碳纤维织物/环氧复合材料销钉连接实验研究

针对1K缎纹碳纤维织物/环氧复合材料层板销钉连接,实验研究了几何参数对连接性能的影响.选取了不同的端距和边距,测试了碳纤维织物复合材料层板W/D(试样宽度/孔直径)及E/D(孔端距/孔直径)对销钉连接强度的影响,并分析了销钉连接的破坏模式.实验结果表明:1K缎纹织物/环氧复合材料层板销钉连接承载能力随着边距E和端距W的增加而增加,当E/D=2,W/D=3时,销钉连接强度达到稳定值,此后增加边距E和端距W值连接强度不会有明显的增加.     

Bearing Behavior of Drilled and Molded-in Holes

Glass/epoxy composites were manufactured using RIFT (Resin Infusion under Flexible Tool), a closed mould process capable of obtaining large and complex forms, by impregnating, under a vacuum, a dry preform placed on a flat rigid mould. At certain points of these composite laminates, molded-in holes were made in the dry perform before the resin infusion phase, using two different methods: displacing or cutting the yarn of the fibers. After the resin treatment, other holes were made in the same laminates by drilling. Single-point pin-loaded specimens, cut from laminates, were tested for different values of specimen width-to-hole diameter ratio (W/D) and edge distance-to-hole diameter ratio. In the results of the experiment, the specimens with molded in holes made by displacing the fiber yarn showed higher bearing strength values.     

三维机织碳纤维/环氧树脂复合材料在两种测量方法下的热响应机制对比

采用瞬态热线法和闪光法分别测量了多种结构参数的三维机织碳纤维/环氧树脂复合材料的导热系数。通过对3D正交机织碳纤维/环氧树脂复合材料的有限元模拟可以看出,3D正交机织碳纤维/环氧树脂复合材料内经纱、纬纱和Z向纱的导热作用在不同的受热形式下会发生变化。采用瞬态热线法测量时,2.5D机织碳纤维/环氧树脂复合材料的导热系数低于2.5D经向增强结构,同时高于3D正交结构,而采用闪光法测量时,2.5D经向增强和3D正交碳纤维/环氧树脂复合材料的导热系数均小于2.5D机织结构。这是由于在使用不同的测量方法时,三维机织碳纤维/环氧树脂复合材料内部相同的纱线系统在导热过程中所起的作用并不相同。随着纤维体积含量的提高,瞬态热线法和闪光法测得的2.5D机织碳纤维/环氧树脂复合材料的导热系数都在不断提高。由于经纱的屈曲,采用闪光法测量时,导热性能提升更加明显。研究结果表明,三维机织碳纤维/环氧树脂复合材料在不同受热形式下具有不同的热响应机制。     

Fatigue of a 3D Orthogonal Non-crimp Woven Polymer Matrix Composite at Elevated Temperature

Tension-tension fatigue behavior of two polymer matrix composites (PMCs) was studied at elevated temperature. The two PMCs consist of the NRPE polyimide matrix reinforced with carbon fibers, but have different fiber architectures: the 3D PMC is a singly-ply non-crimp 3D orthogonal weave composite and the 2D PMC, a laminated composite reinforced with 15 plies of an eight harness satin weave (8HSW) fabric. In order to assess the performance and suitability of the two composites for use in aerospace components designed to contain high-temperature environments, mechanical tests were performed under temperature conditions simulating the actual operating conditions. In all elevated temperature tests performed in this work, one side of the test specimen was at 329 °C while the other side was open to ambient laboratory air. The tensile stress-strain behavior of the two composites was investigated and the tensile properties measured for both on-axis (0/90) and off-axis (±45) fiber orientations. Elevated temperature had little effect on the on-axis tensile properties of the two composites. The off-axis tensile strength of both PMCs decreased slightly at elevated temperature. Tension-tension fatigue tests were conducted at elevated temperature at a frequency of 1.0 Hz with a ratio of minimum stress to maximum stress of R = 0.05. Fatigue run-out was defined as 2 × 10⁵ cycles. Both strain accumulation and modulus evolution during cycling were analyzed for each fatigue test. The laminated 2D PMC exhibited better fatigue resistance than the 3D composite. Specimens that achieved fatigue run-out were subjected to tensile tests to failure to characterize the retained tensile properties. Post-test examination under optical microscope revealed severe delamination in the laminated 2D PMC. The non-crimp 3D orthogonal weave composite offered improved delamination resistance.     

Characterisation of carbon fibres recycled from carbon fibre/epoxy resin composites using supercritical n-propanol

Three different PAN based carbon fibres (Toray T600S, T700S and Tenax STS5631) were recycled from epoxy resin/carbon fibre composites using supercritical n-propanol. The recycled carbon fibres were characterised using single fibre tensile tests, SEM, XPS and micro-droplet test. The tensile strength and modulus of the recycled carbon fibre was very similar to the corresponding as-received carbon fibres. However, the surface oxygen concentration decreased significantly, which caused a reduction of the interfacial shear strength with epoxy resin.     

