Influence of Fabric Parameters on Microstructure,Mechanical Properties and Failure Mechanisms in Carbon-Fibre Reinforced Composites

The effects of fibre/matrix bonding, fabric density, fibre volume fraction and bundle size on microstructure, mechanical properties and failure mechanisms in carbon fibre reinforced composites （plastic and carbon matrix） have been investigated. The microstructure of unloaded and cracked samples was studied by optical microscopy and scanning electron microscopy （SEM）, respectively whereas the mechanical behaviour was examined by 3- point bending experiments. Exclusively one type of experimental resole type phenolic resin was applied. A strong fibre/matrix bonding, which is needed for high strength of carbon fibre reinforced plastic （CFRP） materials leads to severe composite damages during the pyrolysis resulting in low strength, brittle failure and a very low utilisation of the fibres strain to failure in C/C composites. Inherent fabric parameters such as an increasing fabric density or bundle size or a reduced fibre volume fraction introduce inhomogenities to the CFRP＇s microstructure. Results are lower strength and stiffness whereas the strain to failure increases or remains unchanged. Toughness is almost not affected. In C/C composites inhomogenities due to a reduced bundle size reduce strain to failure, strength, stiffness and toughness. Vice versa a declining fibre volume fraction leads to exactly the opposite behaviour. Increasing the fabric density （weight per unit area） causes similar effects as in CFRPs.     

Surface analysis of carbon fibres modified with PVAL coating and the composite interfaces

High-resolution X-ray photoelectron spectroscopy (XPS) has been used to analyse the fibre surface and composite interfaces with and without polyvinyl alcohol (PVAL) coating (both fibres being commercially surface-treated and sized). Major functional groups on the fibre surface are also identified by Gaussian curve-fitting of carbon peaks to study the correlations of surface chemistry with the observed failure mechanisms of the uncoated and coated fibre composites. The main difference in the fracture surface of the fibre composites with and without the coating is that the latter has a significant amount of silicon (about 6 at% concentration) associated with the epoxy matrix, but silicon is almost absent in the PVAL-coated fibre composites. This suggests that the debonding mechanism in the uncoated fibre composite, which has a strong interfacial bonding, is controlled by the combination of cohesive failure of the matrix material and adhesive failure at the interface. In contrast, the PVAL coating promotes adhesive failure due to the weak bonding at the fibre-matrix interface. This observation is consistent with SEM observations in that the uncoated fibre composite consists of significant deformation of matrix material which covers the majority of the fracture surface and tiny epoxy resin particles adhering to the debonded fibre surface, whereas the coated fibre composite shows a lesser amount of matrix deformation with relatively clean fibre surface.     

Polyamide coating on carbon fibres and potential application in carbon/kevlar/epoxy hybrid composites

Hybrid unidirectional composite materials, consisting of alternately laminated layers of Kevlar-49 fibres and carbon fibres in an epoxy resin, have been studied. Before embedding, the carbon fibres were coated with a Nylon 6,6 film by an interfacial in-situ polymerization technique. Emphasis is given to the mechanical properties of the hybrid composites based on coated carbon, with those based on uncoated carbon, for various values of partial volume fraction of the carbon fibres, V_cf, polyamide content deposited on the carbon fibres, C_N, and total fibre (Kevlar + carbon) volume fraction, V_f.     

Failure micromechanisms in continuous carbon-fibre/epoxy-resin composites

This study is concerned with the influence of the strength of the fibre/matrix interface on the strength and failure process in uniaxial arrays of carbon fibres in an epoxy resin. A batch of high-strength carbon fibres has been supplied with several levels of an oxidative surface treatment to produce composites with various interface strengths. Tensile tests have been conducted on single fibres, on loose bundles and on tows impregnated with an epoxy resin. Further tests have been conducted to estimate the interface strength. A hybrid-tow test configuration has then been used to follow the sequence of failure within a single tow of the carbon fibre in a uniaxial composite. The results indicate that the fibre strength is affected only slightly by the surface treatment, the strength of impregnated tows is reduced, and their mode of failure and that of the hybrid tows is affected significantly.     

