Carbon fibre compressive strength and its dependence on structure and morphology

The axial compressive strength of carbon fibres varies with the fibre tensile modulus and precursor material. While the development of tensile modulus and strength in carbon fibres has been the subject of numerous investigations, increasing attention is now being paid to the fibre and the composite compressive strength. In the present investigation, pitch- and PAN-based carbon fibres with wide-ranging moduli and compressive strengths were chosen for a study of fibre structure and morphology. A rayon-based carbon fibre was also included in this study. Structural parameters (L _c, L_a(0), L _a(90), orientation parameter Z, and the spacing between graphitic planes d(00, 2)) were determined from wide angle X-ray spectroscopy (WAXS). Fibre morphology was characterized using high-resolution scanning electron microscopy (HRSEM) of fractured fibre cross-sections. The mechanical properties of the fibres, including compressive strength, the structural parameters from WAXS, and the morphology determined from HRSEM are reported. The influence of structure and morphology on the fibre compressive strength is discussed. This study suggests that the width of the graphitic sheets, the crystallite size perpendicular to the fibre axis (L _c and L _a(0)), and crystal anisotropy play significant roles in accounting for the large differences in compressive strengths of various carbon fibres.     

Characterization of microvoids in polyacrylonitrile-based carbon fibres

The microvoids in PAN-based carbon fibres covering a wide range of crystallite size were measured by the method using small-angle X-ray scattering on fibre bundles, and the fractional content of voids and the parameters representing the cross-sectional size of voids perpendicular to the fibre axis were determined. The variation in the shape and size distribution of voids with crystallite thickness was considered by introducing an elliptical crosssection model and a cross-sectional size distribution model. The electron density distribution in the inside of a void was also considered. It is concluded that the electron density difference between a void and the solid surrounding the void is relatively larger in the periphery than in the inner part of a void, and that the electron density in the inner part of a void decreases with increasing crystallite thickness.     

Optical properties of vinyon copolymer fibre

Birefringence for vinyon fibres have been investigated. The principal refractive index parallel and perpendicular to the stretch direction were found. Double-beam and multiple-beam interferometry methods were applied. The double-beam technique was applied to the mean birefringence and the mean refractive indices for plane-polarized light parallel and perpendicular to the fibre axis. The diffraction of a He-Ne laser beam was used to measure the dimensional parameters, transverse sectional shape, and area of the fibre. The fibre area was used to calculate the principal refractive indices. The Becke-line method was used to measure the refractive indices of the outer layer of the fibre (skin) for polarized light parallel and perpendicular to the fibre axis. The results were found to be in good agreement with published data.     

The sputtering of rough cylinders; applications to the thinning of fibres for transmission electron microscopy

Thinning by ion-bombardment is a useful technique for the preparation of carbon and other fibres for transmission electron microscopy. It is shown that the presence of axial ridges on the initial fibre leads to the development of striations perpendicular to the axis of the thinned specimen. An analysis of the variation of ion incidence angle on a rotating cylinder enables the change in shape of a fibre on sputtering to be explained; scanning electron microscopy of sputtered fibres confirms these conclusions. A mechanism for the formation of striations perpendicular to pre-existing ridges on the fibre is proposed.     

Finite element study of compressively loaded fibres fractured during impact

This paper examines the compressive behaviour of plies with fibres previously fractured during impact. The analysis is conducted using finite element (FE) modelling in ABAQUS 6.7. Two- and three- dimensional models are used to consider the possibility of fibre penetration and “brooming” of fractured fibres, or fibre buckling. A parametric study of the influence of the input parameters of the Drucker Prager plasticity model was also conducted, to enable a better understanding of the model. This was used to capture the triaxial stress state in the matrix surrounding the fibres. The results suggest that fibre buckling is more likely to occur due to the geometry of fibres and fibre spacing in carbon fibre composites, but fibre penetration could still occur in regions of low fibre volume fraction.     