Shear strength of polymers and fibre composites: 2. carbon/epoxy pultrusions

Pultrusions were made with carbon fibres and an epoxy resin. Three different curing agents were used, so that the matrices were resins with different glass transition temperatures. The composites were tested for shear strength at different temperatures, so that the effect of the resin shear strength on composite shear strength could be observed, with a fixed fibre architecture. It was found that the composite was always much stronger than the resin both for the 0 and 90° fracture modes. The 90° fracture surfaces contained many broken fibres, and shear hackles were observed in the resin-rich regions. These suggested that shear failure (rather than tensile failure) took place in the Iosipescu test for the 90° specimens. It was concluded that the fibre architecture played a dominant role in the composite shear strength, with interphase effects being involved also.     

Effect of hydrothermal ageing on bond strength

Changes in the fibre/resin interfacial zone due to hydrothermal ageing can be detected using the TRI microbond technique for measuring interfacial shear strength. The procedure involves exposing fibre/resin microdroplet assemblies to the specified environmental condition for a given time and comparing their interfacial shear strengths with those of unaged specimens. We have investigated the effect of hydrothermal exposure on Kevlar^® 49, AS4 carbon and E-glass fibres embedded in Epon 828 thermosetting resin and two thermoplastic resins (polyethylene and polycarbonate). For the fibres embedded in Epon 828 and polycarbonate resins, consistent and significant bond strength reductions (13–50%) were obtained after refluxing in water at 88°C. These reductions could be partially or completely reversed, depending on the fibre/resin system. In contrast, interfacial shear strengths for the same three fibres embedded in polyethylene resin were increased irreversibly by 36 to 46% upon exposure to water at 88°C for 24 h. Evaluation of these results suggests that the mechanisms of bond strength changes due to hydrothermal ageing may be different for various combinations of fibres and resins.     

Accelerated thermal ageing of epoxy resin and 3-D carbon fiber/epoxy braided composites

This paper reports the accelerated thermal ageing behaviors of pure epoxy resin and 3-D carbon fiber/epoxy braided composites. Specimens have been aged in air at 90 °C, 110 °C, 120 °C, 130 °C and 180 °C. Microscopy observations and attenuated total reflectance Fourier transform infrared spectrometry analyses revealed that the epoxy resin oxidative degradation only occurred within the surface regions. The surface oxidized layer protects inner resin from further oxidation. Both the resin degradation and resin stiffening caused by post-curing effects will influence the compression behaviors. For the braided composite, the matrix ageing is the main ageing mode at temperatures lower than glass transition temperatures (T_g) of the pure epoxy resin, while the fiber/matrix interface debonding could be observed at the temperatures higher than T_g, such as the temperature of 180 °C. The combination of matrix degradation and fiber/resin interface cracking leads to the continuous reduction of compressive behaviors.     

Negative Poisson's ratios in angle-ply laminates: theory and experiment

A series of composite panels has been prepared by laminating unidirectional prepreg tapes of epoxy resin reinforced with continuous carbon fibres. Each panel was a balanced, symmetrical laminate with the plies alternating at ± θ to a reference direction where θ = 0, 10, 15, 20, 25, 30 and 40°. The full set of nine elastic constants was determined for each panel using ultrasonic velocity measurements. The experimentally determined elastic constants were then compared with theoretical predictions obtained using standard laminate theory. The Poisson's ratios of the composites were of particular interest in showing negative values for θ in the range between 15 and 30°, as predicted by the theory.     

Bolted Joints in Three Axially Braided Carbon Fibre/Epoxy Textile Composites with Moulded-in and Drilled Fastener Holes

Experimental behaviour of bolted joints in triaxial braided (0°/±45°) carbon fibre/epoxy composite laminates with drilled and moulded-in fastener holes has been investigated in this paper. Braided laminates were manufactured by vacuum infusion process using 12 K T700S carbon fibres (for bias and axial tows) and Araldite LY-564 epoxy resin. Moulded-in fastener holes were formed using guide pins which were inserted in the braided structure prior to the vacuum infusion process. The damage mechanism of the specimens was investigated using ultrasonic C-Scan technique. The specimens were dimensioned to obtain a bearing mode of failure. The bearing strength of the specimens with moulded-in hole was reduced in comparison to the specimens with drilled hole, due to the increased fibre misalignment angle following the pin insertion procedure. An improvement on the bearing strength of moulded-in hole specimens might be developed if the specimen dimensions would be prepared for a net-tension mode of failure where the fibre misalignment would not have an effect as significant as in the case of bearing failure mode, but this mode should be avoided since it leads to sudden catastrophic failures.     

