The effect of environment on the fatigue of bonded composite joints. Part 1: testing and fractography

In this work, the effect that test environment and pre-conditioning had on the fatigue behaviour of CFRP/epoxy lap-strap joints was investigated. It w as shown that the fatigue resistance of the lap-strap joints did not vary s ignificantly until the glass transition temperature, T-g, was approached, a t which point a considerable reduction in the fatigue threshold load was ob served. It was also noted that absorbed moisture resulted in a significant reduction in the T-g of the adhesive. This must be taken into account when selecting an adhesive to operate at elevated temperatures. The locus of fai lure of the joints was seen to be highly temperature dependent, transferrin g from primarily in the composite adherend at low temperatures to primarily in the adhesive at elevated temperatures. It was also seen that as the cra ck propagated along the lap-strap joint, the resolution of the forces at th e crack tip tended to drive it into the strap adherend, which could result in complex mixed mode fracture surfaces. (C) 2000 Elsevier Science Ltd. All rights reserved.