Environmental Durability of Adhesively Bonded Joints

The goal of this project was to evaluate the environmental durability of adhesively bonded aircraft joints using fracture mechanics and the strain energy release rate concept. Three bonded aerospace material systems, two epoxies and one polyimide, were investigated. Adhesive specimens were tested for tensile and toughness behavior. Bonded joint specimens were subject to Mode I, Mode II, and mixed mode fracture tests and to Mode I fatigue tests. Prior to testing, selected specimens were exposed for up to 10,000 hours to isothermal and thermally cyclic conditions simulating aircraft service environments. Analysis was accomplished using finite element programs and closed-form solutions. Environmental exposure caused reductions in the failure strain, strength, and toughness, of the adhesive specimens and in the toughness and fatigue threshold of the bonded joint specimens. Specimens exposed to high temperature and humidity prior to testing and those tested at low temperatures indicative of high altitude operations experienced the most significant toughness losses. The fatigue crack growth rate sensitivity appeared to be unaffected by environmental exposure. Results are discussed in terms of their relationship to bonded joint design and should prove valuable to efforts aimed at extending the lives of aging aircraft using bonded repairs as well as to efforts focused on using adhesive bonding for future aerospace structures.Air Force Inst of Tech Wright-Patterson AFB OH is the author of 'Environmental Durability of Adhesively Bonded Joints', published 1997 under ISBN 9781423580980 and ISBN 1423580982.