Shrinkage Control in Fibre Reinforced Polymers I: Toughness Improvements Expected

Abstract

The fracture toughness of polymer composites made with glass and carbon fibres is not very great; they tend to split and delaminate easily and can also break easily across the fibres. While delamination resistance can be markedly improved by using tough polymers, this is not the case with across-the-grain toughness. Instead, the polymer shrinkage stress needs to be reduced. In this paper an earlier theory on the effect of the polymer shrinkage stress on toughness is developed further to eliminate some approximations and to take account of the fibre debonding process. It is shown that debonding can be a factor which limits the toughness achievable. Thus, although very large increases in toughness are possible if the shrinkage stress is reduced to an optimum value, the actual toughness that can be achieved is determined by the fibre-matrix bond strength. Consequently a well bonded composite can only be toughened to a limited extent: further improvements are dependent on weakening the bond, with consequent loss in shear strength. Nevertheless, there is a window of opportunity for toughness improvement if the shrinkage pressure can be controlled by chemical or other means.