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.     

A new glassy carbon fibre

It is shown that phenol-hexamine polymers may be extruded from the melt to produce fibres which may be carbonised to form fine high-strength glassy carbon fibres with a tensile strength of up to 2 GNm^–2 (300 000 Ib in^–2) after 900° C heat-treatment. The fibres have a specific modulus of 5 Mm compared with 14 Mm for carbonised polyacrylonitrile fibres and 3 Mm for silica glass fibres. Both strength and modulus increase rapidly with decrease in diameter. The fibres are subjected to no special surface treatment after extrusion but electron microscopy indicates the presence of a thin textured sheath surrounding a true glassy carbon core in the final fibre. The fibres have the advantages of glassy carbon (inertness to chemical attack, resistance to abrasion) and give promise of a new range of cheap high-strength carbon fibres derived from coal tar fractions.     

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).     

Recycling effects on microstructure and mechanical behaviour of PEEK short carbon-fibre composites

The effect of recycling on microstructure and mechanical properties has been evaluated for injection-moulded poly-ether-ether-ketone (PEEK) composites reinforced with 10% and 30% short carbon fibres. Microstructure characterization was carried out by determining fibre length distributions, PEEK molecular weight, and by SEM observations of fracture surfaces before and after processing. These studies reveal degradation of fibres and matrix during recycling. Tensile Youngs modulus and strength, as well as impact strength reductions are presented for recycled composites.     

Effect of hot-pressing time and post-heat treatment on the microstructure and mechanical properties of SiC-fibre-reinforced glass-ceramic composites

Nicalon-SiC-fibre-reinforced (35 vol %) lithium-aluminosilicate (LAS) glass-ceramic composites were fabricated by a slurry-infiltration process followed by hot pressing at 1400°C and 10 MPa for varying soaking times. The ultimate strength and elastic modulus of the as-fabricated composites, as determined by four-point flexural tests, increased rapidly with the densification time, saturating after 30 min at 550 MPa and 130 GPa, respectively. Longer hotpressing times caused a decrease in the elastic modulus via fibre degradation. A carbon-rich interfacial layer formed between the fibres and the matrix, the thickness of which reached a maximum of 400 nm after 30 min soaking time. The flexural strength of post-heat-treated composites in air decreased by a factor of approximately four, due to oxidation and removal of the carbon content of the interfacial layer. The silica-rich bridges left behind between the fibres and the matrix contributed to brittle fracture of the composite.     

Matrix-dominated tensile behaviour of unidirectional T300/914 and structural modelling of the material

Unidirectional T300/914 carbon epoxy composite has been tested under transverse tensile loading at various temperatures and test rates. The test programme revealed a different viscoelastic response for the bulk resin modulus from that of the composite modulus and associated fractographic examination showed the structures of the resin and composite to be complex. The resin consisted of an epoxy-based particulate phase embedded within a thermoplastic-based connecting phase. The addition of carbon fibres resulted in the formation of an irregular epoxy-based interphase region. A parameter, , has been defined that easily characterizes the viscoelastic behaviour of the composite. Structural models have been successfully developed for both the resin and the composite and a cure mechanism suggested. Any plastic deformation occurring on failure was within the connecting phase and the weakest point of the composite was identified as being the particle/interphase boundary. Property modelling has successfully accounted for the interphase influence on the transverse tensile modulus and the composite transverse tensile strength was found to be higher than the bulk resin tensile strength.     

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.     

Influence of carbide formation on the strength of carbon fibres on which silicon and titanium have been deposited

Silicon or titanium was deposited on the filaments of carbon fibres by chemical vapour depositions and the reactions between the deposited silicon or titanium and the carbon fibres were investigated below 1300° C. Between the silicon and the carbon fibres, -SiC layers formed at rates of 1.5 to 3 nm in 3 h at 1300° C. These rates were 10^–4 times that of the TiC formation by the reaction of titanium with carbon fibre. Furthermore, the effect of the reaction on fibre strength was investigated. By reaction with silicon, the carbon fibre at a carbonized stage decreased in strength at the beginning of the reaction, but afterwards it recovered to the original level. The carbon fibre at a graphitized stage maintained its original strength after heat treatment for several hours at 1300° C. With the TiC-coated carbon fibres, the carbon fibres decreased in strength following the relation _m d ^–1/2, where d is the thickness of the TiC layer.     

