The effect of fibre-matrix interface strength on the impact and fracture properties of carbon-fibre-reinforced epoxy resin composites

Uniaxial fibre-reinforced epoxy resin composites have been produced containing up to Vf=08 of type I, II, III and IV carbon fibres in both untreated and surface-treated conditions. Their mechanical properties have been determined, and it has been shown that the fibre-matrix interface is strongest when the type II and III treated fibres are employed and weakest for the type I untreated fibres. The strength and stiffness of the composites approach that predicted by a rule of mixtures law at low Vf, but tend to fall below this when Vf>06. This fall off is greater in the case of composites of low fibre-matrix interface strength. The impact resistance (unnotched) is related to the strain energy potential of the fibre, Vf, and to a lesser extent to the fibre-matrix interface strength. A parameter U*=σf2Vf/Ef is proposed as an indicator of impact resistance performance. The principal effect of the interface strength is to modify the mode of failure. At high levels of strength the failure is brittle and relatively little energy is absorbed. At low levels of strength a multiple delamination occurs, energy absorption is about three times as high as in the brittle case, but extensive disintegration of the specimen occurs. At intermediate values of interface strength a progressive failure occurs which appears to be the best practical compromise; energy absorption is relatively high and disintegration is avoided. The composites having the best impact resistance were those containing 06 Vf type II or III untreated fibre. These gave impact energies of similar 100 kJ m−2 and had interlaminar shear strengths of 50-60 MN m−2.