The thermal expansion of carbon fibre-reinforced plastics

Interferometric measurements of the linear thermal expansion coefficients of epoxy resin DLS 351/BF₃400 are reported over the approximate temperature range 90 to 500 K. Corresponding measurements in directions parallel and perpendicular to the fibres are also reported for unidirectional composite bars of Courtaulds HTS carbon fibre in this resin, at nominal fibre volume contents of 50, 60 and 80%. The results are qualitatively similar to earlier observations upon resin ERLA 4617/mPDA-based specimens, but effects associated with resin softening occur at significantly higher temperatures in the case of resin DLS 351/BF₃400. Current theoretical models account successfully for the influence of fibre volume fraction in the range 0.5 to 0.8 upon the value of the coefficient of thermal expansion at room temperature, within the limitations imposed by experimental uncertainty, provided that appropriate values are assigned to the linear thermal expansion coefficients of the fibres. In the directions parallel ( ^f ) and perpendicular ( ^f ) to the fibre axis these values have been selected from the ranges –10×10^–7< ^f <–9×10^–7 and 0.5×10^–5< ^f <1.9×10^–5. It is concluded that a more rigorous appraisal must await the availability of independent information concerning the directional thermoelastic properties of carbon fibres.     

The thermal expansion of carbon fibre-reinforced plastics

Interferometric measurements of the linear thermal expansion coefficients between approximately 90 and 400 K of a series of unidirectional and bidirectional specimens of epoxy resin ERLA 4617/mPDA reinforced with Courtaulds HTS and HMS carbon fibres are reported for three mutually perpendicular directions. The room temperature results have been employed, in association with related data, to confirm that the main features of the dependence of the linear thermal expansion coefficients of these composites upon the thermal and elastic properties of the constituents and the orientations of the fibres within the matrices conform with expectations based upon current models of the thermal expansion and elastic behaviour of composite solids. At higher temperatures the results clearly show marked changes in the temperature dependence of the dimensional behaviour which result from the relaxation accompanying the softening of the resin.     

The influence of distribution between fibre orientations upon the thermal expansion characteristics of carbon fibre-reinforced plastics

The linear thermal expansion coefficients of nine laminates of carbon fibre-reinforced plastic containing fibres angled at 0°, ± 45° and 90° to the measurement direction in proportions from 0% to 100%, and of the matrix resin Fibredux 914C, have been measured between approximately −200°C and +200°C. Predicted values for bidirectional and tridirectional laminates from unidirectional data agree well with experimental values for (0°, 90°) and (± 45°, 90°) laminates. The thermal expansions of these composites are more dependent upon thermal history than those of similar composites based upon ERLA 4617/m PDA, DLS 351/BF₃ 400 and Code 69 resins, particularly at higher temperatures in the more highly internally stressed directions.     

The thermal expansion of carbon fibre reinforced plastics

Measurements of the principal linear thermal expansion coefficients of a tridirectional (–45°, 0°, +45°) carbon fibre reinforced plastics laminate are reported in the approximate temperature range 90 K to 500 K. A quantitative evaluation of the in-plane results in thermoelastic terms has yielded an agreement with these results consistent with the approximations and uncertainties involved. The qualitative agreement with expectations based upon the behaviour of unidirectional and bidirectional laminates is also demonstrated. The account concludes with an examination of some effects which are peculiar to multidirectional laminates.     

Hardness anisotropy in carbon fibre-reinforced plastics

Hardness tests have been performed on continuous carbon fibre-reinforced thermoset resin and on short fibre-reinforced thermoplastic, using conical and biconical indenters. It is shown that, in addition to the expected anisotropy in the continuous material, there is significant anisotropy in the reinforced injection moulded thermoplastic. The biconical indenter appears to furnish unique information regarding the local plastic properties of the latter material, with particular reference to surface damage.     

The thermal expansion of carbon-reinforced plastics

We compare the convenience offered by alternative ways of recording thermal expansion data. Limitations imposed on the values, for every day engineering, of the linear thermal expansion coefficients of laboratory produced carbon fibre reinforced plastics are assessed. It is concluded that the commonly encountered levels of moisture content, voids and effects due to the misalignment and non-uniform distribution of fibres do not provide cause for serious concern in most applications. The practical consequences of thermo-elastic stress are deemed to be more important. It is concluded that provided sensible account is taken of the experience accumulated over the years from the production and application of carbon fibre reinforced plastics, laminate theory may be confidently applied to laboratory data in order to gain a good idea of the behaviour expected from new constructions under normal operating conditions.     

Compression fracture of unidirectional carbon fibre-reinforced plastics

The effect of volume fraction and tensile strength of fibres, temperature and stress concentrators on the compression strength and fracture mode of unidirectional CFRP was studied. The cause of kinking is different for composites reinforced by low-(<3 GPa) and high-strength fibres. If fibre strength is high, the kink is initiated by composite splitting followed by fibre bend fracture in the tip of the split. In the case of low-strength fibres, kinking is initiated by compressive fracture of the fibres. The effect of stress concentrators on the CFRP compressive strength is described by linear fracture mechanics. In the presence of defects, fracture is a result of the emergence of splits near a hole. As the critical stress of splitting growth initiation reduces in proportion to the square root of the defect size, the Griffith criterion describes the composite compressive fracture. At elevated temperature, failure is caused by fibre buckling. The fracture band in this case is oriented perpendicular to the fibre direction. Carbon fibre compressive strength may be measured by the loop method. Bending a strand of carbon fibres glued to the elastic beam gives a fibre-controlled upper limit of the composite compressive strength.     