Compression strength of carbon,glass and Kevlar-49 fibre reinforced polyester resins

The compression behaviour of a series of polyester resins of various compositions and in different states of cure has been investigated. Their mechanical characteristics having been established, the same range of resins was then used as a matrix material for a series of composites reinforced with carbon, glass and aromatic polyamide fibres. The composites were unidirectionally reinforced, having been manufactured by pultrusion, and were compression tested in the fibre direction after a series of experiments to assess the validity of a simple testing procedure. Rule of Mixtures behaviour occurred in glass-polyester composites up to limiting volume fractions (V _f) of 0.31 for strength and 0.46 for elastic modulus, the compression modulus being equal to the tensile modulus, and the apparent fibre strength being in the range 1.3 to 1.6 GPa at this limiting V _f. At a V _f of 0.31 the strengths of reinforced polyesters were proportional to the matrix yield strength, _my, and their moduli were an inverse exponential function of _my. For the same matrix yield strength a composite with an epoxy resin matrix was stronger than polyester based composites. At V _f=0.30, Kevlar fibre composites behaved as though their compression modulus and strength were much smaller than their tensile modulus and strength, while carbon fibre composites were only slightly less stiff and weaker in compression than in tension. The compression strengths of the polyester resins were found to be proportional to their elastic moduli.     

Influence of short-fibre reinforcement on the mechanical and fracture behaviour of polycarbonate/Acylonitrile Butadiene Styrene polymer blend

The present study investigated the dependence of various mechanical and fracture properties on the volume fraction, f, of the reinforcing glass fibres in Polycarbonate/Acylonitrile Butadiene Styrene (ABS) blends. The addition of glass fibres enhanced the ultimate tensile strength and modulus and reduced elongation (both to yield and to break) and total work of fracture. The elastic modulus was not significantly affected by the loading mode although the ultimate strength was significantly affected, giving flexural strength values of 1.5–1.6 times greater than tensile strengths. The elastic modulus and strength were linear functions of f and thus followed the principle of rule of mixtures. The presence of weldlines in specimens had an adverse effect on most tensile properties except for the elastic modulus. Linear elastic fracture mechanics could not be used to assess the resistance to crack propagation of the present range of materials because their behaviour violated the principle assumptions upon which the theory is based. An alternative method was employed where the total work of fracture and the work of fracture corresponding to the maximum load were plotted as a function of initial crack length. These plots were reasonably linear for the polymer and its composites giving values of the resistance to steady state crack propagation JT and the J integral of maximum load Jm respectively. Values of JT and Jm decreased with increasing f.     

Tensile testing of ceramic fibres by video extensometry

A computer-assisted video extensometer was used to measure the Young's modulus and tensile strength of commercially available alumina fibres (11.5 m in diameter). The results showed excellent agreement with manufacturers reported values (384 ± 12 GPa and 3132 ± 296 MPa for Young's modulus and tensile strength, respectively). The machine was initially tested with 100 m diameter SiC monofilaments to identify optimum experimental conditions. The mechanical properties of these fibres were independent of the crosshead speed and fibre length. This non-contacting extension measuring system allowed testing of fragile materials and a submicron resolution could be achieved with a high-precision CCD camera. The results in term of precision and resolution therefore meet the requirements for strain measurements in mechanical ceramic materials testing.     

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.     

Fracture of aluminium-coated carbon fibres

A degradation in the ultimate tensile strength (UTS) of aluminium-coated carbon fibres was associated with the formation of a reaction layer of aluminium carbide during annealing treatments 475° C for high tensile fibres (HT) and 550° C for high modulus fibres (HM). It was established that for a given annealing treatment, the UTS depended on the square root of the original coating thickness and proposed that fracture was controlled by cracks in the aluminium carbide, with a specific surface energy () and intrinsic crack length (c ₀) of 2.33 J m^–2 and 30 nm for HT fibres, and of 0.64 to 0.77 J m^–2 and 20 nm for HM fibres.     

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.     

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     

High-temperature compatibility of carbon fibres with nickel

A study has been made of the elevated temperature degradation of a number of carbon fibre types coated with nickel by a variety of methods (electroless, electrolytic, carbonyl and physical vapour deposition). At high temperatures, Ni-coated fibres undergo a transformation of structure to crystalline graphite with a consequent loss of strength and elastic modulus. Resistance to this recrystallization is related to the fibre type and structure and increases in the order HTS PAN-based, HM PAN-based, HM rayon-based. For PAN-based fibres the resistance increases with the degree of structural order and orientation. The recrystallization of HTS fibres is consistent with a simple model of dissolution and reprecipitation controlled by diffusion of carbon in nickel. To explain the higher stability of HM fibres an additional factor must be introduced. For example, their behaviour can be explained in terms of a highly stable surface layer between about 0.1 and 0.5m thick. Rapid recrystallization occurs when the nickel breaks through this layer e.g. by dissolution. The recrystallization was not greatly affected by the type of nickel coating but the recrystallization temperature of HM fibres was considerably reduced by a small proportion of air in the heat-treatment atmosphere. HTS fibres were not affected in this way but the fibres were severely weakened through surface attack by both air and hydrogen at temperatures well below the recrystallization temperature.     

