Abstract
The strength and failure characteristics of polyamide 6.6 filled with both glass and carbon fibres has been studied. It has been shown that provided there exists a moderately strong interface between the fibre and matrix, stiffness enhancement is as predicted by a modified rule of mixtures, but that strength is limited by an embrittlement of the material, which results in premature fracture at low strain. Failure mechanisms in a simple model composite were studied using optical microscopy and this suggested that matrix cracking initiated by the fibres was the principal cause of this premature fracture. Experiments in which fibre length distributions were determined both before and after fracture confirmed that failure sequences involving gross fibre fracture did not occur. This hypothesis has been confirmed by careful observation of the mechanical properties, together with optical and scanning microscopy and acoustic emission techniques, of the fibre filled thermoplastic. Limited experimental evidence suggests a more beneficial combination of properties might be obtained by using a mixture of glass and carbon fibres in the composite.