Transmission electron microscopic investigations on SiC- and BN-coated carbon fibres

Carbon fibres were coated with layers of silicon carbide (SiC) and boron nitride (BN) by conventional chemical vapour deposition. The SiC films were deposited by thermal decomposition of methyltrichlorosilane, whereas the BN films were deposited using the stepwise disproportion reaction of boron chloride with ammonia. Samples for electron microscopic investigations were prepared by separating film from fibre or by conventional mechanical thinning and subsequent ion milling of cross sections of coated fibres. Bright- and dark-field images of both planar and cross-sectional electron microscopic investigations on the fibre coatings gave detailed information on film thickness and morphology. High-resolution images improved the structural information of electron diffraction patterns. Crystal dimensions in the SiC film vary between 10 and 40 nm. Electron diffraction revealed the crystal structure to be a mixture of disordered hexagonal 2H-SiC and cubic -SiC. High-resolution images showed the (1 1 1)-planes to be preferred for deposition. In BN films, a hexagonal turbostratic structure similar to turbostratic carbon was observed. Apart from amorphous regions, nanocrystalline parts were detected, which have a higher structural perfection in the stacking sequence of their (0 0 2)-planes compared to the (0 0 2)-planes of the turbostratic carbon fibre. High-resolution images located the film-fibre interface that was confirmed by electron energy loss spectroscopy.     

Flexural fatigue and surface abrasion of Kevlar-29 and other high-modulus fibres

This investigation deals with some flexural fatigue and abrasion studies of Kevlar-29, glass and carbon fibres. The test methods included in the study are fatigue by pure flexing, buckling and rotation over a wire, and abrasion by rubbing against a rotating rod. Kevlar-29 fibres were found to perform well in these tests because they could survive the relatively high bending strains by yielding in axial compression. Carbon and glass fibres, although unable to survive at these high strains, did perform well when very low bending strains and tensions were used. Kevlar-29 fibres were found to be less abrasion-resistant than glass fibres, probably because of their low radial strength. The fracture morphologies of Kevlar-29 fibres in nearly all these tests showed axial splitting, confirming indications of low strength in the fibre perpendicular to its axis.     

Microstructure and micromechanics of the interface in carbon fibre reinforced Pyrex glass

Detailed microstructural studies have been carried out on composites consisting of Pyrex glass reinforced with carbon fibres. Analysis of the fibre-matrix interface showed that some reaction had taken place during fabrication of the composite and that a carbide or oxycarbide layer had formed between the glass and the carbon fibre. The measured interlaminar shear strength of the composite indicated that the layer was not a source of weakness and appeared to be well bonded to the matrix. Substantial fibre pull-out had occurred, however, to expose clean fibre surfaces and smooth sockets. These observations led to the conclusion that the interfacial shear process was confined substantially to the outer layers of the carbon fibre. Confirmatory evidence for the low interfacial friction stress was available from micro-indentation tests which showed fibre displacement relative to the matrix at loads of less than 10 kPa. Heat treatment of the composite at 500°C in air caused preferential oxidation of the carbon fibre. Where fibres met the specimen surface, oxidation had proceeded down the fibre to produce a smoothly tapering shape. The rate of oxidation was estimated to be 3 m h^–1 parallel to the fibre axis, but much less than this in a direction perpendicular to the fibre, 0.5 m h^–1, due to the relatively slow diffusion rate of oxygen through glass.     

Evaluation of interfacial properties between carbon fibres and semicrystalline thermoplastic matrices in single-fibre composites

The interfacial properties between pitch-based carbon fibre and semicrystalline thermoplastic matrices have been investigated by using the fragmentation test on single-fibre composites. For this purpose, fibres with seven different degrees of surface oxidation were prepared. From the fragmentation test, it was found that oxidization of carbon fibre reduces the fibre fragment length. Further, the length is also influenced by the nature of resin used as matrix. The morphology of crystallites formed on the fibres has been studied. Based on these results, the interfacial properties of carbon fibre and thermoplastic resins are discussed.     

High modulus/high strength poly-(p-phenylene benzobisthiazole) fibres

Wide-angle X-ray diffraction studies of poly-(p-phenylene benzobisthiazole) fibres from Part 1 of this work were undertaken to examine fibre structural changes associated with the heat treatment process and which contribute to the observed significant enhancement of mechanical properties. Crystallite size perpendicular to the fibre axis increases from approximately 2 nm in as-spun fibres to 10 to 12 nm in fibres heat treated at temperatures above 600 C. Fibre tensile strength was found to increase with this increase in the extent of the lateral molecular order. However, tensile modulus and tensile strength did not depend directly on heat treatment parameters but rather indirectly through the effect of applied tension during heat treatment on the overall axial orientation. Higher values of fibre tensile modulus and tensile strength were exhibited by the more highly oriented fibres.     

