纺织复合材料预制体成型过程无损检测技术研究进展

纺织复合材料多为各向异性材料,其力学性能很大程度上取决于成型后预制体内纤维的取向.为确保预制体成型后纤维的取向符合产品设计的要求,目前已有多种无损检测技术为纺织复合材料预制体成型过程及质量的检测提供服务.本文结合纺织复合材料预制体织造技术的发展趋势及预制体成型过程对无损检测的需求,就目前广泛用于科研和产业化生产当中的多...     

Hierarchical composites reinforced with robust short sisal fibre preforms utilising bacterial cellulose as binder

A novel robust non-woven sisal fibre preform was manufactured using a papermaking process utilising nanosized bacterial cellulose (BC) as binder for the sisal fibres. It was found that BC provides significant mechanical strength to the sisal fibre preforms. This can be attributed to the high stiffness and strength of the BC network. Truly green non-woven fibre preform reinforced hierarchical composites were prepared by infusing the fibre preforms with acrylated epoxidised soybean oil (AESO) using vacuum assisted resin infusion, followed by thermal curing. Both the tensile and flexural properties of the hierarchical composites showed significant improvements over polyAESO and neat sisal fibre preform reinforced polyAESO. These results were corroborated by the thermo-mechanical behaviour of the (hierarchical) composites, which showed an increased storage modulus and enhanced fibre–matrix stress transfer. Micromechanical modelling was also performed on the (hierarchical) composites. By using BC as binder for short sisal fibres, added benefits such as the high Young’s modulus of BC, enhanced fibre–fibre and fibre–matrix stress transfer can be utilised in the resulting hierarchical composites.     

Analysis of the elastic properties of an interpenetrating AlSi12-Al2O3 composite using ultrasound phase spectroscopy

An interpenetrating composite fabricated by squeeze-casting a eutectic aluminium-silicon alloy into a porous alumina preform is studied in this work. The preform was fabricated by pyrolysis of cellulose fibres used as pore forming agent, pressing of the green ceramic body and subsequent sintering of alumina particles. The resulting preform had both micropores within the ceramic walls and macropores between those walls, which were infiltrated by the liquid metal. Composites with alumina contents varied in the range of 18-65 vol.% were studied. Three longitudinal and three shear elastic constants of the composites were determined using ultrasound phase spectroscopy on rectangular parallelepiped samples. Complete stiffness matrix of one sample was determined by modifying the sample geometry by cutting at the corners of the sample and subsequent ultrasonic measurements. All composites exhibit a moderately anisotropic behavior, which can be attributed to a non-random pore orientation distribution caused by uni-axial pressing of the preforms prior to sintering. The experimental results are compared with several theoretical micromechanical models.     

Characterising Mechanical Properties of Braided and Woven Textile Composite Beams

The focus of this paper is on the manufacture of textile composite beams and on the determination of their mechanical properties. This includes investigating the effects of fibre orientation on the mechanical properties of braided and woven textile composites. Composites were manufactured from nominally identical constituents and identical consolidation processes, leaving as the only variables, variations caused by the different fibre architecture of the preform. The repeatability and, hence, reliability of this approach is demonstrated. Results obtained show that fibre architecture affects composite strength and extensibility. Composites with woven preforms are practically linear up to catastrophic failure while composites with braided preforms exhibit non-linearity prior to failure. Also the mechanical properties of the textile composite beams were determined. Results show that by tailoring the braid angle and pick density of braided and woven composite performs, the mechanical properties of the composite beams can be controlled to suit end-use requirement.     

Novel method for functionalising and patterning textile composites: Liquid resin print

The paper reports a novel method of integrating resin into continuous textile reinforcement. The method presents a print of liquid reactive resin into textile preforms. A series of targeted injections forms a patch which upon consolidation and curing transforms into a stiff region continuously spanning through preform thickness. Enhancing the injected resin with conductive phase allows creating a pattern of patches with controlled dimensions and added functionalities. Patterned composites reveal features which are not typical for conventional composites such as fibre bridged interfaces, regular thickness variation, and gradient matrix properties. The presented study explores the role of these features in (a) the mechanical behaviour of these materials, focusing on their deformation and failure mechanisms in tension, and (b) the feasibility of adding functionality by printing electrically conductive resins containing carbon nano-tubes (CNT). It was shown that resin print is a promising method for local functionalization of structural composites.     