Graphite-fibre/carbide-matrix composites—II: Structure and electrical properties

The measurement of electrical properties of ceramic-matrix composites supplies data which can be used directly with information about the structure of the composites. The structures of graphite-fibre/carbide-matrix composites may be varied within large intervals of appropriate parameters, as shown in a previous publication. A corresponding variety of behaviour patterns of such materials in the electrical field had been expected and was actually observed. The matrices of the composites were boron, niobium, and tantalum carbides, and the fibres were Kulon and VMN-4. The electrical conductivity of graphite-fibre/carbide-matrix composites has been determined, including that at cryogenic temperatures. A preliminary series of experiments gives the characteristics of piezo-resistance of the C/NbC composites. The experimental data yield a set of characteristics of the fibre, matrix, and composite structure. The conductivity characteristics of the graphite fibres and carbide matrices obtained in this way correspond to the expected ones. That also includes a type of conductivity behaviour revealed by the temperature dependence of the conductivity. Graphite fibre behaviour is of the metallic type, while boron carbide is a semiconductor-type material. The conductivity of pure matrices differs essentially from that of the matrices obtained as an extrapolation of the conductivity versus fibre volume fraction dependence to zero fibre content. Active diffusion of carbon from the fibre/matrix interface makes the stoichiometry of a carbide better and its conductivity higher. But a distinctive dependence of the piezo-resistance of the C/NbC composites on the fibre volume fraction may lead to an assumption about the possibility of the existence of another mode of influence of the carbon fibres on the conductive properties of carbides. Calculation of the ineffective length of a fibre (from the viewpoint of conductivity) yields an estimate of the conductivity of the interface. It is clearly connected to the structure of the interface.     

Interface characterization by TEM,AES and SIMS in tough SiC (ex-PCS) fibre-SiC (CVI) matrix composites with a BN interphase

The fibre-matrix interfacial zone formed during the isothermal/isobaric chemical vapour infiltration processing of SiC fibres (ex-polycarbosilane)/boron nitride/SiC matrix composites has been analysed by TEM/electron energy loss spectroscopy, Auger electron spectroscopy, and secondary ion mass spectroscopy. In the composites, the boron nitride interphase (deposited from BF₃-NH₃) is made of turbostratic boron nitride, almost stoichiometric but containing some oxygen (less than 5 at %). The boron nitride layer stacks are randomly orientated except in the very vicinity of the fibre surface where they lie almost parallel to the substrate. The long chemical vapour infiltration treatment at 1000 °C used to infiltrate the SiC matrix acts as an annealing treatment for the metastable ex-polycarbosilane fibres which gives rise to the growth of an SiO₂/carbon amorphous double layer at the boron nitride/fibre interface. Deflection of microcracks arising from the failure of the brittle SiC-matrix occurs at the boron nitride/SiO₂ interface considered to be the weaker link in the matrix/boron nitride interphase/SiO₂/carbon/fibre sequence. It is suggested that the combination of the boron nitride layered interphase and SiO₂/carbon fibre decomposition products might play an important role in determining the propagation path of microcracks in the fibre/matrix interfacial zones and could be responsible, at least to some extent, for the non-brittle behaviour of such composites.     

Suppression of crack growth in hybrid fibre composites

A theoretical analysis based on the assumed form of the strain field surrounding a crack bridged by reinforcing elements has been used to examine the growth of a crack propagating transversely to the fibres in hybrid fibre composites. An intermingled carbon fibre/glass fibre polymer matrix system has been considered. Two situations have been investigated. In the first of these the effect of the addition of carbon fibres on the development of cracks resulting from the failure of the glass fibres by stress corrosion has been studied. The analysis indicates that crack growth can be severely inhibited by a 5% volume fraction of type III carbon fibres. The analysis has been used also to investigate the process by which strong high failing strain glass fibres inhibit the growth of cracks caused by the fracture of localized clusters of low failing strain carbon fibres. The predictions of this analysis agree with existing experimental data on glass fibre/carbon fibre hybrids.     

Oberflächenbehandlung von Kohlenstoffasern und mechanische Eigenschaften von Laminaten

Surface Treatment of Carbon Fibres and Resulting Composite Properties In composites carbon fibres are used as reinforcing fibres with thermosetting and thermoplastic resins as martices. These carbon fibres differ strong in their micro-structure and therefrom in fibre properties. To achieve sufficiant composite properties special carbon fibre surface treatment methods are necessary. This paper describes a systematic study on oxidative surface treatment of carbon fibres by wet-, dry- and anodic oxidation. Further investigations by matrix variation show us the influence of matrix strength on the mechanical composite properties. Finally it is shown that in case of impact load composite fracture behaviour is controlled only by the fibre itself.     