Stress disturbance due to broken fibres in metal matrix composites with non-uniform fibre spacing

Premature fracture of weaker fibres causes stress disturbances in composites. These disturbances are affected by non-uniformity of fibre spacing. In order to evaluate quantitatively how the disturbances in metal matrix composites are affected by the extent of non-uniformity of fibre spacing, a method of calculation is presented on the basis of two-dimensional shear lag analysis. Static tensile stress concentrations in the intact fibres to broken fibres, tensile stress distribution along the fibre axis in the broken and intact fibres and shear stresses between broken and intact fibres were calculated by the method presented, using some examples. It is shown quantitatively that the spacing between broken and intact fibres and that between intact and next fibres has a significant influence on tensile stress concentrations in intact fibres and also on the shear stresses between broken and intact fibres: the narrower the former spacing and the wider the latter spacing, the higher become both tensile and shear stress concentrations. This tendency is enhanced when the number of broken fibres is large and when the strain hardening of the matrix is high.     

Static and fatigue characterisation of new basalt fibre reinforced composites

Basalt reinforced composites are recently developed materials. These mineral amorphous fibres are a valid alternative to carbon fibres for their lower cost, and to glass fibres for their strength. In order to use basalt reinforced composites for structural applications, it is necessary to perform a mechanical characterisation. With this aim in the present work experimental results of several static and fatigue tests are described. Two polymeric matrices are taken into account, vinylester and epoxy, to assess their influence on the evaluated parameters. In parallel to these mechanical tests, also the thermal answer of the specimens to mechanical loads is evaluated by means of thermography. This experimental technique allows defining the composite local heating during the application of mechanical loads and its behaviour in details. Final discussion on obtained results is proposed focussing the attention on basalt fibre composite behaviour, and comparing mechanical properties of BFRP with other composite materials in glass and carbon fibres.     

The crystallization and interfacial bond strength of nylon 6 at carbon and glass fibre surfaces

The nucleation and crystallization of nylon at the interface in glass-fibre and carbon-fibre reinforced nylon 6 composites has been investigated by electron microscope studies of sectioned and etched bulk specimens and solution cast and melt crystallized thin films. The fracture energies of the composites were obtained from tensile strength tests and the interfacial bond strengths were calculated from fibre pullout measurements. The fibres are shown to nucleate a columnar structure at the interface with marked differences between the structures nucleated by glass fibres and by carbon fibres and also between that nucleated by type I and type II carbon fibres. The structure around glass fibres was non-uniform and influenced to some extent by the presence of the size coating on the fibre surface. In the carbon-fibre composites the columnar structure was due primarily to physical matching of the graphite crystallites. Surface treatment of the carbon fibres to improve chemical bonding is shown to have a significant effect on bond strength which cannot be explained in terms of the columnar structure at the fibre surface. The treated fibres gave rise to only small amounts of fibre pull-out and low fracture energies whereas the untreated fibres showed extensive pull-out which was reflected in high fracture energies.     

Microstructural inhomogeneity in carbon fibres

Variations in the layer plane configurations of PAN-based carbon fibres have been studied. The predominant configuration in the surface zones of all fibres is a general circumferential alignment. In highly graphitized fibres, the type of internal structure depends sensitively on fibre thickness. A duplex structure, comprising a core of radially oriented graphite layers and an outer circumferentially aligned zone, is only detected in fibres having diameters of approximately 8m. Thicker fibres contain more complex configurations, while thin fibres consist entirely of circumferentially aligned layers. Structural parameters associated with each layer alignment are reported.     

Reflection from natural fibres: Determination of the scale angle profile

Light of wavelength 632.8~nm scattered from a single natural fibre onto an observation plane perpendicular to the fibre axis was observed to form rings which converge to a point defined by the incident beam. Using a series cone model for natural fibres, these rings are shown to be due to reflection from a curved surface inclined at some angle to the fibre axis, such as scales found on the fibre surface. Measurements of relative intensity and position associated with prominent rings in this observation plane were made. These measurements were used to determine all possible scale angles illuminated by the incident laser beam. Results show that although some 300 scales were illuminated only a few intensity peaks corresponding to prominent rings were detected. Therefore, suggesting that a few prominent scales occur repeatedly within the illuminated section on the fibre. Results also show that both positive and negative scale angles exist along a single fibre. Ranges of scale angles obtained for coarse, fine and super fine wool, mohair and cashmere fibres were found to be: 0.2° to 8.6° ± 0.05°, 0.5° to 7.6° ± 0.05°, 2.2° to 3.0° ± 0.05°, 0.1° to 3.6° ± 0.05°, 0.1° to 2.6° ± 0.05° and respectively.     

