Flow characteristics of carbon fibre moulding compounds

This paper presents the development of a low-cost carbon fibre moulding compound using an automated spray deposition process. Directed Fibre Compounding (DFC) is used to produce charge packs directly from low cost carbon fibre tows and liquid epoxy resin. A range of material and process related parameters have been studied to understand their influence on the level of macroscopic charge flow, in an attempt to produce a carbon fibre moulding compound with similar flow characteristics to conventional glass fibre SMCs.Charge packs covering just 40% of the mould can be effectively used to process DFC, without detrimentally affecting void content, fibre distribution and mechanical properties. Tensile stiffness and strength values of 36 GPa and 320 MPa are reported for isotropic materials (100% charge coverage), which increase to 46 GPa and 408 MPa with flow induced alignment (50% charge coverage) at 50% fibre volume fraction.     

Experimental characterization of permeability and fibre wetting for liquid moulding

Liquid moulding processes are unique in that resin is infused into a dry fibre preform. Appropriate wet-out of the reinforcing fibres is thus a necessity for the achievement of good composite properties. For this class of manufacturing methods, both macroscopic flow, as related to Darcy's Law and characterized by permeability, and microscopic flow, as related to fibre wet-out, are important. The current research investigates factors affecting permeability and fibre wet-out as related to liquid moulding. Specifically, it is shown that fabric permeability is dependent on the type of test fluid used. Surface tension and contact angle measurements indicate that interactions at the microscopic level between fibre and test fluid account for these differences in permeability. The investigation illustrates the competing nature of macroscopic and microscopic flow in liquid moulding.     

Sheet moulding compound (SMC) from carbon fibre recyclate

This study reports on the first attempt to develop a new sheet moulding compound (SMC) using low cost carbon fibre (CF) recyclate as the reinforcing component in place of conventional glass fibres. A newly designed manufacturing system allows large coarse recyclate fibrous materials to be utilised within existing SMC production lines, and with careful control of virgin and recyclate fibre distribution, a viable SMC grade utilising recycled CF can be obtained.     

Process modelling for liquid moulding of braided preforms

Prediction of the fibre architecture within reinforcement preforms is required to model subsequent processing and performance characteristics. In recent years, a number of studies has been published in which forming or draping of fabric reinforcements has been examined, and links between draping and subsequent processing and mechanical properties have been established. Similar work on 2D braiding is limited, with existing models restricted to cylindrical geometries or surfaces of revolution. This paper presents a model for the braiding of a general mandrel cross-section, which is composed of a number of flat facets. A relatively simple geometric procedure is used, based on the paths of key braid tows that bound each flat facet. Locking or jamming of the braid is accounted for by considering the effects of tow spacing and braid angle on the fibre architecture. Experimental results are presented for a range of geometries to demonstrate the validity of the model, including a range of rectangular mandrels and a prototype automotive component. In all cases agreement is reasonably good, given the appropriate locking angle. The consequences of variations in fibre architecture on subsequent flow are demonstrated. The relationship between ply angle and the principal permeabilities is shown to be different from that observed for woven fabric reinforcements. At low braid angles, flow within the large inter-tow spacings appears to dominate the measured permeability. Finally the effect of fibre architecture on the subsequent eprocessing is demonstrated using a PC-based flow model. This demonstrates that the associated variations in permeability and porosity may result in a non-uniform flow pattern during liquid moulding.     

