Tensile recoil measurement of compressive strength for polymeric high performance fibres

Recoil forces acting on the broken ends of a fibre after tensile failure are known to cause substantial damage to polymeric high performance fibres. This damage is the result of compressive stresses developed during snap-back, or recoil, whose magnitude exceeds the compressive strength of the fibre. An analysis describing the axial stress history experienced by a fibre following a tensile failure has been performed and the results have led to the development of a simple, single filament, recoil technique for measuring fibre compressive strength. A number of polymeric high performance fibres were examined using the technique and compressive strengths measured are in excellent agreement with values obtained from composite tests.     

The effect of nanostructure upon the compressive strength of carbon fibres

The relationship between the structure and the compressive strength of carbon fibres has been studied in detail. In order to determine the compressive strength, a combination of single-fibre composite tests and Raman spectroscopy was employed. It was found that the compressive stress–strain curves showed nonlinear behaviour, with modulus softening in compression. The compressive strengths for the fibres with a modulus ≥400 GPa were measured as ≤2 GPa and those with a modulus <400 GPa were >2 GPa. We have introduced a model to explain this behaviour that assumes that the fibres behave as composites consisting of both crystallites and amorphous carbon. It is suggested that the compressive strength is controlled by the critical stress for kinking the crystallites in the fibres. Hence, the compressive strength of carbon fibres is found to depend upon the shear modulus of the fibres and the orientation of the crystallites within them.     

Structure of mesophase pitch-based carbon fibres

The structure of five samples of commercially available carbon fibres with ultra-high modulus produced from mesophase pitch was studied by the complementary techniques of high resolution electronmicroscopy, X-ray diffraction and transverse magnetoresistance effect. The fibres with high strength and elongation to failure were found to be composed of turbostratic carbon structure, which was different from the three-dimensional graphite structure in ultra-high modulus carbon fibres. Transmission electron microscope examination revealed that the mesophase pitch-based fibres with high strength have a basic structure unit with folded sheets arranged nearly parallel to the fibre axis similar to those of high modulus carbon fibres produced from PAN. The present fold structure was suggested to contribute consequently to the lower graphitizability of the fibres and to the strong effects on the fibre strength. By controlling the microstructure, it is expected that the crystallographic as well as the mechanical properties could be improved significantly even from the same kind of precursor materials such as mesophase pitch.     

Orthogonal structure in mesophase pitch-based carbon fibres

Some carbon fibres showed the presence of an orthogonal structure when cross-sections were viewed with light or transmission electron microscopy. This structure was formed from a re-ordering of layer planes as a result of the interaction of mesophase with a stainless steel, square-mesh filter above the spinnerette, using laboratory-scale equipment. Where the mesophase was comparatively viscous the structure persisted and was present in the undrawn and drawn fibre; with mesophase of higher fluidity the orthogonal structure was preserved only in fragmentary or distorted form, or not at all. The structure inferred from these microscopical studies differs from that of an orthogonal pattern previously reported. Fibres spun with a filter appear to have improved mechanical properties, compared to those where no filter Was used.     

The strength of carbon fibres

The available information on the structure and properties of high strength carbon fibres is reviewed, and some new data are presented, showing the effects of boron doping and neutron irradiation on the properties of PAN-based carbon fibres.Theories relating the Young's modulus of the fibre to its microstructure are examined, and it is concluded that their relationship is qualitatively understood. Variations in electrical resistivity with different treatments may also be explained satisfactorily, again, in qualitative terms.The strength of carbon fibres is less well understood, however. It has been suggested that the fibre strength is governed by the presence or absence of stress-raising flaws, but while it is clear that such flaws can markedly reduce the strength, there is no clear estimate of the strength of an unflawed fibre.In this paper, we examine an alternative failure mechanism, initiated by shearing of the graphite crystallites in the fibre, and we conclude that such a mechanism may control the strength of the more graphitic fibres. Increases in the strength of carbon fibres may thus be achieved, not only by reducing the number and severity of the flaws, but also by applying the principles of solid-solution or dispersion hardening, and by reducing the graphite crystallite size (grain refining).     

