Hybrid effects in composites: conditions for positive or negative effects versus rule-of-mixtures behaviour

Abstract

A positive or negative hybrid effect in hybrid composites is defined as a positive or negative deviation of a certain mechanical property from the rule-of-mixtures behaviour. The question of hybrid effects is first examined with special hybrids which have been chosen so that the effect of the fibre-matrix interface is minimized. The hybrids examined consisted of two types of carbon fibres with different mechanical properties but similar surface treatments. The results of all the mechanical properties examined (modulus, strength, stress intensity factor, fracture energies) under quasi-static and fast testing conditions do not show any synergism. In view of these results a second hybrid system of E-glass fibre/AS carbon fibre-reinforced epoxy has been chosen. In this system both the mechanical properties of the fibres and the interface which they form with the resin are entirely different. None of the mechanical properties, excluding the fracture energies, show any signs of a hybrid effect. The fracture energy results, however, show the existence of a negative hybrid effect. A theory which sets upper and lower bounds for the hybrid effect is proposed, and the conditions for the occurrence of either a positive or a negative effect are discussed.