Effects of moisture and thermal cycling on in-plane shear properties of graphite fibre-reinforced cyanate ester resin composites

The weight change and retention of in-plane shear (+/-45 degrees) strength of graphite fibre-reinforced cyanate ester resin matrix composites have been estimated on exposure to high humidity and thermal cycling, respectively. Cyanate ester resin matrix composites absorbed a remarkably small amount of moisture on exposure to high humidity. However, the degree of moisture absorption underwent a rather sudden increase to a new equilibrium level after prolonged exposure. A morphology study showed the occurrence of extensive cracking in the matrix/interface region in the form of delamination between plies as well as translaminar cracking within plies. The phenomenon is believed to be caused by weakening of the fibre-matrix interface, which was confirmed by microscopic analysis of fracture surfaces. A sudden moisture gain associated with extensive matrix/interface cracking was found to reduce the in-plane shear strength and fatigue lifetime at a given stress amplitude. The slope of the S-N curve was lower for wet specimens, implying a higher growth rate of local cracks as well as delamination. The rate of in-plane shear strength degradation was also measured on static exposure to dry heat as well as after thermal cycling to a peak temperature of 150 or 204 degrees C. At a frequency of 10 min/cycle and for a relatively short duration, the effect of thermal cycling seems to be represented by the cumulative sum of thermal oxidation effects at the peak temperature. Copyright (C) 1996 Elsevier Science Limited