Microstructure and mechanical properties of pitch-based carbon fibres

The microstructure of a series of mesophase pitch-based carbon fibres have been examined using X-ray diffraction, electron microscopy and Raman spectroscopy. It has been shown that the mechanical properties of the fibres are related directly to the response of this microstructure to deformation and, in particular, that the Young's modulus and tensile strength of the fibres are controlled directly by the fibre microstructure. It has also been shown that Raman spectroscopy can be a useful technique for not only characterizing the microstructure of the fibres but also for following molecular deformation in the fibres. It was found that the position of the 1580 cm^–1 Raman band for the fibres shifted with the application of stress and that the rate of shift per unit strain was proportional to the Young's modulus of the fibres. It was also shown that this reflected the higher degree of stressing of the graphite plane in the higher modulus fibres, consistent with recently developed theories which attempt to explain the dependence of the mechanical properties of carbon fibres upon the degree of orientation of the graphite planes.     

The strength of pitch-based carbon fibre at high temperature

The strengths of some high modulus pitch-based carbon fibres have been determined up to 1300 °C in both air and nitrogen atmospheres. The fibres possessed Young's moduli of 700 GPa and were 10 m in diameter. The strength of the fibres was seen to be gauge length dependent but to a lesser extent than is usual with PAN-based carbon fibres. The fibre strength was observed to increase with temperature as did the Weibull modulus.     

Crack formation in mesophase pitch-based carbon fibres: Part II Detailed structure of pitch-based carbon fibres with some types of open cracks

Several mesophase pitch-based carbon fibres showing radial textures in their transverse alignments were observed by high resolution scanning electron microscope (HR-SEM) to elucidate the principal factor that induces the open crack in the transverse sections of the fibre along its fibre axis. The HR-SEM images of transverse sections exhibited various features in the alignment and shapes of the domains, although they were approximately arranged in the radial direction. The alignment of the domains was often variable in the locations from the outer part to the centre. Linear domains radially oriented in the outer part of the transverse section, that induce the circumferential shrinkage at the spinning and further heat-treatment steps, were essential for the development of open cracks with the PAC-man shaped fibre. Non-radial alignment or non-linear, bent or loop domains in the outer parts prohibited the development of the crack; these are observed in the fibres with the radial skin-random core and the onion skin-radial core alignments. Fibres without cracks in their as-spun or stabilized states usually showed shallow cracks even after the graphitization because the bent and loop domains in the intermediate and centre parts prohibited cracks from propagating into the centre part because the graphitic shrinkage along the domain shapes cannot be linear. This revised version was published online in November 2006 with corrections to the Cover Date.     

Effects of surface and sizing treatments on axial compressive strength of carbon fibres

Attempts have been made to estimate the fibre axial compressive strength of pitch-based graphitized fibres, and the effects of surface- and size-treatment on compressive strength was investigated. The estimated compressive strength of fibres decreases with increasing temperature. This decrease in compressive strength may be accounted for by a decrease in the radial compression force owing to a decrease in the residual thermal stress and a decrease in Young's modulus of the resin matrix. There is a linear relationship between the estimated compressive strength and radial compression force in a temperature range from room temperature to 80 °C. The real compressive strength of the fibres, determined by extrapolating this straight line until the radial compression force is zero, increases with increasing shear yield strength at the fibre-matrix interphase. In order to obtain reinforcing fibres with a higher compressive strength, it will be necessary to surface- and size-treat the fibres.     

Anodic surface oxidation mechanisms of PAN-based and pitch-based carbon fibres

In order to clarify the differences in the anodic surface oxidation mechanisms of PAN-based and pitch-based carbon fibres, the fibres were oxidized in an electrolyte and characterized using the coulostatic method, X-ray photoelectron spectroscopy, laser Raman spectroscopy, and X-ray diffraction. The interfacial bonding strength to an epoxy resin was evaluated based on the interlaminar shear strength (ILSS). The results showed a good correlation between the differential double layer capacities, which were measured with the coulostatic method, and the ILSS values of PAN-based high tensile strength carbon fibres (PAN-HTCFs), PAN-based high modulus carbon fibres (PAN-HMCFs), and pitch-based high modulus carbon fibres (pitch-HMCFs). Their morphologies for the anodic oxidation were as follows: PAN-HTCFs are anodized homogeneously; pitch-HMCFs are selectively oxidized and promote crevice etching; and PAN-HMCFs resist crevice etching due to the many defects in the hexagonal network.     

X-ray measurements and the structure of polyacrylonitrile- and pitch-based carbon fibres

X-ray measurements were carried out on polyacrylonitrile- and pitch-based carbon fibres. The crystallites, disordered regions and microvoids in these carbon fibres were evaluated quantitatively by applying the methods previously proposed by the present authors. The structural parameters evaluated are the 1 1 plane spacing of the carbon layer, the average, standard deviation and asymmetry of the distribution of interlayer spacing, the stacking regularity parallel to the layer plane, the layer extents parallel and perpendicular to the fibre axis, the stacking height, the crystallite orientation, the volume fractions of crystallites, disordered regions and microvoids, the variation of the electron density in a microvoid, and the size parameters of the void cross-section perpendicular to the fibre axis, such as the area, radius of gyration, chord length and thickness. The mutual relationships of these structural parameters are presented, and parameters sensitive to the nature of the starting materials are pointed out.     