The processing and characterization of sintered metal-reinforced aluminium matrix composites

The current investigation involves the fabrication and characterization of an aluminium metal matrix composite reinforced with sintered metal preforms. Two types of metallic preforms were used (steel and stainless steel) and a variety of squeeze casting conditions were investigated using systematic design-of-experiments techniques to determine the effect of casting conditions on the composite microstructure and mechanical properties. It was observed that a detrimental reaction phase containing iron, aluminium and silicon formed around the metallic preform particles, with a lower volume fraction of reaction phase forming at the lower melt casting temperature. This reaction phase appears to promote premature fracture by facilitating crack initiation and propagation. The stainless steel-reinforced composites had a smaller volume fraction of reaction phase and exhibited superior properties compared to the steel-reinforced composites. This revised version was published online in November 2006 with corrections to the Cover Date.     

C/ C坯体对 C/ C2Cu复合材料摩擦磨损行为的影响

采用无压熔渗方法制备炭纤维整体织物/炭2铜 (C/ C2Cu) 复合材料 , 在 MM22000型环2块摩擦磨损试验机上考察复合材料的摩擦磨损性能 , 利用扫描电子显微镜观察分析磨损表面形貌 , 研究 C/ C坯体对材料的摩擦磨损行为的影响及机制。结果表明 : 随着 C/ C坯体密度的增加 , 摩擦系数及 C/ C2Cu材料自身和对偶的磨损量均降低 ; 采用浸渍/炭化 ( I/ C) 坯体的 C/ C2Cu材料摩擦系数及自身和对偶件的磨损量均高于采用化学气相渗透(CVI) 坯体的试样; 摩擦面平行于纤维取向的试样摩擦系数低于垂直于纤维取向的试样 , 但磨损率较高。     

Fabrication of particulate aluminium-matrix composites by liquid metal infiltration

Aluminium-matrix composites were fabricated by liquid metal infiltration of porous particulate reinforcement preforms, using AlN, SiC and Al₂O₃ as the particles. The quality of the composites depended on the preform fabrication technology. In this work, this technology was developed for high-volume fraction (up to 75%) particulate preforms, which are more sensitive to the preform fabrication process than lower volume fraction whisker/fibre preforms as their porosity and pore size are much lower. The technology developed used an acid phosphate binder (with P/Al molar ratio=23) in the amount of 0.1 wt% of the preform, in contrast to the much larger binder amount used for whisker preforms. The preforms were made by filtration of a slurry consisting of the reinforcement particles, the binder and carrier (preferably acetone), and subsequent baking (preferably at 200 °C) for the purpose of drying. Baking in air at 500 °C instead of 200 °C caused the AlN preforms to oxidize, thereby decreasing the thermal conductivity of the resulting Al/AlN composites. The reinforcement-binder reactivity was larger for AlN than SiC, but this reactivity did not affect the composite properties due to the small binder amount used. The Al/AlN composites were superior to the Al/SiC composites in the thermal conductivity and tensile ductility. The Al/Al₂O₃ composites were the poorest due to Al₂O₃ particle clustering.     

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。     

Experimental and prediction of sintered Cu–W composite by using artificial neural networks

High-energy mechanical milling was used to mix Cu and W powders. Cylindrical preforms with initial preform density of 85% were prepared using a die and punch assembly. The preforms were sintered in an electric muffle furnace at 750 °C, 800 °C, 850 °C, and subsequently furnace cooled and then the specimens are hot extruded to get 92% preform density. Scanning Electron Microscope and X-ray diffraction observations used to evaluate the characteristics. The pore size reduction during extrusion was studied using Auto CAD 2010. Neural networks are employed to study the tribological behavior of sintered Cu–W composites. The proposed neural network model has used the measured parameters namely the weight percentage of tungsten, sintering temperature, load and sliding distance to predict multiple material characteristics, hardness, specific wear rate, and coefficient of friction. The predicted values from the proposed networks coincide with the experimental values. In addition, a relative study between the regression analysis and the networks revealed that the artificial neural networks can predict the tribological characteristics of sintered Cu and W composites better than regression polynomials within a very few percent error.     