Fabrication and testing of natural fibre composites: Vakka, sisal, bamboo and banana

A study has been carried out to investigate the tensile, flexural and dielectric properties of composites made by reinforcing vakka as a new natural fibre into a polyester resin matrix. The fibres extracted by retting and manual processes have been used to fabricate the composites. These composites are tested for tensile, flexural and dielectric properties and compared with those of established composites like sisal, bamboo and banana made under the same laboratory conditions. The composites are fabricated up to a maximum volume fraction of fibre of 0.37 in the case of tensile testing, and 0.39 for flexural and dielectric testing. It has been observed that the tensile properties increase with respect to volume fraction of fibre for vakka fibre composite and are also more than those of sisal and banana composites and comparable to those of bamboo composites. The flexural strength of vakka fibre composite is more than that of banana composite and is closer to sisal fibre composite with respect to the volume fraction of fibre, where as the flexural modulus is much higher than those of banana and sisal fibre composites and also very much closer to bamboo fibre composites. The dielectric strength of vakka fibre composite increases with increase in volume fraction of fibre in the composite unlike the case of sisal, bamboo and banana composites. The dielectric strength being a unique feature of vakka fibre composite, can be suggested for electrical insulation applications.     

The influence of fibre surface properties on the mode of failure in carbon-fibre/epoxy composites

A series of mechanical tests have been performed on composites consisting of high-strength carbon fibres in an amine-cured epoxy resin. A comparison has been made between composites containing untreated, commercially treated (electrochemically), and plasma treated fibres. While both treatments improve interfacial adhesion, the manner in which the composite fails is totally different. In composites that contain electrochemically treated fibres failure is, in most cases, matrix dominated, whereas interfacial failure always occurs in samples made from plasma-treated fibres. This behaviour can be explained in terms of the nature of the fibre surface after each type of treatment.     

Failure characteristics in carbon/epoxy composite tows

In this paper the failure mechanisms of unidirectional aligned carbon fibre/epoxy composites are investigated. Experimental results are presented for the strength of carbon/epoxy composite tows, as well as for single carbon fibres supplied in the sized and unsized condition. Laser Raman spectroscopy was used in this study to assess the effect of fibre breaks on the stress distribution within a composite. Fibre stress mapping of composite tows using laser Raman spectroscopy showed redistribution due to fibre failure and a value of the stress concentration factor, K_r, was obtained. The results were analysed using a Weibull distribution for the strength of the reinforcing fibres and composite.     

In State of Art: Mechanical and tribological behaviour of polymeric composites based on natural fibres

In this article, a comprehensive literature review on the mechanical and tribological behaviour of polymeric composites based on natural fibres is introduced. The effects of volume fraction, orientations, treatments and physical characteristics of different types of natural fibres on the mechanical and tribological properties of several thermoset and thermoplastic polymers are addressed. The effects of the tribological operating parameters (applied load, sliding velocity and sliding distance) on the frictional and wear performance of natural fibre polymer composites are demonstrated. The collected date and analyses revealed that volume fraction, orientations, type of treatment and physical characteristics of the natural fibres significantly influence the mechanical and tribological behaviour of composites. The most influence key in designing natural fibre/polymer composite is the interfacial adhesion of the fibre with the matrix. NaOH chemical treatment found to be the most useful treatment method to enhance the interfacial adhesion of the natural fibres with the matrix, while other techniques exhibited either no effect or deterioration on the fibre strength. Frictional characteristics of the natural fibre composites are poor and solid lubricants are recommended to reduce the friction coefficient of the materials.     

Tensile strength of discontinuous fibre-reinforced composites

A stochastic Monte-Carlo approach, based on Eyring's chemical activation rate theory, is used to study the factors controlling the tensile strength of discontinuous fibre-reinforced composites. The model explicitly takes into account the local distribution of stress near fibre ends. Both the fibre and the matrix are allowed to break during fracture of the composite. The stress-strain curves and the modes of failure of the composite are found to be strongly dependent on the volume fraction and aspect ratio of the fibres. The importance of adhesion at the fibre/matrix interface is also studied. The results are compared with available experimental data.     

