Compressive properties of single-filament carbon fibres

Compressive properties of mesophase pitch-based carbon fibres (NT-20, NT-40 and NT-60) were measured using the tensile recoil test and the elastica loop test. The NT-40 fibre with a 400 GPa tensile modulus showed a smaller loop compressive yield strain and a larger recoil compressive strength compared to these values obtained from the longitudinal compression test on its unidirectional composites. Further, the recoil compressive strength of this fibre was higher than that of PAN-based carbon fibre with a corresponding modulus. Under the ideal conditions in the tensile recoil test, the strain energy was conserved before and after recoil, and the initial tensile stress and the recoil compressive stress do not coincide when fibre stress-strain behaviour is non-linear, and the non-linearity in compression and in tension is different. The difference between the composite compressive strength and the recoil compressive strength of NT-40 was quantitatively explained by taking account of the fibre compressive stress-strain non-linear relation. The difference between the loop compressive yield strain and the composite compressive strain to failure was also explained by this non-linearity.     

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.     

Compressional behaviour of carbon fibres

Spectroscopic-mechanical studies have been conducted on a range of carbon fibres by bonding single filaments on the top surface of a cantilever beam. Such a loading configuration allows the acquisition of the Raman spectrum of carbon fibres and the derivation of the Raman frequency strain dependence in tension and compression. Strain hardening phenomena in tension and strain softening phenomena in compression were closely observed. The differences in the slopes of the Raman frequency versus applied strain curves in tension and compression respectively, have been used to obtain good estimates of the compression moduli. A method of converting the fibre Raman frequency versus strain data into stress-strain curves in both tension and compression, is demonstrated. Values of fibre stress and fibre modulus at failure in compression compare exceptionally well with corresponding estimates deduced from full composite data. The mode of failure in compression has been found to depend upon the carbon fibre structure. It is demonstrated that certain modifications in the manufacturing technology of PAN-based fibres can lead to fibres which show resistance to catastrophic compressive failure without significant losses in the fibre compressive modulus.     

A novel method for measuring direct compressive properties of carbon fibres using a micro-mechanical compression tester

A novel method has been developed for measuring direct compressive properties such as strength and elasticity of a series of mesophase-pitch-based and PAN-based carbon fibres about 10 m in diameter by uniaxial and transverse compression tests using a micromechanical tester. The fibres were shaped into cylindrical specimens, with their size ratio of length to diameter kept at about 2 to 3, by separating them from a thin film made by polishing the cut faces of a strand of carbon fibres with epoxy resin as a matrix. Individual cylindrical specimens were stood up or laid down on a glass plate without any fixer for the measurements of axial and transverse compression properties of fibres, respectively. The fibres exhibited non-linear elasticity, with the compressive modulus decreasing with compressive deformation. The direct axial compressive strengths of pitch-based carbon fibres were found to be marginally lower than the indirect ones, whereas there was no significant difference between the two strength values for PAN-based fibres. The pitch-based fibres exhibited smaller average values of axial compressive strength than the PAN-based fibres. The transverse compressive strength, which decreases with an increase in elasticity of carbon fibres, exhibited a considerably lower average value than that of the axial compressive strength. Further, the axial compressive strength was found to be smaller than the direct tensile strength for the fibres.     

Compressional behaviour of carbon fibres

The axial compressive strain to failure of various types of PAN-based carbon fibres was measured by applying small and defined compressive loads to single filaments which have been bonded to a rectangular polymer cantilever beam and parallel to its long edge. By monitoring the Raman frequencies along the fibre with the 2 m laser probe of a Raman microscope, the critical compressive strain required for first fibre failure could be assessed and the residual load that each type of fibre supported after first failure, could be measured. Estimates of the compressive moduli for all fibres could, also, be obtained by considering the dependence of the Raman frequency upon compressive strain in the elastic region. The critical compressive strain to failure was found to decrease with fibre modulus and its value was, approximately, equal to 50% of the tensile fracture strain. However, for some low-modulus fibres the compressive strain to failure was found to approach the tensile fracture strain. The initial compressive moduli of high-modulus fibres were estimated to decrease up to a maximum of 10% with respect to their tensile moduli, whereas more significant reductions were found in the case of intermediate and low-modulus fibres.     