Glass fibre sizing/matrix interphase formation in liquid composite moulding: effects on fibre/matrix adhesion and mechanical properties

The glass fibre sizing/matrix interactions in a liquid composite moulding environment have been investigated to develop an understanding of the effect of wetting between sized fibre surfaces and the reacting liquid matrix, the structure of the resulting interphase and its effect on fibre/matrix adhesion and composite properties. The overall research objectives in this programme are to develop an experimental protocol for measurement of the key wetting parameters of the fibre and matrix; to quantify the effect of wetting on the mechanical properties of the composites formed by liquid composite moulding; and to formulate a time-, temperature- and geometry-dependent model of the liquid composite moulding process which incorporates the surface properties of the fibre, its finish and the reactive polymer matrix.Results are presented for a series of sized glass fibres consisting of incompatible and compatible sizings, studied with a reacting vinyl ester matrix system. Surface free energy analysis was conducted to characterize the fibre surfaces. Axial and transverse wicking rates were measured to quantify the change in total surface free energy of the fibre/matrix systems. The sizing surface free energy and its solubility in the matrix strongly affected the fibre/matrix interphase formation and consequently the fibre/matrix adhesion and composite shear and flexural strengths.     

Characterising wood fibre mats as reinforcements for liquid composite moulding processes

Liquid composite moulding (LCM) processes are commonly used techniques for the manufacture of advanced composite structures. This study explores the potential of wood fibres as reinforcement for LCM preforms, considering discontinuous fibre mats produced using four different methods. Modified paper manufacturing techniques were employed to produce two types of wet formed mats, the other two being manufactured using dry methods. The dry compaction response of these mats has been investigated, required compression loads being measured up to a fibre volume fraction of 0.4. A complex non-elastic compression response was observed which has significant influence on forces generated within moulds. Saturated compaction tests were also carried out, the samples infiltrated with two different test fluids. A significant reduction in compaction load was observed due to wood softening when using a water based fluid. On the other hand, a non-water-based solution had little less influence on the compaction of the wood fibre mats. In addition, permeability of all four types of mats was measured as a function of fibre volume fraction. Reinforcement permeability and compaction response data are required to simulate LCM processes.     

Experimental investigation of compression moulding of glass/PA12-PMI foam core sandwich components

The manufacturing of sandwich components from pre-consolidated glass/polyamide 12 faces and polymethacrylimide foam core by compression moulding has been studied. A statistical experiment design was used initially to identify the dominant process parameters in terms of interfacial bond strength, evaluated using the transverse tensile test method. In a subsequent extended study, the influence of face temperature and moulding pressure on the mechanical properties of the face–core interface and the sandwich construction as a whole, were characterised in terms of mode I interfacial fracture toughness, shear strength, shear stiffness, and flexural rigidity. These properties were evaluated using a modified double cantilever beam test, shear test, and four point bend test.     

Effects of fibre surface silanisation on silica fibre/phenolics composites produced by resin transfer moulding process

Abstract

The effects of fibre surface silanisation on silica fibre/phenolics composites produced by the resin transfer moulding (RTM) solution impregnation route were investigated. Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and micro-Wilhelmy method were used to evaluate the surface properties of silanised silica fibre. The interlaminar shear strength (ILSS) measurements and morphological observations of the silica fibre/phenolics composites were also performed. The interactions occurring between silica fibre and the components of phenolic resin solution affect the dynamic adsorption behaviour of phenolic resin onto fibre reinforcement. The competitive adsorption of ethanol as solvent onto silica fibre suppresses that of phenolic resin. Fibre surface silanisation by γ-aminopropyl-triethoxysilane (γ-APS), γ-glycidoxypropyl-trimethoxysilane (γ-GPS) and γ-methacryloxypropyl-trimethoxysilane (γ-MPS) leads to the improvement of mechanical interfacial properties of silica fibre/phenolics composites on one hand and decreases the inhomogeneities of resin distribution and mechanical interfacial properties at different regions of the RTM product on the other hand.     

Citric acid separation using a hollow fibre liquid membrane system

Citric acid is the bulk chemical produced in the largest volume by fermentation using Aspergillus niger in submerged cultures. It can be made from sugar cane juice, or cane or beef molasses, and acid concentrations up to around 15% can be achieved.     