Relation between axial compressive strength of reinforcing fibres and fibre diameter

Attempts were made to estimate the fibre axial compressive strength of pitch-based graphitized and polyarylate fibres, and the relationship between the compressive strength and fibre diameter was investigated. The estimated compressive strength of fibres decreases with increasing temperature. This decrease in compressive strength may be accounted for by a decrease in the radial compressing force. There is a linear relationship between the estimated compressive strength and radial compressing force in a temperature range from room temperature to 80°C. The real compressive strength of the fibres, determined by extrapolating this straight line until the radial compressing force is zero, increases with decreasing fibre diameter, but remains almost unchanged at a diameter range smaller than 10 m. In order to obtain reinforcing fibres having a higher compressive strength, it will be necessary to prepare fibres having a diameter smaller than 10 m.     

Electron microscopy study of mesophase pitch-based graphite fibres

A comprehensive electron microscopic investigation of the structure of the graphitic sheet in mesophase pitch-based fibres is presented.In situ brightfield and (00l) darkfield observation of the sheets in sub micrometre fibres reveals a finely striated structure, associated with three-dimensional order. (hkl) darkfield imaging of the sheets in their edge-on and face-on orientations indicates that the striations correspond to the edge view of a mosaic of graphite grains. The grains have lateral dimensions of 100 to 200 nm on average but are only a few atomic layers thick.In situ lattice imaging of the fibre edges indicates a very high level of lattice perfection of the (00l) domains below the fibre surface, quite in line with the outstanding mechanical and thermal properties of this type of fibres. A variety of surface defects are revealed. Preferential orientation effects of the sheet texture on the fibre electron diffraction pattern are described.     

The compressive deformation of cross-linkable PPXTA fibres

A study has been conducted on the compressive deformation behaviour of thermally cross-linkable poly(p-1,2-dihydrocyclobuta phenylene terephthalamide) (PPXTA) fibres. The morphology of the failure zones was examined by scanning electron microscopy and dark-field transmission electron microscopy. On increasing the heat-treatment temperature from 260–400°C, and therefore with increasing cross-link density, fewer kinks per unit length were displayed after compressive deformation. The kink specific energy was estimated to increase by a factor of 30, as determined by quantitative measurements of kink density at a given strain and of the critical strain to kink formation. Thus, the intermolecular cross-links still allowed deformation to proceed by kinking, but significantly raised the energy of kink formation. Finally, rupture zones were predominantly observed in axially compressed PPXTA fibres heat-treated at 440°C. Compressive failure of the fibres changed from kink-dominated failure to brittle rupture with increased heat-treatment temperature, evidently as the result of cross-linking or of chain degradation. A dislocation model of the kink boundary developed by Vladimirov et al. was analysed and critically compared with our data. The analysis of this theory with our experimental results suggested that the dramatic change in compressive behaviour with cross-linking was due to a transition from fine intermolecular shear to blocky interfibrillar shear. This revised version was published online in November 2006 with corrections to the Cover Date.     

碳纤维复合材料层合板压缩性能的相关影响因素

碳纤维作为一种新型的增强材料,具有优良的理化性能,已在多个领域广泛应用.由于单向碳纤维复合材料层合板的压缩强度较低,其在结构复合材料方面的应用受到限制,提高其压缩性能成为关键.本文综述了影响碳纤维复合材料层合板压缩性能的相关因素,详细介绍了纤维弯曲、孔隙率、纤维体积分数、树脂性能等影响因素对碳纤维复合材料层合板的重要性,以及各影响因素之间的关系等,为提高碳纤维复合材料层合板压缩性能的研究提供了参考.     