The effect of gauge length on tensile strength and Weibull modulus of polyacrylonitrile (PAN)- and pitch-based carbon fibers

Carbon fibers are widely used as a reinforcement in composite materials because of their high specific strength and modulus. Current trends toward the development of carbon fibers have been driven in two directions; ultrahigh tensile strength fiber with a fairly high strain to failure (~2%), and ultrahigh modulus fiber with high thermal conductivity. Today, a number of ultrahigh strength polyacrylonitrile (PAN)-based (more than 6 GPa), and ultrahigh modulus pitch-based (more than 900 GPa) carbon fibers have been commercially available. In this study, the tensile strengths of PAN- and pitch-based carbon fibers have been investigated using a single filament tensile test at various gauge lengths ranging from 1 to 250 mm. Carbon fibers used in this study were ultrahigh strength PAN-based (T1000GB, IM600), a high strength PAN-based (T300), a high modulus PAN-based (M60JB), an ultrahigh modulus pitch-based (K13D), and a high ductility pitch-based (XN-05) carbon fibers. The statistical distributions of the tensile strength were characterized. It was found that the Weibull modulus and the average tensile strength increased with decreasing gauge length, a linear relation between the Weibull modulus, the average tensile strength and the gauge length was established on log–log scale. The results also clearly show that for PAN- and pitch-based carbon fibers, there is a linear relation between the Weibull modulus and the average tensile strength on log–log scale.     

High-performance aromatic polyimide fibres

A new polyimide has been synthesized from 3,3,4,4-biphenyltetracarboxylic dianhydride (BPDA) and 2,2-dimethyl-4,4-diaminobiphenyl (DMB). A high-strength, high-modulus, high-temperature fibre has been developed from this polyimide via a dry-jet wet spinning method. The tensile strength of BPDA-DMB fibres is 3.3 GPa and the tensile modulus is around 130 GPa. The compressive strength of the fibres has been investigated through a tensile recoil test (TRT), while the fibre morphology after compression has been studied via polarized light microscopy (PLM) and scanning electron microscopy (SEM). From the TRT measurements, we have observed that the compressive strength of this fibre is 665 (±5) MPa, which is higher than those of other aromatic polymer fibres. The effect of fibre diameter on the compressive strength of BPDA-DMB fibres is not substantial. The critical compressive strain for this fibre at which the kink bands start appearing under the observation of PLM is at 0.51–0.54%. Subglass relaxation processes have been observed and the measure of an apparent relaxation strength may serve as one of the factors which significantly affect the compressive strength of the fibres. Tensile tests of pre-compressed fibres reveal a continuous loss in tensile strength (up to 30%) with increasing the compressive strain (up to 2.6%). PLM and SEM observations show that during the compression BPDA-DMB fibres form regularly-spaced kink bands at ±60 ° (±2 °) with respect to the fibre axis. The kink band density initially increases with the compressive strain, and reaches a maximum at around 1.1%. Further increase of the compressive strain decreases this density due to the merge of the neighbouring bands. The size of kink bands also correspondingly increases within this compressive strain region. The morphological observation via SEM implies the existence of a skin-core structure and microfibrillar texture which are common features in polymer fibres.     

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.     

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.     

The recoil compressive strength of pitch-based carbon fibres

The recoil compressive strengths of pitch-based carbon fibres with folded-radial and flat-layer textures were compared. Using the recoil test method, it was shown that the compressive strengths of pitch-based carbon fibres with folded-radial textures are superior to pitch-based carbon fibres with flat-layer textures at all modulus levels. Analysis of the failed fibre ends revealed that the folded-radial texture appeared to inhibit shearing of the basal planes. This may account for the superior compressive strengths of pitch-based carbon fibres with folded-radial textures. Procedural differences in the recoil test were shown to influence the calculated recoil compressive strength of pitch-based carbon fibres. Microscopic analysis of the recoil test specimens revealed that some energy is absorbed in the area where the fibre is secured to the support tab.     

Compressive and torsional behaviour of Kevlar 49 fibre

The mechanical anisotropy of an aromatic polyamide fibre, Kevlar 49, was studied in tension, compression and torsion. A new technique involved applying small and defined compressive strains to filaments by bonding them to one side of a beam which is subsequently bent to compress the fibres. Using scanning electron and optical microscopy, fibres were shown to form regularly-spaced helical kink bands at 50 to 60° to the fibre axis after the application of small axial compressive strains. Tensile tests of previously-compressed fibres revealed only a 10% loss in tensile strength, after application of as much as 3% compressive strain. A torsion pendulum apparatus was used to measure the shear modulus and an apparent shear strength of fibres. A loss of tensile strength after the application of large (> 10%) torsional shear strains coincided with a loss in recoverable shear strain due to longitudinal fibre splitting. Ratios of tensile-to-compressive strength, tensile-to-shear strength and tensile-to-shear moduli of 51, 171, and 701, respectively, were measured for Kevlar 49.     

