Compressibility of carbon woven fabrics with carbon nanotubes/nanofibres grown on the fibres

Growth of carbon nanotubes (CNT) or carbon nano-fibres (CNF) on carbon fibrous substrates is a way to increase the fracture toughness of fibre reinforced composites (FRC), with encouraging results reported in the recent years. If these nano-engineered FRC (nFRC) are destined to leave laboratories and enter industrial-scale production, a question of adapting the existing composite manufacturing methods will arise. The paper studies compressibility of woven carbon fibre performs (two types of fabrics) with CNT/CNF grown on the fibres using the CVD method. The results include pressure vs thickness and pressure vs fibre volume fraction diagrams for one and four layers of the fabric. Morphology of the nFRC is studied with SEM. It is shown that the pressure needed to achieve the target fibre volume fraction of the preform increases drastically (for example, from 0.05 MPa to more than 0.5 MPa for a fibre volume fraction of 52%) when CNT/CNF are grown on it. No change in nesting of the fabric plies is noticed. The poor compressibility can lower the achievable fibre volume fraction in composite for economical vacuum assisted light-RTM techniques and increase the pressure requirements in autoclave processing.     

Pressure infiltration of sol/gel processed short fibre ceramic matrix composites

Using pressure infiltration short fibre-reinforced ceramic matrix composites with uniform and dense microstructure can be made. Based upon this processing technique, composites composed of silica sol, alumina particles, and alumina short fibres were fabricated. The related processing parameters studied in this work include infiltration rate, fibre volume fraction Vf, particle size, and infiltrate viscosity. An optimal infiltration rate was 4 mm min-1, at which rate the composite with Vf of 8.1% and particle size of 3 m has the highest green density. An equilibrium between particle packing strength and applied load must be obtained during the infiltration to obtain high green density and composite strength. The influence of fibre volume fraction and particle size on composite green density is in a synergistic manner because it involves particle–fibre interactions, fibre–fibre interactions, and sedimentation. Furthermore, the increase of sol viscosity results in more sedimentation in the infiltrate and lower composite green density. The fracture toughness of composites is 38% higher than that of monolithic alumina. © 1998 Chapman & Hall     

3D-Quantification of the distribution of continuous fibres in unidirectionally reinforced composites

Synchrotron microtomography is carried out for continuous C-fibre reinforced aluminium and a continuous C-reinforced polymer. The local volume fraction as well as the orientation distribution of the reinforcement are analysed three dimensionally for both composites using self-developed calculation methods. Representative elements for the analysis of the local volume fraction are determined by using two-point probability functions. The results show that regions with smaller volume fractions tend to form channels along the fibre bundles for both composites. Channels of high volume fractions, representing touching fibres (local volume fraction >55 vol%), are identified for the polymer matrix composite. The regions with high volume fraction >50 vol% tend to form clusters in the case of the metal matrix composite. The orientation of the reinforcement is followed throughout the volume of both composites. The results show preferential orientations within each bundle of the fibre reinforced metal. The orientation of the reinforcement is more homogeneous in the fibre reinforced polymer and the largest misorientations are found within the channels separating fibre bundles. The characterisation methods developed in this work can be used to evaluate quality criteria adopted in the stage of development of the composites.     

Numerical prediction and experimental characterisation of meso-scale-voids in liquid composite moulding

Pressure gradients that drive the resin flow during liquid composite moulding (LCM) processes can be very low while manufacturing large composite parts. Capillary pressure becomes the predominant force for tow impregnation and thus meso-scale-voids can be generated, reducing the part quality. In contrast, micro-voids are created at high resin pressure gradients. In this work, a numerical method is presented to predict the creation of meso-scale-voids and their evolution. Experimental validation is conducted by measuring void content of produced composite parts with micro-computed tomography (μ-CT). Additionally, the void content as a function of the modified capillary number Ca^* is determined and the influence of the fibre volume content in the bundles on the meso-scale- and micro-void content is studied.     