Ultrasonic wave propagation in carbon fibre-reinforced plastics

The attenuation and velocity of the longitudinal and shear waves in unidirectional carbon/epoxy composites have been measured as a function of the fibre volume fraction over the frequency range, 1.84 to 11.9 MHz, using the pulse-echo technique. The decrease of attenuation with fibre volume fraction suggested that the high attenuation in the composites was caused by viscoelastic losses in the epoxy matrix rather than scattering losses by the fibre. The attenuation increased with frequency, while the velocity was found to be independent of frequency.     

The thermal expansion of carbon-fibre reinforced plastics

The thermal expansion characteristics of a series of carbon-fibre fabric reinforced plastic laminates over the approximate temperature range 90 K to 440 K have been determined. The reinforcements included Morganite Type II fibres in a plain weave and a two-by-two twill weave and Courtaulds Grafil E/XAS fibres in a two-by-two twill weave, a five-shaft satin weave and in an unwoven unidirectional disposition. The results show that the ratio of fibre tow densities in the principal fibre directions, the crimp in the reinforcing fibres and the laminate stacking sequence all influence the magnitudes and temperature dependences of the linear thermal expansion coefficients, as well as the detailed manner in which the dimensions respond to changes of temperature. Volume shrinkage effects resulting from temperature cycling are also reported. The linear thermal expansion coefficients of Courtaulds Grafil E/XAS carbon fibres in directions parallel, ^f , and perpendicular, _¶ ^f , to the fibre axis have been estimated as ^f =–2.6× 10^–7 K^–1 and ^f =2.6×10^–5 K^–1.     

The thermal conductivity of carbon fibre-reinforced composites

Measurements of the thermal conductivity between approximately 80 and 270 K of a series of unidirectional and bidirectional specimens of epoxy resin DX210/BF₃400 reinforced with Morganite high modulus (HMS) and high strength (HTS) carbon fibres are reported for in-plane and out-of-plane directions. The main features of the results conform with expectations based upon known structural properties of the fibres and predictions based upon current theoretical models. Employing the results for the composites in association with results for the pure resin, the account concludes with an assessment of some of the heat transmission characteristics of the fibres.     

Predicting the compressive engineering performance of carbon fibre-reinforced plastics

This paper examines the compressive strength data of a recent experimental study [Smith FC. The effect of constituents’ properties on the mechanical performance of fibre-reinforced plastics. PhD thesis. Centre for Composite Materials, Imperial College, April 1998] concerned with the evaluation of a range of engineering properties of continuous carbon fibre/epoxy composites subjected to static tensile and compressive loading. A plastic fibre kinking analysis [Budiansky B. Micromechanics. Comput Struct 1983;16(1):3–12] and a linear softening cohesive zone model (CZM) [Soutis C. Compressive failure of notched carbon fibre–epoxy panels. PhD thesis. Cambridge University Engineering Department, UK, 1989; Soutis C, Fleck NA, Smith PA. Failure prediction technique for compression loaded carbon fibre–epoxy laminates with an open hole. J Comp Mat 1991;25(5):1476–1498] are used for the prediction of the unnotched and open hole compressive strength (OHC) of unidirectional and multidirectional laminates made of six different commercially available CFRP prepregs. Damage introduced by drop-weight (low-velocity) impact is modelled as an equivalent open hole and the cohesive zone model [Soutis C. Compressive failure of notched carbon fibre–epoxy panels. PhD thesis. Cambridge University Engineering Department, UK, 1989; Soutis C, Fleck NA, Smith PA. Failure prediction technique for compression loaded carbon fibre–epoxy laminates with an open hole. J Comp Mat 1991;25(5):1476–1498] is applied to estimate compression-after-impact (CAI) strength. The unnotched strength is accurately predicted from the knowledge of initial fibre misalignment and the shear yield stress of the composite, while the difference between the theoretical and experimental OHC and CAI strength results in most cases is less than 10%.     

Non-destructive inspection of carbon fibre-reinforced plastics using eddy current methods

The possibilities and limitations of eddy current methods for the inspection of carbon fibre-reinforced plastics have been investigated. The electrical properties of these composites lead to modifications of current methods applied to metals; in particular the required frequencies are much higher, 1–30 MHz for weave and 10–500 MHz for unidirectional reinforcements. A proper design of the probes is essential. Single turn coils are adequate, and it is shown how a higher sensitivity and suppression of the lift-off effect can be obtained. Fibre orientations can be detected conveniently with dedicated probes using a polar scan technique. The eddy currents are insensitive to delaminations. Only for unidirectional reinforcements and extensive delamination, over 20%, is the effect large enough to show up in C-scan images. In contrast, fibre fracture is readily detected for unidirectional and weave reinforcements, with a lower limit of about 8% fracture. As a result, eddy current methods are useful to establish the type of defect when a composite is damaged. This is confirmed by comparing eddy current and ultrasonic measurements on composites with impact damage.     