Fabrication of gradient distribution alumina short fibre reinforced aluminium alloy

Gradient distribution alumina short fibre reinforced 6061 aluminium alloy have been fabricated by taking advantage of preform compressive deformation during squeeze casting. Pressure was applied mechanically by a punch. Velocity of the punch, pre-heat temperature of the preforms and pouring temperature were controlled during the infiltration of molten 6061 alloy into alumina short fibre preforms. The distribution of hardness along the infiltration direction in the composites was measured and the distribution of volume fraction along the infiltration direction was calculated by the hardness. Velocity of the inflow, pre-heat temperature of the preform, pouring temperature of the molten metal, binder content of the preform and volume fraction of fibres, all have a very great effect on the gradient distribution of alumina short fibres in the aluminium alloy composites.     

C/C坯体密度对2D C_f/SiC复合材料结构和性能的影响

通过理论计算,探究C_f/SiC复合材料密度与C/C坯体密度的相关性;而后采用碳纤维布叠层制作2D C/C坯体,经先驱体浸渍裂解工艺增密,制得密度分别为0.98、1.06、1.12g/cm~3的C/C坯体,通过液相渗硅法反应合成2DC_f/SiC复合材料,探究C/C坯体密度对其结构和性能的影响。与理论计算结果来对比。研究结果表明:试验结果与理论数学计算结果基本一致。随着C/C坯体密度的增加,C_f/SiC复合材料的密度出现先上升后下降的趋势,当C/C坯体密度大于0.98g/cm~3后,复合材料的弯曲强度随着C/C坯体密度的增加而降低,C/C坯体密度为0.98g/cm~3时,2DC_f/SiC复合材料结构和性能较优。     

Flow modeling of the VARTM process including progressive saturation effects

Modeling of vacuum based liquid composite molding methods (e.g., VARTM) relies on good understanding of closely coupled phenomena. The resin flow depends on the preform permeability, which in turn depends on the local fluid pressure; the preform compaction behavior, and the membrane stresses in the vacuum bag. It has also been shown that for many preforms there is a significant unsaturated region behind the flow front, and that the flow in this region affects the overall flow behavior of the process. Studies of preform compaction have shown that the preform stiffness, as well as being non-linear and exhibiting significant hysteresis, is dependant on the fluid saturation. For this reason most researchers model the preform compaction based on the pressure-compaction behavior of saturated preforms during unloading. This assumption leads to an effective discontinuity in preform thickness at the flow front, which is not observed in actual experiments. In this paper an improved compaction model incorporating the saturation dependence of the compaction pressure in the partially saturated region, is used in a one-dimensional model of the VARTM process. The model gives physically more realistic results for the thickness in the flow front region, and an improved model for the consolidation of the preform at the end of infusion.     

Transient gas flow technique for inspection of fiber preforms in resin transfer molding

A transient gas flow method was developed to determine the quality of fibrous preforms in resin transfer molding (RTM) prior to resin injection. The method aims at detecting defects resulting from preform misplacement in the mold, accidental inclusions, preform density variations, race tracking, shearing, etc. Unlike the previously developed method based on steady-state gas flow, the new method allows for the acquisition of continuous time-varying pressure data from multiple ports during a single test. The validity of the method was confirmed by one-dimensional flow experiments.     

Effective elastic, piezoelectric and dielectric properties of braided fabric composites

Composites based upon 3D textile preforms have found broad structural application. This paper presents an analytical methodology for functional composites using piezoceramic fibers in a 3D braided preform. The effective elastic, piezoelectric and dielectric properties of 2-step braided composites with a polymeric matrix have been investigated. In the analytical approach, the effective properties of the braider and axial yarns of the unit cells are determined first using a 3D connectivity model. Then, the effective properties of the 2-step braided composite are predicted using an averaging technique. Results of a numerical example illustrating the variation of elastic, piezoelectric and dielectric constants with the braider yarn angle are provided. Textile preforming technique in general offers the potential of near net shape forming and 3D fiber placement. The present work provides the analytical basis for 3D piezoceramic textile composites.     