An assessment of methods to determine the directional moisture diffusion coefficients of composite materials

An assessment was made of methods of determining the directional moisture diffusivities in a unidirectional carbon–fibre/epoxy composite with a distinctly anisotropic microstructure. Experimental data were collected for samples cut from 25 mm thick panels in three orthogonal directions during conditioning in water and humid environments. Using the full 3D Fickian solution and an optimisation process on the full data set was the best method of determining the diffusivities. Diffusivity across the fibres was 40% and through thickness diffusivity was 13% of that along fibres, which was found to be close to that of the unreinforced resin. A critical analysis was made of methods of approximating the 3D Fickian solution.     

A novel method for measuring direct compressive properties of carbon fibres using a micro-mechanical compression tester

A novel method has been developed for measuring direct compressive properties such as strength and elasticity of a series of mesophase-pitch-based and PAN-based carbon fibres about 10 m in diameter by uniaxial and transverse compression tests using a micromechanical tester. The fibres were shaped into cylindrical specimens, with their size ratio of length to diameter kept at about 2 to 3, by separating them from a thin film made by polishing the cut faces of a strand of carbon fibres with epoxy resin as a matrix. Individual cylindrical specimens were stood up or laid down on a glass plate without any fixer for the measurements of axial and transverse compression properties of fibres, respectively. The fibres exhibited non-linear elasticity, with the compressive modulus decreasing with compressive deformation. The direct axial compressive strengths of pitch-based carbon fibres were found to be marginally lower than the indirect ones, whereas there was no significant difference between the two strength values for PAN-based fibres. The pitch-based fibres exhibited smaller average values of axial compressive strength than the PAN-based fibres. The transverse compressive strength, which decreases with an increase in elasticity of carbon fibres, exhibited a considerably lower average value than that of the axial compressive strength. Further, the axial compressive strength was found to be smaller than the direct tensile strength for the fibres.     

Investigation of the long term effects of moisture on carbon fibre and epoxy matrix composites

The aim of this work is to investigate the long term effects of moisture on the interface between a carbon fibre and an epoxy matrix. High modulus carbon fibres were used to prepare single fibre model composites based on an epoxy resin. The samples were immersed in the seawater and demineralised water and their moisture uptake behaviour was monitored. The equilibrium moisture content and diffusion coefficients for the samples were determined. DSC has been used to analyse the moisture effects on glass transition temperature and thermal stability of the pure epoxy specimens. These results showed a reduction in the glass transition temperature (T_g) after moisture absorption. Tensile tests were also carried out for the epoxy specimens and a general decrease in the mechanical properties of the epoxy matrix was observed. Raman spectroscopy was used to observe the effects of moisture on the axial strain of the carbon fibre within the composite and stress transfer at the interface as a function of exposure time. The results show that the decrease in the mechanical and interfacial properties of the model composites under the seawater immersion is more significant than under demineralised water immersion.     

Investigations on d.c. conductivity behaviour of milled carbon fibre reinforced epoxy graded composites

This paper reports the d.c. conductivity behaviour of milled carbon fibre reinforced polysulphide modified epoxy gradient composites. Milled carbon fibre reinforced composites having 3 vol. % of milled carbon fibre and poly sulphide modified epoxy resin have been developed. D.C. conductivity measurements are conducted on the graded composites by using an Electrometer in the temperature range from 26°C to 150°C. D.C. conductivity increases with the increase of distance in the direction of centrifugal force, which shows the formation of graded structure with the composites. D.C. conductivity increases on increase of milled carbon fibre content from 0·45 to 1·66 vol.%. At 50°C, d.c. conductivity values were 1·85 × 10⁻¹¹, 1·08 × 10⁻¹¹ and 2·16 × 10⁻¹² for samples 1, 2 and 3, respectively. The activation energy values for different composite samples 1, 2 and 3 are 0·489, 0·565 and 0·654 eV, respectively which shows decrease in activation energy with increase of fibre content.     

Preparation of carbon microparticle assemblies from phenolic resin using an inverse opal templating method

Three-dimensionally (3D) well-ordered carbon microparticle assemblies with different particle morphologies were fabricated by infiltration of phenolic resin solution into SiO₂ inverse opal structures and subsequent carbonization. The effect of the phenolic resin solution concentration and the carbonization temperature on the morphology of the fabricated carbon microparticles was investigated. At a carbonization temperature of 1000 °C, carbon microparticles with interlocked bridges were obtained when the concentration of phenolic resin solution is 40 wt% and hollow carbon microparticles with opened window channels were obtained at a concentration of 30–35 wt%. When the carbonization temperature was decreased to 500 °C, carbon microparticles with interlocked bridges also were observed, even when the phenolic resin concentration was 30 wt%. The structures and properties of the carbon microparticles and their assemblies were characterized using SEM, XRD, and N₂ adsorption.     