Mechanical properties of injection moulded styrene maleic anhydride (SMA) Part II Influence of short glass fibres and weldlines

The dependence of the various mechanical and fracture properties on the volume fraction ofshort glass fibres in the styrene maleic anhydride (SMA) polymer was investigated. Special attention has been given to describing the dependence of various mechanical properties on the volume fraction of the glass fibres, f by way of the rule of mixtures. It was found that, strength, elastic modulus and fracture toughness, all follow a simple rule-of-mixtures of the form Qc=Qff+Qm(1–f), where Qc is the measured quantity for the composite, Qm and Qf are the corresponding values for the matrix and the fibre, respectively, and is the overall efficiency of the fibres, taking into account the orientation and the length of the fibres in the composite. It was also found that, while the presence of the weldline had no significant effect upon elastic modulus, its presence significantly reduced tensile strength and the fracture toughness of SMA and its composites. © 1998 Kluwer Academic Publishers     

Studies on nickel coated carbon fibres and their composites

Uniform and continuous coating of nickel was given to the carbon fibres by cementation, electroless or electroplating techniques. The coating thickness was ranged between 0.2 and 0.6 m for all the three methods used. Coating thickness less than 0.2 m showed discontinuous coating of nickel over the fibre surface. Beyond 0.6 m thickness, nickel deposited in den-drite form over the continuous coating. For continuously coated fibres, the ultimate tensile properties of electroless coated fibres were near to uncoated carbon fibres suggesting adherent and defect free coating; while fibres coated by electrolytic and cementation process exhibited lower ultimate tensile strength (UTS) properties. The tensile fracture of the cementation coated fibres suggested degradation of the fibres. In composites, prepared by dispersing the coated fibres in pure aluminium matrix, no appreciable fibre-metal interaction was observed. NiAl₃ intermetallics were observed around and adjacent to the carbon fibres. Sometimes carbon fibres were found embedded in massive NiAl₃ intermetallics suggesting that fibre surface can also act as nucleating centre for these precipitates.     

Characterization of the interface in a carbon-epoxy composite using transmission electron microscopy

Two commercial kinds of unsized pan-based carbon fibres (high strength and high modulus) were subjected to electrochemical treatments (oxidative and non-oxidative) and to a nitrogen plasma. After embedding in an epoxy resin, they were sectioned by ultramicrotomy (diamond knife), and the interface between the resin matrix and the fibre was studied by the lattice fringes method (TEM). The adhesion of the fibre to the matrix depends on the surface treatment and the external microtexture of the fibre. High-modulus fibres (plasma etching) show a good adhesion only when the carbon layers are perpendicular to the interface. High-strength fibres exhibit two types of adhesion. Some treatments yield a surface which mainly presents carbon layers parallel to the interface with a rugosity of about 1–2 nm. The second type of adhesion consists of fibre-matrix interpenetration. In this case carbon layers are slightly exfoliated and perpendicular to the interface. For most treatments, interfacial shear stress was determined by the pull-out test. We found no correlation between TEM observations and shear stress data. Consequently, we were unable to discuss adhesion from the transmission electron microscopy results. It would probably be more reliable to consider the problem with a concept based on interface toughness. However, as a first step, we have defined a new parameter, the contact index, and have shown the relations between the contact index, the morphology of the interface and the interfacial shear stress determined by the pull-out test.     

Influence of short glass fibres and weldlines on the mechanical properties of injection-moulded acrylonitrile–styrene–acrylate copolymer

The present study investigated the dependence of various mechanical and fracture properties on the volume fraction, f, of reinforcing glass fibres in acrylonitrile–styrene–acrylate (ASA) copolymer. The addition of glass fibres enhanced the ultimate strength and modulus as measured in both tension and flexure but reduced the total work of fracture. The elastic modulus was not affected by the loading mode. The ultimate strength in flexure was found to be always greater than in tension by a factor of about 1.3. Both properties were found to be a linear function of f following the rule of mixtures:Pc=Pff+Pm(1–f)where Pc is the measured property for the composite, Pf and Pm are the corresponding values for the fibre and the matrix, respectively, and is the overall efficiency of the reinforcing fibres. Addition of glass fibres to ASA polymer reduced both the notched and the unnotched impact strengths. Linear elastic fracture mechanics were used to determine values of the fracture toughness and the strain energy release rate. The fracture toughness did not change significantly with f, whereas the strain energy release rate decreased with increasing f. The presence of weldlines in the specimens had an adverse effect on all tensile properties except for the elastic modulus. The weldline integrity parameter for the modulus was between 1 and 0.95, and for strength it was between 0.87 and 0.20, decreasing linearly with increasing f. © 1998 Kluwer Academic Publishers