Compression testing of carbon fibre-reinforced plastics exposed to humid environments

A compression test jig has been developed which may be used in the measurement of the compressive strength of both unidirectional and multidirectional carbon fibre-reinforced plastics. The jig has the merit that small specimens may be tested without the necessity of end-tabbing. This feature makes its use in test programmes where the material undergoes environmental exposure prior to tests particularly advantageous. The results of tests on the compressive strength of waisted unidirectional specimens support the utility of the test technique in comparison with alternative methods. The results of compression tests on unwaisted specimens of multidirectional material demonstrates that the test technique is able to detect changes in compressive stress due to environmental exposure and test conditions.     

The thermal expansion of carbon/carbon composites

The thermal expansion from 20 to 180°C of three samples of woven cloth based carbon/carbon composites and one sample of unidirectional carbon/epoxy (cfrp) composite (with high modulus fibre) has been measured. In the direction perpendicular to the fibres the cfrp has much the highest expansion, but parallel to the fibres its expansion is considerably less than that parallel to one set of fibres in the C/C samples. One C/C sample contained rayon-based fibre and the other two pan-based fibre. The differences in their behaviour can be explained by assuming that the rayon-based material is not so well aligned crystallographically as the pan material.     

Improving the fracture energy of carbon fibre-reinforced plastics by delamination promoters

The effect of delamination promoters on the Charpy impact energy and other mechanical properties of carbon fibre-reinforced plastic has been studied. These delamination promoters consist of thin sheets of metallic or organic materials which make the multiple splitting of samples at the time of impact, a process involving a greater energy absorption, much easier.     

Thermal conductivity measurements of some glass fibre- and carbon fibre-reinforced plastics

Thermal conductivity experiments are reported on unidirectional composites over the temperature range –150 to 130 ° C. Results for R-glass/Fibredux 914 specimens, and for carbon fibre GY-80/Code 69 resin specimens, in directions parallel and perpendicular to the fibre directions have been interpreted using standard theories. Experiments were also made on specimens in which the carbon fibres made angles of 30 °, 45 ° and 60 ° to the temperature gradient. An attempt was made to understand these data using a finite difference model.     

Dynamic properties of aligned short carbon fibre-reinforced plastics in flexure and torsion

Theoretical and experimental studies of the dynamic material properties of short aligned carbon fibre-reinforced plastics are described. It is shown that, by correct choice of fibre aspect ratio and volume fraction, the damping of composites can be improved whilst retaining high modulus of elasticity. Both flexural and shear properties have been studied and experimental techniques for dynamic measurements are described. It is shown how material loss factors may be deduced from specimen loss factors and results are presented for a range of fibre lengths from 0.25 to 3.0 mm. Moulding techniques for manufacturing short aligned fibre-reinforced plastic rods and beams to a required fibre distribution and alignment have been developed.     

The effect of thermal ageing on carbon fibre-reinforced polyetheretherketone (PEEK)

The morphological properties of carbon fibre-reinforced PEEK (APC-2) laminates subjected to long-term thermal ageing and cycling treatments have been studied using differential scanning calorimetry and wide-angle X-ray diffraction techniques. This work sought provided a structural explanation for measured changes in mechanical properties. Annealing at 120 ° had no effect but significant changes in crystal content and crystal perfection occurred at 250 ° and 310 °C. At 250 °C, for short ageing periods, both crystal growth and perfection processes were observed. For longer ageing periods, there was a multiple increase in capital perfection. Initial ageing at 310 °C caused rapid crystal growth, but further ageing resulted in a reduction in measured crystal contents due to thermal degradation.     

Compression strength of hybrid fibre-reinforced plastics

Pultruded fibre composite rods have been made from polyester resin and mixtures of carbon, Kevlar and glass fibres. The strengths and elastic moduli of the hybrids were not always as predicted by the mixtures rule, moduli in particular often being lower than expected. The addition of relatively little carbon fibre to a Kevlar composite drastically reduces its ductility. Glass has a less significant effect, and some glass-Kevlar composites show a secondary elastic modulus.     

The effect of thermal ageing on carbon fibre-reinforced polyetheretherketone (PEEK)

The static and dynamic mechanical properties of carbon fibre-reinforced PEEK (APC-2) laminates subjected to long-term thermal ageing and cycling treatments have been studied using three-point bend flexure tests. Results are discussed with respect to morphological changes and degradation analysis. S/N curves were modelled using fatigue modulus degradation data. Ageing laminates at high temperatures, for long time periods, between the glass transition temperature, T _g, and the melting temperature, T _m, caused a significant reduction in mechanical properties. However, for short ageing periods, a crystal-perfection process occurs which enhanced the low stress level fatigue resistance of both laminate geometries.