Structure of round-shaped methylnaphthalene-derived mesophase pitch-based carbon fibres prepared by spinning through a Y-shaped die hole

Structures of as-spun, stabilized, carbonized and graphitized fibres prepared by spinning a methylnaphthalene-derived mesophase pitch through a Y-shaped die hole at 295 °C, was examined by combining optical, scanning electron and transmission electron microscopy from the macro- and microscopic view points. The prepared round-shaped fibre spun through a Y-shaped spinning die hole at 295 °C exhibited excellent tensile and compressive strengths of 410 and 70 Kg mm^–2, respectively, after graphitization at 2500 °C. The stabilized fibre consisted of densely packed anisotropic domains in very random alignment, of which transverse domains and longitudinal features appeared as bent, multi-bent and looped, and endless thin stripes, respectively. The size of domain in the transverse section ranged above 100 nm in length and below 100 nm in thickness, respectively. Further heat treatment (carbonization and graphitization) slightly reduced the dimension and deformed the shape of domains to shrink and to have more sharp edges at their bends according to the graphitic growth; however, the shapes and distribution of domains in transverse section were basically unchanged. High-resolution SEM and TEM observations of the domain confirmed the existence of smaller units of graphitic layers in their assemblies which were more closely arranged in the domain. Such a sub-unit was defined as a micro-domain. TEM revealed that the micro-domain was composed of more than one unit of graphitic layers in the graphitized fibre. Most of them were around 10 nm thick and 10–100 nm long. The thickness of micro-domains was observed to be smaller than the value ofL _c (002), 23 nm, in the same graphitized fibre. Micro-domains have not yet been identified in the stabilized fibre, while TEM suggested some stackings of hexagonal planes. A number of voids (micro- and meso-voids) up to 40 nm diameter were formed at the intra- or inter-domain locations, due to the graphitic shrinkage and evolution of volatile matter by the heat treatments. Micro-voids of around 5 nm diameter were formed within a domain. The better mechanical performances of the present fibre spun through a Y-shaped die hole were ascribed to the homogeneous distribution of looped or bent domains in the transverse section (random nature of transverse alignment). Such a random alignment may also lead to the least number of macro-voids and cracks in the fibril.     

Through-thickness compressive strength of a carbon/epoxy composite laminate

As carbon-fiber-reinforced composite materials are increasingly used for heavy-duty self-lubricating bearings, their through-thickness compressive strength (TTCS) has become an important parameter because the TTCS depends on the weave type and stacking sequence of laminates regardless of their tribological properties.In this work, the TTCS of a carbon/epoxy composite bearing material was measured with respect to weave type, stacking sequence, and direction of cut from the laminate. The tests showed that, for unidirectional laminate, cylindrical specimens resulted in the most reliable data of TTCS. However, for woven fabric laminate, cubic specimens with the edge length greater than twice the repeating unit gave reliable results.     

Mixed resin and carbon fibres surface treatment for preparation of carbon fibres composites with good interfacial bonding strength

The objective of this work is to improve the interlaminar shear strength of composites by mixing epoxy resin and modifying carbon fibres. The effect of mixed resin matrix’s structure on carbon fibres composites was studied. Anodic oxidation treatment was used to modify the surface of carbon fibres. The tensile strength of multifilament and interlaminar shear strength of composites were investigated respectively. The morphologies of untreated and treated carbon fibres were characterized by scanning electron microscope and X-ray photoelectron spectroscopy. Surface analysis indicates that the amount of carbon fibres chemisorbed oxygen-containing groups, active carbon atom, the surface roughness, and wetting ability increases after treatment. The tensile strength of carbon fibres decreased little after treatment by anodic oxidation. The results show that the treated carbon fibres composites could possess excellent interfacial properties with mixed resins, and interlaminar shear strength of the composites is up to 85.41 MPa. The mechanism of mixed resins and treated carbon fibres to improve the interfacial property of composites is obtained.     

Temperature dependent torsional properties of high performance fibres and their relevance to compressive strength

A simple arrangement for the measurement of torsional moduli of high performance fibres as a function of temperature has been reported. Torsional moduli and damping factors have been measured on a number of polymeric [Kevlar, poly(p-phenylene benzobisoxazole) (PBO), poly(p-phenylene benzobisthiazole) (PBZT) and Vectran] and carbon fibres [pitch and PAN based, and one bromine intercalated pitch based carbon fibre] as a function of temperature (room temperature to 150 °C, range) and as a function of vacuum level (1.1–80 ×10³ Pa). At these vacuum levels damping in the fine fibres is mainly due to aerodynamic effects. In general PAN based carbon fibres have higher torsional moduli than pitch based carbon fibres. Kelvar 149, PBO and PBZT fibres have comparable room temperature torsional moduli, while the torsional modulus of Vectran fibre is very low, probably due to the torsional flexibility of the -COO- group. In the above temperature range, torsional moduli of both pitch and PAN based carbon fibres do not change significantly, while for polymeric fibres they decrease; a small decrease is observed for PBO and PBZT, and a significantly higher decrease is observed for Vectran. Relationships between compressive strength and torsional moduli have been discussed