Reduction of Thermal Residual Strains in Adhesively Bonded Composite Repairs

Many military and commercial aircraft are being called upon to fly well beyond their original intended service lives. This has forced the United States Air Force (USAF) to increasingly rely on structural repairs to address fatigue induced damage and to extend aircraft useful life. The focus of this research is the use of a high-strength composite patch technique to repair a fatigue crack on an aluminum aircraft structure. This study investigates the thermal residual strains that occur as a direct result of the coefficient of thermal expansion (CTE) mismatch between the repair patch and the underlying cracked metallic structure to which the patch is bonded. This research examines the response of a precracked 24 inches x 6 inches x 0.125 inch 7075-T6 aluminum panel repaired with a 15-ply graphite/epoxy patch. Two adhesives: EA 9696 and FM 73M were used with varying cure cycles. The hypothesis is that by reducing cure temperatures the CTE mismatch will be less dramatic thus yielding a more robust repair with a comparable fatigue crack growth rate. The research concluded that reducing the cure cycle temperature could decrease the thermal residual strains by as much as 26.5% between the graphite/epoxy composite patch and structure when FM 73M adhesive is used to bond them together and 7.4% when EA 9696 is used. The research also concluded that a lower cure cycle temperature did not detrimentally affect the panels' fatigue crack growth rates.Air Force Inst of Tech Wright-Patterson AFB OH School of Engineering and Management is the author of 'Reduction of Thermal Residual Strains in Adhesively Bonded Composite Repairs', published 2003 under ISBN 9781423512813 and ISBN 1423512812.