Axial compression fracture in carbon fibres

Axial compression fracture of carbon fibres was studied by embedding single fibres in epoxy resin and compressing the specimens parallel to the fibre axis. By careful optical monitoring of the fibre surface the earliest stages of fracture were identified leading to estimates of the fibre axial compression failure strengths. Compression strength decreases markedly from about 2.2 GN m^?2 for moderately oriented fibres to <1 GN m^?2 for highest modulus filaments. The trend towards decreasing compression strength with increasing anisotropy is explained on the basis of an increasing fibre microfibrillar nature. However fracture morphology studies show that the unduly rapid strength decrease results from an increasing degree of fibre outer layer ordering which accompanies increasing axial anisotropy in carbon fibres since cracking occurs first on the more highly aligned filament surfaces. It is suggested that fibre compression fracture changes from a shear to a microbuckling or kinking mode with increasing fibre anisotropy, where the latter initiates in individual, well-aligned but uncoupled microfibrils. The similarity of fine axial compression fractures in oriented carbon fibres to those found in elastica loop experiments is noted as are the possible implications which the low strain-to-failure in compression of very high modulus fibres might have for practical composites.     

Off-axis fatigue cracking behaviour in notched fibre metal laminates

The off‐axis fatigue cracking behaviour of notched fibre metal laminates under constant amplitude loading conditions was investigated experimentally and numerically. It was found that the off‐axis fatigue crack initiation life decreased as the off‐axis angles increased. This indicated that the off‐axis laminates raised the applied stress level in the aluminium (Al) layer and subsequently resulted in earlier cracking in the Al layer. The off‐axis fatigue crack initiation lives of notched fibre metal laminates were predicted using lamination theory and an energy‐based critical plane fatigue damage analysis from the literature. After a crack initiated in the Al layer, it was observed that the crack path angles of the off‐axis specimens were neither perpendicular to the fibre nor to the loading direction. A finite‐element model was established for predicting the crack path angles.     

Sinusoidal Microbending Loss of Orthogonal Polarized Modes in a Single-mode Step-index Fibre

Sinusoidal microbending losses of the HE₁₁ orthogonal polarized modes in a single-mode step-index fibre are studied and compared. The loss of the mode with polarization in a plane perpendicular to the plane of microbending is found to be higher than the loss of the orthogonal polarized mode with polarization in a plane parallel to the plane of microbending. The microbending loss difference of these two orthogonal polarized modes is more significant around the perpendicular directions with respect to the fibre axis than that in nearly parallel directions corresponding to different length periods of sinusoidal microbending.     

Influence of spatial correlations on crack bridging by frictional fibres

The effect of fibre interaction on matrix cracking in a unidirectional fibre-reinforced composite is analyzed. It is assumed that the matrix material contains a crack in a plane perpendicular to the fibres. Fibres, remaining intact, debond from the matrix and then act as bridging ligaments in the crack wake. The debonding process is accompanied by frictional sliding governed by a Coulomb friction law. Fibres are considered to be randomly located in the transverse plane. The fibre axial stress and longitudinal displacement are expressed in terms of the solution to a model problem for a single fibre in an ambient stress field due to all other fibres and applied load. The stress field produced by the other fibres is described using an ensemble averaging procedure. The radial distribution function g(r) that provides a quantitative measure of the correlations between the positions of different fibres is evaluated numerically from the Percus-Yevick equation for hard disks. The dependence of the fibre axial stress on the relative fibre-matrix displacement is examined for different values of the volume fraction of fibres. The resulting stress-displacement law is compared with results for other choices of the function g(r) and with a law given by a concentric cylinder model.     

Structural change of carbon-fibres at high temperatures under load

Creep tests were performed with carbon fibre‐bundles (Toho Tenax, HTA 5131) in a temperature range from 1500 to 1800 °C. The fibre‐bundles were electrically heated in vacuum (pressure 10^?4 mbar) and tensile load was applied in a hydraulic testing machine. The creep parameters were obtained from varying temperature and loading conditions. Accompanying structural investigations were performed with X‐rays (SAXS and WAXD). An increasing orientation of the graphene planes along the fibre (and thus the loading axis) could be observed with increasing temperature and load. No structural change and no creep, however, was observed for carbon fibres stabilized by an appropriate heat treatment.