Characterization of fibre/matrix interfaces in carbon/carbon composites

The present paper proposes an approach to characterizing fibre/matrix (F/M) interface in carbon/carbon (C/C) composites with respect to both modes of loading that may be expected: opening or shearing. Push-out and tensile tests were used. The former tests involve the shearing mode whereas the latter ones involve the opening one. Push-out tests use a diamond indenter to load the fibres. The interface sliding shear stress was obtained from the load-fibre displacement curve. The tensile tests were conducted on specimens having fibres oriented at 90° with respect to loading direction in order to preferentially open the interfaces. Interface opening strength was extracted from the composite tensile stress–strain behaviour. The specimens were examined under load and after ultimate failure by optical microscopy (OM). The mechanical properties of the F/M interfaces were then discussed.     

激光对碳纤维及碳纤维/环氧树脂复合材料性能影响

采用高能激光束对聚丙烯腈（PAN）基碳纤维进行表面改性。利用SEM、EDS、FTIR、XRD、万能试验机等表征手段,对改性前后碳纤维微观形态、成分变化、物相结构、力学性能进行表征,系统地研究了激光束对碳纤维微观组织变化、性能变化等的影响规律,探索激光束对碳纤维的作用机制。结果表明,碳纤维经激光表面改性后,其表面的粗糙度和比表面积增加,碳纤维的浸润性得到提升,且激光束的功率越高、扫描速度越低,碳纤维浸润性越好。改性后的碳纤维化学成分、微观结构及官能团种类没有改变;改性后的碳纤维官能团种类没有改变,说明激光改性过程主要以物理过程为主;激光改性没有改变碳纤维的微观结构,改性后微晶尺寸略有减小,有利于改善碳纤维与环氧树脂的界面黏结性能。激光表面改性碳纤维/环氧树脂复合材料的拉伸强度和冲击强度均有不同程度的提高,当碳纤维质量分数为0.2wt%、激光改性功率为150 W时,碳纤维/环氧树脂复合材料的拉伸强度提高了59%,冲击强度提高了52%。     

Production of carbon fibres from a pyrolysed and graphitised liquid crystalline cellulose fibre precursor

Regenerated cellulose fibres, spun from a liquid crystalline precursor, were pyrolysed at temperatures in the range 400–2,500?°C. Raman spectroscopy and X-ray diffraction showed that the degree of graphitisation of the fibre increased with increasing temperature. Electron microscopy, however, suggested that the fibres have a skin–core structure. This observation was confirmed by micro-Raman analysis, whereupon the ratio of the intensities of the D and G bands shows that the skin consists of a graphitised structure, whereas the core consists of significantly less graphitised material. The contributions of the graphitised skin and the inner core to the potential mechanical properties of the fibres were also assessed by following the position of the 2D Raman band during tensile deformation of the fibre. The Raman band shift rate against strain was used to evaluate the fibre modulus, which suggested a modulus of ~140 GPa for the skin and 40?GPa for the core, respectively. If this incomplete graphitisation could be overcome, then there is potential to produce carbon fibres from these novel precursor materials.     

Enhancement of optical contrast on carbon/carbon fibre composites

Journal of Materials Science Letters -     

Preparation of carbon fibre reinforced aluminium via ultrasonic liquid infiltration technique

Abstract

An ultrasonic liquid infiltration technique has been developed for the fabrication of carbon fibre reinforced aluminium (CF/Al) precursor wires. The principal effect of ultrasound on aluminium infiltration into carbon fibres is considered to be caused by cavitation. The acoustic power required to produce cavitation in the present experimental system has been approximately calculated to be about 150 W, which is much greater than the requirement, several tens of watts, for overcoming the capillary pressure among carbon fibres. The observations on the morphology of the CF/Al precursor wires show that there are generally four states of infiltration: totally non-infiltrated, non-infiltrated in the centre and in some local regions of the wires, and completely infiltrated. It is found that carbon fibres can be sufficiently impregnated by molten aluminium given the appropriate application of ultrasound. Furthermore, a single fibre tensile test shows that there is no strength degradation of carbon fibres after aluminium infiltration. The CF/Al precursor wires obtained have an average fibre volume fraction of 26%. The maximum longitudinal tensile strength of the CF/Al wires is 605 MN m^?2, which implies a fibre strength transfer efficiency of 0·76.