Effect of magnesium on the composition,microstructure and mechanical properties of carbon fibres

This work was undertaken in order to provide more detailed information on the chemical and mechanical behaviour of carbon fibres during the elaboration of graphite-magnesium composite materials. For this purpose, PAN-based T300, pitch-based P55 and P100 carbon fibres were isothermally heat treated, at temperatures ranging from 450 to 700 °C, under a saturated vapour pressure of magnesium. The composition, microstructure and tensile strength of the resulting samples were characterized by chemical and electron probe microanalysis, Raman spectrometry, X-ray diffraction and mechanical test of single filaments. From the results obtained, it has been concluded that highly graphitized fibres such as pitch-based P55 or P100 are not affected by long-time annealing in the presence of magnesium vapour, whereas impure and disorded fibres such as PAN-based T300 undergo some chemical and microstructural modifications decreasing their mechanical properties.     

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.     

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.     

Transverse loading of monofilament reinforced microcomposites: a novel fragmentation technique for measuring the fibre compressive strength

Carbon/J-polymer single fibre composite samples were tested under tensile conditions with the fibre direction perpendicular to the tensile loading axis. The Poisson ratio effect induced a compression strain field in the fibre, resulting in a fragmentation phenomenon similar to that observed in a fibre subjected to tensile loading. This observation introduces a novel technique for the measurement of the compressive strength of single fibres, calculated either from the stress at first break, or from the Weibull scale parameter obtained from the fragmentation data produced at various stress levels. The special sample loading configuration used here also provides the first measurement of the effect of the length of the fibre on its compressive strength value.     

Anodic oxidation of pitch-precursor carbon fibres in ammonium sulphate solutions: the effect of fibre surface treatment on composite mechanical properties

Tonen HM pitch-based carbon fibres were electrochemically treated in solutions of ammonium sulphate using a pilot plant surface treatment apparatus. Embedded single fibre and short beam shear specimens prepared with these treated fibres exhibited strengths over 300% greater than those made with untreated fibres. The surface treatment did not, however, result in improvements in the longitudinal compressive strength. This suggests that the compressive strength is not limited by the shear strength of the fibre/matrix interface.     

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.     

Tensile and compressive properties of flax fibres for natural fibre reinforced composites

Mechanical properties of standard decorticated and hand isolated flax bast fibres were determined in tension as well as in compression. The tensile strength of technical fibre bundles was found to depend strongly on the clamping length. The tensile strength of elementary flax fibres was found to range between 1500 MPa and 1800 MPa, depending on the isolation procedure. The compressive strength of elementary flax fibres as measured with a loop test lies around 1200 MPa. However, the compressive strength can be lowered severely by the decortication process. The standard decortication process induces kink bands in the fibres. These kink bands are found to contain cracks bridged by microfibrils. The failure behaviour of elementary flax fibres under compression can be described as similar to the failure behaviour of a stranded wire.     

Analysis of the fragmentation test for carbon-fibre/epoxy model composites by means of Raman spectroscopy

The interfaces between high-modulus PAN-(T50) and mesophase pitch-based (P55) carbon fibres and an epoxy matrix have been studied by using the conventional fragmentation test in conjunction with polarised-light optical microscopy. Raman spectroscopy has also been used to follow stress transfer from the matrix to the fibres for the same fragmentation geometries. The level of fibre/matrix adhesion and mechanisms by which the stress is transfered from the matrix to the fibres has been determined from both the stress birefringence patterns and strain-induced Raman band shifts in the fibres. The values of interfacial shear strength have been determined by means of both the conventional analysis and the Raman technique. It is found that the Raman method gives a much more detailed picture of stress transfer in the test specimens and that the two methods give somewhat different values of the interfacial shear strength. The values of interfacial shear stress have been discussed with respect to fibre surface energy, surface chemistry and surface morphology. It was found that the surface chemical functional groups appear to have no direct correlation with interfacial shear strength. Furthermore, it appears that mechanical interlocking due to surface roughness could contribute to the higher values of interfacial shear strength determined for the PAN-based fibre.     

The compression strength of composites with kinked,misaligned and poorly adhering fibres

Experiments carried out on pultruded fibre reinforced polyester resins show that, at moderate fibre volume fractions, the compressive strength of aligned fibre composites depends linearly on the volume fraction. The strength falls off when the fibre volume fraction,V _f=0.4 with Kevlar and high strength carbon fibres. The effective fibre strength atV _f<0.4 is much less than the tensile strength but it is close to the tensile strength with E-glass fibres and high modulus carbon fibres. Poor adhesion between fibres and matrix reduces the compressive strength, as does kinking the fibres when the fibre radius of curvature is reduced to below 5 mm. Misalignment of the fibres reduces the compressive strength when the average angle of misalignment exceeds about 10° for glass and carbon fibres. However, with Kevlar no such reduction is observed because the compression strength of Kevlar reinforced resin is only a very little better than that of the unreinforced resin.