester yarn in a zigzag geometry on the resulting mechanical performance of a non-crimped fabric (NCF) carbon fibre-reinforced epoxy composite manufactured by vacuum-assisted resin transfer moulding is presented. This study was aimed at generating a solid foundation regarding the overall performance level of stitched NCF composites and at identifying critical property changes. The comprehensive evaluation of the mechanical composite properties includes static as well as dynamic tests of the in-plane properties as well as a characterisation of the interlaminar properties such as apparent interlaminar shear strength (ILSS) and compression after impact (CAI). It is demonstrated that mechanical properties such as the tensile and compression stiffness and CAI strength are not degraded by the chosen stitching parameters, whereas the tensile and compression strength, ILSS as well as the tensile fatigue behaviour are reduced as a result of pronounced localised fibre ondulations. A direct comparison to properties of a commonly used 5H satin woven fabric composite verifies that the overall performance of these particular stitched NCF composites must be enhanced with regard to the identified key criteria to meet the level required for aircraft applications and in order to maintain the performance advantage of NCF composites as compared to standard woven fabrics in general. Promising approaches include the use of different yarn materials based on soluble thermoplastics and/or modified stitching parameters.     

Pulsed chemical vapour infiltration of SiC to three-dimensional carbon fibre preforms

Three-dimensional carbon fibre preforms were infiltrated with silicon carbide from a gas system of CH₃SiCl₃-H₂ using a process of pressure pulsed chemical vapour infiltration. To infiltrate to a deep level, the temperature had to be lowered to 870–900°C, and the hold time per pulse below 1.0 s. Three-dimensional carbon fibre preforms partly filled with SiC fine powder were compared with those without filler. The weight of the preforms increased linearly with increasing number of pulses up to 10⁵ when no filler was present. However, the weight increase slowed down above 8×10⁴ pulses when the filler was used. Preforms with and without SiC filler showed three-point flexural strengths of 160 and 80 MPa after CVI of 10⁵ pulses, respectively. In order to improve the strength, a denser filling of SiC powder is necessary.     

Direct deflection method for determining refractive-index profiles of polymer optical fiber preforms

We present a method for determining the refractive-index profile of polymer optical fiber preforms through a direct-deflection measurement. The method is simple to use, compact, and has good resolution. The profile is obtained from the deflection data by numerically integrating the differential-ray equation for a radial refractive-index gradient. Corrections for topographical deviations are also discussed. Results for both graded-index and step-index fibers are presented.     

Doping of optical fiber preforms via porous silica layer infiltration with salt solutions

A process is described for reproducible deposition of porous layers uniform along the preform axis, and the effect of the nature of the solvent on the infiltration of salt solutions into the porous layer is analyzed in relation to the fabrication of fiber preforms with controlled doping level. Data are presented on the variation of the retention volume in the porous layer with sintering temperature.Translated from Neorganicheskie Materialy, Vol. 41, No. 3, 2005, pp. 363–368.Original Russian Text Copyright © 2005 by Khopin, Umnikov, Guryanov, Bubnov, Senatorov, Dianov.This revised version was published online in April 2005 with a corrected cover date.This revised version was published online in April 2005 with a corrected cover date.     

Preparation of fibrous activated carbons from wood fiber

Short fibrous hollow activated carbons with a high aspect ratio were prepared by carbonization and carbon dioxide activation of softwood and hardwood fibers. The softwood activated carbon consisted mostly of micropores even though the degree of burn-off during the activation became higher. In the case of the hardwood fiber, on the other hand, meso- and macropores as well as micropores were formed and the ratio of these large pores increased with an increasing degree of burn-off. The adsorbed amount of water and toluene vapor on these fibrous activated carbons was compared with that of the commercial activated carbon fibers (ACFs). These fibrous activated carbons showed a high adsorption capacity comparable to that of the ACFs.     

纺织复合材料纤维预制体力学性能测试方法研究进展

纺织复合材料具有质量轻、强度高,可设计性强等诸多优势,在航空航天领域得到广泛应用.纺织预制体的纤维结构对复合材料的最终力学性能有着决定性影响.然而,预制体的纤维结构在织造过程中不可避免地会发生宏观尺寸和微细观结构的变形,甚至产生褶皱缺陷.纺织预制体作为一种柔性骨架,其变形机制十分复杂.采用试验测试来表征预制体的力学变形...