MST/1715     

Preparation, characterisation and processing of carbon fibre/polyamide-12 composites for selective laser sintering

Aiming at developing carbon fibre/polyamide-12 (CF/PA) composite powders for manufacturing high-performance components by selective laser sintering (SLS), the preparation, characteristics and sintering process of the composite powders and mechanical properties of sintered components were studied. Surfaces of the carbon fibres were treated by the oxidation modification and coated with polyamide-12 through the dissolution-precipitation process to provide good interfacial adhesion and homogenous dispersion within the polyamide-12 matrix. The particle size and micro-morphology analyses show that the CF/PA composite powders with 30 wt%, 40 wt% and 50 wt% carbon fibres present the suitable powder sizes and format for SLS. The incorporation of carbon fibres into the polyamide-12 matrix decreases the initial melting temperature and consequently lowers the SLS part bed temperatures, implying lower energy requirement and less thermal degradation in the sintering process. The CF/PA composites also represent higher thermal stability than the pure polyamide-12. The CF/PA sintered components with 30 wt%, 40 wt% and 50 wt% carbon fibres exhibit the greatly enhanced flexural strengths by 44.5%, 83.3%, 114%, and the flexural modulus by 93.4%, 129.4%, 243.4%, respectively, as compared with the pure polyamide-12 sintered parts. Fractured surface analysis shows that the carbon fibres are encapsulated and bonded well with the polyamide matrix. The complex SLS parts with the thinnest wall of 0.6 mm, the density of 1.09 ± 0.02 g/cm³ and the relatively density of 94.13 ± 1.72% were manufactured using the CF/PA composite powder with 30 wt% carbon fibres. This study demonstrates that the CF/PA composite powders prepared by the surface treatment and dissolution-precipitation method represent suitable interfacial adhesion, filler dispersion, particle sizes and sintering behaviours for SLS and enable the manufacture of complex components with high performance.     

Fabrication of a carbon fibre/aluminium alloy composite under microgravity

Fabrication of a composite material with ultra low density and high stiffness under microgravity is the objective of the present investigation. The composite structure to be obtained is a random three-dimensional array of high modulus, short carbon fibres bounded at contact points by an aluminium alloy coated on the fibres. The material is highly porous and thus has a very low density. The motivation toward the investigation, simulation experiments, choice of the component materials and in-flight experiment during ballistic trajectory of a National Space Development Agency rocket are described herein. Supporting experimental evidence shows that the cohesion between the carbon fibre and the aluminium alloy is excellent, by which the achievement of desired properties of such composites seems probable.     

Characterizing the fibre/matrix interface of carbon fibre-reinforced composites using a single fibre pull-out test

A single fibre pull-out technique has been used to determine the interfacial bond strength of carbon fibres embedded in epoxy resin. A method is presented whereby the maximum interfacial bond shear stress may be evaluated from the simple measurement of the pull-out force and fibre dimensions. Additional information may also be obtained concerning the interface morphology. The results obtained demonstrate the feasibility of the method and good agreement between theory and experiment.     

Testing of single fibre bundles of carbon/carbon composite materials

A method has been developed for excising single fibre bundles of carbon/carbon material from large billets, and of testing them in axial tension to failure. The fibre bundles are strain gauged so that the tensile modulus and complete stress/strain curves to failure can be obtained. Experimental results are presented and discussed.     

Interface morphology of carbon fibre/PEEK composites

The morphology of high-performance thermoplastic composites (APC-2) based on continuous carbon fibres embedded in a poly-ether-ether-ketone matrix is studied by means of scanning electron microscopy. Samples with different degrees of crystallinity obtained using different thermal treatments are investigated. The effect of the crystallinity content seems to be crucial for fibre/matrix adhesion, as can be detected by SEM analysis.