The low velocity impact response of non-woven hemp fibre reinforced unsaturated polyester composites

Hemp fibre reinforced unsaturated polyester composites (HFRUPE) were subjected to low velocity impact tests in order to study the effects of non-woven hemp fibre reinforcement on their impact properties. HFRUPE composites specimens containing 0, 0.06, 0.10, 0.15, 0.21 and 0.26 fibre volume fractions (V_f) were prepared and their impact response compared with samples containing an equivalent fibre volume fraction of chopped strand mat E-glass fibre reinforcement. Post-impact damage was assessed using scanning electron microscopy (SEM). A significant improvement in load bearing capability and impact energy absorption was found following the introduction hemp fibre as reinforcement. The results indicate a clear correlation between fibre volume fractions, stiffness of the composite laminate, impact load and total absorbed energy. Unreinforced unsaturated polyester control specimens exhibited brittle fracture behaviour with a lower peak load, lower impact energy and less time to fail than hemp reinforced unsaturated polyester composites. The impact test results show that the total energy absorbed by 0.21 fibre volume fraction (four layers) of hemp reinforced specimens is comparable to the energy absorbed by the equivalent fibre volume fraction of chopped strand mat E-glass fibre reinforced unsaturated polyester composite specimens.     

Determining the minimum,critical and maximum fibre content for twisted yarn reinforced plant fibre composites

The effect of fibre volume fraction on the physical and tensile properties of aligned plant fibre composites (PFCs) produced via vacuum infusion has been investigated. There is no clear correlation between fibre volume fraction and porosity. However, low fibre content PFCs are prone to intra-yarn voids, while high fibre content PFCs are prone to inter-yarn voids. This is due to changing resin flow dynamics with increasing fibre content.     

Correlation between Young’s modulus and impregnation quality of epoxy-impregnated SWCNT buckypaper

Carbon nanotube (CNT) sheets, also known as buckypaper, have high potential for structural applications due to their high volume fraction of CNT, the strongest and stiffest materials known. In this work, two different techniques, one based on positive pressure and another based on vacuum infiltration, are utilized to impregnate single-walled carbon nanotube (SWCNT) buckypaper sheets of 50–70 μm in thickness, resulting in a Young’s modulus of up to 15.4 GPa. Scanning electron microscopy demonstrates that the vacuum-based technique results in more effective impregnation of the buckypaper than the positive pressure technique. Thermogravimetry analysis of vacuum-impregnated specimens indicated a void content ranging from 5% to 32%. An advanced Mori–Tanaka-based micromechanics technique is also utilized to predict the effect of SWCNT volume fraction and void content on Young’s modulus of nanocomposites. These calculations suggest a higher void content of around 40% for the vacuum-impregnated composites.     

孔隙对碳纤维/环氧复合材料层合板层间剪切疲劳性能的影响

研究了孔隙对碳纤维增强环氧树脂基复合材料层合板［(±45)/0₄/(0, 90)/0₂］_S的静态层间剪切强度和层间剪切疲劳性能的影响。采用不同的热压罐压力制备了孔隙率为0.4%~6.6%的试样。采用显微照相法和图像分析技术对孔隙率和孔隙的微观形貌进行了分析。研究结果表明, 随着热压罐压力的降低, 大孔隙(S>7.85×10-3mm2)所占的比例逐渐增加, 平均孔隙率增加。在孔隙率为0.4%~6.6%时, 每增加1%, 复合材料层压板的层间剪切强度下降2.4%。随着孔隙率的增加, 层压板的疲劳寿命降低。与静态试验相比, 孔隙率对层压板疲劳性能的影响比对静态性能的影响大。大孔隙的存在促进了疲劳裂纹的产生和扩展。      

Pseudo-percolation: Critical volume fractions and mechanical percolation in polymer nanocomposites

Polymer nanocomposites offer a basis for the design and manufacture composite materials with greatly enhanced properties at relatively low volume fractions of the included phase. One underlying mechanism, thought to contribute to these properties is the presence of an interfacial region, ∼15 nm thick, between the polymer matrix and included particles. The size of the interface makes relatively little contribution to the effective properties of composites with micro-sized particles but, because its thickness is comparable to the size of the nanoscaled included phase, its potential impact within nanocomposites is much greater. In particular, percolated nano-microstructures may result at volume fractions below theoretical thresholds, due to connectivity achieved through rod-interface-rod, or ‘pseudo-percolation’, contact. In this work the influence of the interface layer is incorporated into estimates of critical volume fraction through an excluded volume model. Results show a significant reduction in the range of critical volume fractions. These values are incorporated into a mean-field micromechanics model to illustrate mechanical percolation through changes in predicted effective elastic composite properties.     

Representative volume elements for discontinuous carbon fibre composites – Part 2: Determining the critical size

This is the second part in a series of papers investigating the size of representative volume elements for discontinuous carbon fibre composites. An ‘embedded cell’ finite element approach, outlined in Part 1, has been used to determine critical RVE sizes for materials with increasing fibre lengths and fibre volume fractions. Convergence of the results for mechanical properties were seen at RVE edge lengths of four times the fibre length (a/L = 4), irrespective of the fibre volume fraction. The calculated Poisson’s ratios (v₁₂ and v₂₁) were largely independent of the RVE size.     