Influence of fibre architecture on the tensile,compressive and flexural behaviour of 3D woven composites

This paper presents a comprehensive study on the tensile, compressive, and flexural performance of six types of 3D woven carbon-fibre/epoxy composites which were manufactured using a traditional narrow fabric weaving loom and resin transfer moulding. Four orthogonal and two angle-interlock weaves were tested with the primary loading direction parallel to the warp direction. The mechanical performance was found to be affected by the distribution of resin rich regions and the waviness of the load-carrying fibres, which were determined by the fibre architectures. The binding points within the resin rich regions were found to be the damage initiation sites in all weave types under all loading conditions, which were confirmed with both visual observation and digital image correlation strain maps. Among all weave types, the angle interlock weave W-3 exhibited the highest properties under all loading conditions.     

The thermal conductivity of carbon fibre-reinforced composites

Measurements of the thermal conductivity between approximately 80 and 270 K of a series of unidirectional and bidirectional specimens of epoxy resin DX210/BF₃400 reinforced with Morganite high modulus (HMS) and high strength (HTS) carbon fibres are reported for in-plane and out-of-plane directions. The main features of the results conform with expectations based upon known structural properties of the fibres and predictions based upon current theoretical models. Employing the results for the composites in association with results for the pure resin, the account concludes with an assessment of some of the heat transmission characteristics of the fibres.     

Exploring mechanical property balance in tufted carbon fabric/epoxy composites

The paper details the manufacturing processes involved in the preparation of through-the-thickness reinforced composites via the ‘dry preform–tufting–liquid resin injection’ route. Samples for mechanical testing were prepared by tufting a 5 harness satin weave carbon fabric in a 3 mm × 3 mm square pitch configuration with a commercial glass or carbon tufting thread, infusing the reinforced preforms with liquid epoxy resin and curing them under moderate pressure. The glass thread reinforcement increases the compression-after-impact strength of a 3.3 mm thick carbon fabric laminate by 25%. The accompanying drop-downs in static tensile modulus and strength of the same tufted laminate are below 10%. The presence of tufts is also shown to result in a significant increase in the delamination crack growth resistance of tufted double-cantilever beam specimens and has been quantified for the case of a 6 mm thick tufted carbon non-crimped fabric (NCF)/epoxy composite.     

Flexural behavior of three-axis woven carbon/carbon composites

This work examines the processing characteristics and flexural behavior of 3D woven carbon/carbon composites. Two types of the composites have been made, both having 3-axis orthogonal structures. The first combines solid rods along the axial direction. The rod, 1 mm in diameter, is composed of unidirectional carbon fibers and a phenolic resin. The second is a conventional type composed of carbon yarns in all axes. Both preforms were then impregnated by the phenolic resin. Matched molds were used to enhance fiber packing and to cure the resin under a hot press. The green composites were then heat-treated at various temperatures ranging from 200° through 1000° C. The second set of specimens was made by applying multi-cycle impregnation and carbonization. Flexural tests were carried out for these two sets of specimens. Their responses to the load and the induced damage behavior have been examined. The use of rods enhances fiber packing and reduces fiber crimp, leading to higher material performance. Decomposition of the resin due to the heat-treatment results in weak interfacial bonding and compressive failure in axial yarns. The efficiency of densification has been examined. The induced damage configurations vary significantly in these specimens, as a result of the processing. Some unique modes associated with the 3D structure are discussed.     

2.5D fabrics for delamination resistant composite structures

With the introduction of laminates into primary loaded structures, it has become apparent that the delamination failure mode has the potential for being the major life-limiting failure mechanism. Delamination resistance has previously been increased using a number of techniques, including interleafing, rubber-toughened resin systems and stitching. However, all the methods proposed to date have attendant disadvantages, severely limiting their use in practical applications. This paper presents a novel solution—a 2.5-dimensional (2.5D) fabric—which has none of the aforementioned problems. The fabric is manufactured by cutting a simple three-dimensional weave, consisting of two two-dimensional (2D) fabrics connected by interwoven pile threads, to form a ‘hairy’ fabric. These 2.5D fabrics are impregnated with epoxy resin in the normal way, laminated and cured in an autoclave. Results are presented here for Mode I double cantilever beam and Mode II end load split tests performed on the plain 2D glass fabric and the 2.5D fabrics with glass piles. The fabrics were tested in different orientations (0°, i.e., parallel to the pile fibre weave direction, 90° and 45°), and a variety of pile lengths and pile densities were investigated. The presence of the short piles in the matrix-rich region between laminate plies is shown to increase the fracture toughness of the 2.5D composite over the conventional 2D fabric composite by virtue of the energy-absorbing effect produced by the piles in a similar manner to that produced during fibre bridging.