Surface oxidation of carbon fibre on tribological properties of PEEK composites

Abstract

In this work, ozone modification method and air oxidation were used for the surface treatment of polyacrylonitrile (PAN) based carbon fibre. The surface characteristics of carbon fibres were characterised by X-ray photoelectron spectroscopy. The interfacial properties of carbon fibre reinforced PEEK (CF/PEEK) composites were investigated by means of the single fibre pull-out tests. As a result, it was found that IFSS values of the composites with ozone treated carbon fibre are increased by 60% compared with that without treatment. X-ray photoelectron spectroscopy results show that ozone treatment increases the amount of carboxyl groups on carbon fibre surface, thus the interfacial adhesion between carbon fibre and PEEK matrix is effectively promoted. The effect of surface treatment of carbon fibres on the tribological properties of CF/PEEK composites was comparatively investigated. Experimental results revealed that surface treatment can effectively improve the interfacial adhesion between carbon fibre and PEEK matrix. Thus the wear resistance was significantly improved.     

Characterization of composite materials made from discontinuous carbon fibres within the framework of composite recycling

Recycling carbon fibres from waste composite materials would only be efficient if it were possible to separate the fibres and the matrix and to re-use the recycled fibres as new reinforcements. The challenge is to use non-continuous fibres to produce high-strength materials. The formation of defects in “semi-long” fibre composites has not yet been taken into account. In this paper the influence of fibre length and fibre alignment on the strength and the modulus of composite materials is illustrated. It is shown that the presence of defects may be modelled in order to understand what the quality of a second generation composite material would be.     

The impact strength of fibre composites

Two models have been developed which predict the crack initiation energy, notched impact strength and unnotched impact strength of fibre composites. One is applicable to composites containing short fibres and the other to composites containing long fibres. Data obtained with randomly oriented short fibre composites were consistent with the one model. The other model has been verified using composites containing uniaxially oriented long fibres and long fibres oriented randomly in a plane. The success of the model demonstrates that the high notched impact strength with long fibres is due to the redistribution of stress away from the stress concentrating notch, the extra stress that can be held by the fibre relative to the matrix and the work required to pull fibres out of the matrix during crack propagation. The parameters which have been shown to control the fracture energy are composite modulus, fibre length, fibre volume fraction, effective fibre diameter, fibre tensile strength and the coefficient of friction during fibre pull-out from the matrix. The matrix toughness on the other hand usually has no effect at all for composites containing fibres randomly oriented in two dimensions and only a minor effect in exceptional cases. The shear strength of the fibre-matrix bond has only an indirect effect in that it controls the number of fibres which pull out rather than fracture.     

Failure mechanisms and fracture energy of hybrid materials

A shear-lag model of hybrid materials is developed. The model represents an alternating arrangement of two types of aligned linear elastic fibres, embedded in a linear elastic matrix. Fibre and matrix elements are taken to fail deterministically when the axial and shear stresses in them reach their respective strengths. An efficient solution procedure for determining the stress state for arbitrary configurations of broken fibre and matrix elements is developed. Starting with a single fibre break, this procedure is used to simulate progressive fibre and matrix failure, up to composite fracture. The effect of (1) the ratio of fibre stiffnesses, and (2) the ratio of the fibre tensile strength to matrix shear strength, on the composite failure mechanism, fracture energy, and failure strain is characterised. Experimental observations, reported in the literature, of the fracture behaviour of two hybrid materials, viz., hybrid unidirectional composites, and double network hydrogels, are discussed in the framework of the present model.     

Evaluation of interface fracture energy for single-fibre composites

Raman and luminescence spectroscopy have been used for the first time to determine the interface fracture energy for single-fibre composites. By using the measured fibre stress distributions in single-fibre fragmentation composite specimens and a simple energy-balance scheme, the energy for the initiation of interfacial debonding has been estimated for carbon (T50) and α-alumina (PRD-166 and Nextel 610) fibres embedded in epoxy resins. It has been found that the interface fracture energy shows good sensitivity to changes in the level of fibre/matrix adhesion due to surface treatment and sizing of the fibres. It is also found that the values of interface fracture energy correlate well with measured values of interfacial shear strength determined for the same fibre/matrix systems.