Controlled retting of hemp fibres: Effect of hydrothermal pre-treatment and enzymatic retting on the mechanical properties of unidirectional hemp/epoxy composites

The objective of this work was to investigate the use of hydrothermal pre-treatment and enzymatic retting to remove non-cellulosic compounds and thus improve the mechanical properties of hemp fibre/epoxy composites. Hydrothermal pre-treatment at 100 kPa and 121 °C combined with enzymatic retting produced fibres with the highest ultimate tensile strength (UTS) of 780 MPa. Compared to untreated fibres, this combined treatment exhibited a positive effect on the mechanical properties of hemp fibre/epoxy composites, resulting in high quality composites with low porosity factor (α_pf) of 0.08. Traditional field retting produced composites with the poorest mechanical properties and the highest α_pf of 0.16. Hydrothermal pretreatment at 100 kPa and subsequent enzymatic retting resulted in hemp fibre composites with the highest UTS of 325 MPa, and stiffness of 38 GPa with 50% fibre volume content, which was 31% and 41% higher, respectively, compared to field retted fibres.     

Characterising the loading direction sensitivity of 3D woven composites: Effect of z-binder architecture

Three different architectures of 3D carbon fibre woven composites (orthogonal, ORT; layer-to-layer, LTL; angle interlock, AI) were tested in quasi-static uniaxial tension. Mechanical tests (tensile in on-axis of warp and weft directions as well as 45° off-axis) were carried out with the aim to study the loading direction sensitivity of these 3D woven composites. The z-binder architecture (the through-thickness reinforcement) has an effect on void content, directional fibre volume fraction, mechanical properties (on-axis and off-axis), failure mechanisms, energy absorption and fibre rotation angle in off-axis tested specimens. Out of all the examined architectures, 3D orthogonal woven composites (ORT) demonstrated a superior behaviour, especially when they were tested in 45° off-axis direction, indicated by high strain to failure (∼23%) and high translaminar energy absorption (∼40 MJ/m³). The z-binder yarns in ORT architecture suppress the localised damage and allow larger fibre rotation during the fibre “scissoring motion” that enables further strain to be sustained by the in-plane fabric layers during off-axis loading.     

用于RFI工艺的高性能树脂膜的研究

以烯丙基化合物改性双马来酰亚胺为基体,通过加入聚醚砜树脂(PES),制得了适用于树脂膜熔渗(RFI)工艺的高性能树脂基体膜。该树脂膜性质稳定,能满足RFI工艺要求。其固化树脂耐热性及力学性能优良;所制得的玻璃布复合材料综合性能优异;常温下弯曲强度、层间剪切强度分别达到535 MPa和55.5 MPa。      

Compaction of thick carbon/phenolic fabric composites with autoclave method

Carbon/phenolic composites are used in the nozzle parts of solid rocket motors due to their heat-resisting, ablative, and high strength characteristics, which are required to endure the high temperature and pressure of combustion gas passing through the nozzle. But the thick axi-symmetric structure of the composite nozzle induces high thermal residual stresses due to the large difference of coefficient of thermal expansion (CTE) between the in-plane and the out-of-plane. In this work, in order to reduce the through-thickness CTE and the void content, a compression in the thickness direction was applied to the composite prepreg by a compressive jig during manufacturing of composite to supplement the low autoclave pressure. The through-thickness CTE of the fabric composite was calculated by a compaction model and compared with the measured one by thermo-mechanical analysis. The through-thickness CTE changed drastically with respect to the compaction amount, and the void content of the carbon/phenolic fabric composite laminate showed different characteristics from the ordinary fabric laminates with respect to the autoclave pressure and the jig pressure.     

Enhancing damping features of advanced polymer composites by micromechanical hybridization

A hybrid configuration at the micromechanical level is presented and described as a suitable approach to enhance the damping features of advanced polymer composites. A micro-level hybridization was achieved on dry preform reinforcements by embedding visco-elastic fibres within standard carbon tows. Unidirectional composites with two viscoelastic volume fractions (2.5% and 5% vol/vol) were manufactured by a vacuum infusion process and later tested by dynamic mechanical analysis along the principal directions. Final results reveal a significant enhancement (+80% and +56%) of the damping properties, respectively, for the longitudinal and the transverse directions in the case of the highest viscoelastic fibre content.In turn, the elastic properties of the final composite were greatly reduced (−37% and −35%) with respect to the standard composite. Final results support further work in the direction of micromechanical hybridization looking at the potential exploitation of standard textile configurations with different viscoelastic fibre content to enhance damping properties.     

