The Effect of Fibre Surface Treatment on the Failure of Continuous Carbon Fibre/Epoxy Resin Composites

The failure mechanisms of a composite, consisting of continuous, aligned, high strength, polyacrylonitrile (PAN) based carbon fibre in an epoxy resin, under uniaxial tension, have been studied. In order to study the effect of the interphase/interface strength, six different levels of an electrochemical fibre surface treatment were used. Single tows containing approximately 12,000 treated carbon fibres were impregnated to produce composite rods with a fibre volume fraction of 0.55. Lengths of this impregnated tow were also set in the centre of glass-fibre/epoxy resin composite coupons which were used to study the mechanisms of failure of the embedded tows. Acoustic emission was used to monitor all samples and bundle failure was found to occur after a build-up of sub-critical damage events as previously modelled.¹ Microdebond tests demonstrated an initial increase of interfacial strength which levelled out at the higher levels. In impregnated samples with high surface treatments, catastrophic failure occurred with the crack propagating approximately perpendicular to the fibre direction. However, in samples with lower fibre surface treatments, longitudinal splitting (not accounted for in current models), occurred, meaning that a greater length of composite was involved in the final failure process. Acoustic emission has been shown to have an approximately direct relation with the predicted number of single fibre breaks in composite test-pieces; however, there was no significant difference attributable to the different surface treatments. The hybrid test coupons allow a detailed assessment of the failure mechanisms within the impregnated carbon tow. The failure strains of the embedded tow is some 5% higher than that of unsupported tow. The Weibull modulus is of the same order.