Development of titanium-doped carbon–carbon composites

The development of titanium-doped carbon matrix–carbon fibre reinforced composites (CCCs) via liquid impregnation of carbon fibre preforms using mesophase pitch is studied. Two different approaches for introducing the dopant into the carbon material are investigated. One consists of doping the matrix precursor followed by the densification of the preform with the doped precursor. The second approach consists of doping the porous preform prior to densification with the undoped mesophase pitch. Titanium-doped CCCs with a very fine distribution of dopant (in the nanometric scale) are obtained by adding TiC nanoparticles to the matrix precursor. Thermal decomposition of titanium butoxide on the carbon preform prior to densification yields doped CCCs with higher titanium content, although with larger dopant size. The combination of these two methods shows the best results in terms of dopant content.     

Mathematical Analysis of Resin Flow through Fibrous Porous Media

Resin flow through fiber preforms was analyzed mathematically. Closed form solutions for fiber volume fraction distribution and pressure field during resin infusion into fiber preforms were suggested, and a new effective permeability was defined. The effect of preform compressibility on the fiber volume fraction and pressure distributions in resin-saturated region was investigated analytically. The findings show that the compaction behavior of preforms has significant impact on the resin infusion process. The solutions derived analytically in this study can provide insight into a liquid composites molding (LCM) process.     

Experimental preforming of highly double curved shapes with a case corner using an interlock reinforcement

This experimental work concerns the study of the preforming of a specific highly double curved geometry with a triple point (case corner) by the sheet forming process using powdered interlock reinforcement (G1151®). Three different punches (square box, prism, tetrahedron) were used in this study, each of them presenting highly double curved geometry with a case corner. A specific sheet forming device specially designed for the preforming of textile reinforcement was used. The expected shapes with the three punches have been obtained with an optimized blank-holder pressure. No classical defaults such as wrinkling or yarn damage are present in the useful zone of the preforms. However, a new default, not observed for spherical or hemispherical shape has been identified. It concerns the out of plane buckling of yarns. This phenomenon not observed on the square box is visible on some faces and edges of the prismatic and tetrahedron shapes. For the square box, it is easily possible to control the orientation of the yarn within the preform in the faces, whereas this is not possible for triangular faces of the prismatic and tetrahedron shapes. The square box punch is therefore more adapted to preform the highly doubled curved shape with the case corner.     

Failure behavior of three-axis woven carbon/carbon composites under compressive and transverse shear loads

This work examines the responses of three-dimensional carbon/carbon composites under axial compression and transverse shear. By using a 3D weaving technique, two types of preforms with different bundle sizes of the weaving yarns were prepared for assessing its influence on the failure behavior. Carbon yarns were arranged in a three-axis, orthogonal form with interlacing loops on outer surfaces. The carbon matrix was added by using a phenolic resin as the matrix precursor. Resin transfer molding was used for resin impregnation, followed by the curing and carbonization of the resin. The matrix filling was repeated up to five cycles, and the efficiency of the matrix filling was examined. Special fixtures were designed for applying the loads to the 3D composites. Microscopic observations on the induced damage were carried out. The axial yarns were found to undergo bending fracture in the compressive tests. The yarn imperfection, rather than the fiber misalignment, was the major failure-determining factor. The bending of the yarns is analyzed, and the critical value of imperfection that leads to bending fracture is given. The transverse shear resulted in complex but intriguing damage modes, which are closely related to the surface loops and through-thickness yarns. Some keys for preform design to best withstanding the shear are discussed.     

A micromechanical compaction model for woven fabric preforms. Part II: Multilayer

With nesting between adjacent layers and inter-layer packing, the microstructure and the compaction behaviour of a multilayer woven fabric preform are much more complicated than those of a single layer fabric preform. A micromechanical model, based on the hierarchical structure characteristics of woven fabric preforms, was developed to investigate the elastic compaction behaviour of multilayer plain weave fabric preforms. The compaction mechanisms of fabrics at different hierarchical levels including deformation and compaction of yarn cross-section, flattening of yarn waveform, nesting between adjacent layers and inter-layer packing, are considered in an integrated approach in this predictive model. Effects of structural elements at different hierarchical levels on compaction behaviour of multilayer plain weave fabric preforms are investigated in detail. Both the number of layers and shifting are shown to have significant effects on compaction behavior, while the effect of nesting increases as the number of layers increases. The predictions by this model are correlated well with the experimental data.