Abstract
The strength of carbon fibres as single filaments, bundles and in composite form has been investigated. Single filaments have been shown to have a strength distribution to which a two-parameter Weibull distribution may be fitted. The main objective of the work was then to reconcile the observed strength distributions of both simple (mono-fibre type) and hybrid composites to that of the single filaments. This has been done by preparing and testing a range of unidirectional bundles and composites of varying geometry. In all cases, the carbon fibre used was taken from the same batch of 1000 filament tow, thus ensuring a consistent statistical base for the comparison. The hybrid composites consisted of single bundles of carbon-fibre surrounded by glass-fibre in an epoxy matrix. These were compared with impregnated and dry bundles of the same dimensions. Multi-ligament hybrids were also prepared and tested to assess the effects of bundle dispersion. The strength distributions of both dry and impregnated bundles in relation to that of the single fibre have been shown to follow the trends predicted by Coleman and Harlow and Phoenix respectively. The hybrids by comparison show an enhanced strength which is only partially explained by thermal stress arguments. A hypothesis is advanced which proposes that the enhanced strength of a hybridised ligament is due to a larger critical group of individual fibre fractures being required to initiate catastrophic ligament failure than is the case for non-hybrid bundles. The significance of these effects with respect to practical composites is discussed.