Plant fibre composites – porosity and volumetric interaction

Plant fibre composites contain typically a relative large amount of porosity, which considerably influences properties and performance of the composites. The large porosity must be integrated in the conversion of weight fractions into volume fractions of the fibre and matrix parts. A model is presented to predict the porosity as a function of the fibre weight fractions, and to calculate the related fibre and matrix volume fractions, as well as the density of the composite. The model predicts two cases of composite volumetric interaction separated by a transition fibre weight fraction, at which the combination of a high fibre volume fraction, a low porosity and a high composite density is optimal. Experimental data from the literature on volumetric composition and density of four types of plant fibre composites are used to validate the model. It is demonstrated that the model provides a concept to be used in fabrication, process optimisation and design of plant fibre composites.     

Flexural properties of hemp fibre reinforced polylactide and unsaturated polyester composites

In this work, flexural strength and flexural modulus of chemically treated random short and aligned long hemp fibre reinforced polylactide and unsaturated polyester composites were investigated over a range of fibre content (0-50 wt%). Flexural strength of the composites was found to decrease with increased fibre content; however, flexural modulus increased with increased fibre content. The reason for this decrease in flexural strength was found to be due to fibre defects (i.e. kinks) which could induce stress concentration points in the composites during flexural test, accordingly flexural strength decreased. Alkali and silane fibre treatments were found to improve flexural strength and flexural modulus which could be due to enhanced fibre/matrix adhesion.     

Sustainable food packaging: Valorising wheat straw fibres for tuning PHBV-based composites properties

The present work aims at investigating the impact of wheat straw fibres (WSF) size, morphology and content on the process-ability and functional properties (mechanical properties and water vapour permeability) of PHBV-based composites. For that purpose, three types of fibres obtained by successive grindings (from the micrometric up to the millimetric scale) were used. It was shown that the highest possible filler level was all the more high when decreasing fibre size (over 50 wt% in the case of micrometric fibres), due to reduced film heterogeneity and improved fibre wetting by the polymer. As regards functional properties, increasing fibre size and/or content led to a significant degradation of ultimate tensile properties, while Young’s modulus was not significantly affected. At the same time, water vapour transmission rate was significantly increased from 11 up to 110 g m⁻² day⁻¹, which could extend the applicability of PHBV/WSF composites as food packaging materials to respiring fresh products.     

Effect of consolidation pressure on volumetric composition and stiffness of unidirectional flax fibre composites

Unidirectional flax/polyethylene terephthalate composites are manufactured by filament winding, followed by compression moulding with low and high consolidation pressure, and with variable flax fibre content. The experimental data of volumetric composition and tensile stiffness are analysed with analytical models, and the composite microstructure is assessed by microscopy. The higher consolidation pressure (4.10 vs. 1.67 MPa) leads to composites with a higher maximum attainable fibre volume fraction (0.597 vs. 0.530), which is shown to be well correlated with the compaction behaviour of flax yarn assemblies. A characteristic microstructural feature is observed near the transition stage, the so-called local structural porosity, which is caused by the locally fully compacted fibres. At the transition fibre weight fraction, which determines the best possible combination of high fibre volume fraction and low porosity, the high pressure composites show a higher maximum performance in terms of tensile stiffness (40 vs. 35 GPa). The good agreement with the model calculations (fibre compaction behaviour, and composite volumetric composition and mechanical properties), allows the making of a property diagram showing stiffness of unidirectional flax fibre composites as a function of fibre weight fraction for consolidation pressures in the range 0–10 MPa.     

A novel approach to fabricate high volume fraction nanocomposites with long aligned carbon nanotubes

Conventional micro-fiber-reinforced composites provide insight into critical structural features needed for obtaining maximum composite strength and stiffness: the reinforcements should be long, well aligned in a unidirectional orientation, and should have a high reinforcement volume fraction. It has long been a challenge for researchers to process CNT composites with such structural features. Here we report a method to quickly produce macroscopic CNT composites with a high volume fraction of millimeter long, well aligned CNTs. Specifically, we use the novel method, shear pressing, to process tall, vertically aligned CNT arrays into dense aligned CNT preforms, which are subsequently processed into composites. Alignment was confirmed through SEM analysis while a CNT volume fraction in the composites was calculated to be 27%, based on thermogravimetric analysis data. Tensile testing of the preforms and composites showed promising mechanical properties with tensile strengths reaching 400 MPa.