Effect of Curing Cycle on Fatigue Life of Cracked AA7075-T6 Aircraft Sheet Repaired with a Boron/Epoxy Composite Patch

Abstract

Fatigue failure cracking under fluctuating stressing has been a major concern of engineering for many years. Fatigue failures have become more common and thereby significantly influencing engineering design as sources of vibration and dynamic loading of components have increased. The repairs of damaged metallic structures using bonded composite patch have been proved as an economical method to increase the fatigue life of damaged structures. This paper deals with an experimental study on the fatigue life of AA7075-T6 aluminium alloy plates reinforced with single-sided boron/epoxy patches. The study focuses on the effect of different curing cycles on the fatigue life of reinforced/repaired structure. Bonds were tested with four different curing cycles. Distortion behaviour of bonded repair is studied. The influence of the curing cycle conditions on fatigue life is presented. Also, the repair patch effectiveness on the stress state at the crack tip in increasing the fatigue life of the unrepaired structure is shown. The obtained results show that repair of the cracked plate can be done effectively by using bonded composite patches. It is shown to decrease the crack growth rate to enhance the fatigue life of the structure by 70%. Elevated curing temperatures enhance the adhesion between the metal surface and the composite patch and overall durability. However, elevated cure temperature may result in distortion with bending curvature. The optimum combination of temperature and duration for the curing cycle is of importance to reduce the distortion.