Lowering Thermal Residual Stresses in Composite Patch Repairs in Metallic Aircraft Structure

Thermal residual stressesinduced by the bonding of composite patch repairs causeadverseeffectson the fatigue performance of patch repairs. The reduction of residual stresses was attempted by modifying the cure cycle of the adhesive, FM73M. An effective temperature drop D Teff was determined to quantify thermal residual stresses. Variouscurecyclesfortheadhesivewereinvestigated empirically to determinethe curetimeand curetemperature combination that can achieve a 100% degree of cure and the desired mechanical properties. An efe cient two-step curecyclewasdevelopedforbonding compositepatchestocracked aluminumplates. Theeffectofreduced residual stresson the fatiguelifeof cracked aluminum plates repaired with symmetrical compositepatcheswasinvestigated experimentally and numerically. I. Introduction L IKE other load-bearing structures, aging aircraft may contain damaged or cracked structural components resulting from fatigue andcorrosionduringservice,and thedegradedstructuralcomponents must be replaced or repaired to extend their service lives. Because of thesuperiorpropertiesofadvancede ber composites and the maturing adhesive bonding technology, composite patch repair isnowrecognizedasanefe cientandeconomicalrepairtechnology. 1 Because bonded repairs using high-performance composite materials offer high efe ciency and enhanced structural integrity, extensive works of composite patching on metallic aircraft components have been performed and reported. Examples include Hercules aircraft wings, Mirage wing skins, an Orion fuselage skin, an F-111 wing pivot e tting, a B-767 lower fuselage, and an MD-82 leading-edge slat that were repaired with boron/epoxy patches. 1;2 Because highperformance adhesives used in bonding require elevated cure temperatures, signie cant thermal residual stresses in the repair result from the mismatch of thermal expansion coefe cients between the composite patch and the cracked aluminum structure. The induced thermal residual stresses in the aluminum structure are tensile and tend to open the crack, thus inducing a substantial level of stress intensityatthecracktip.Evidently,theresidualstressescanreducethe effectiveness of the patch on the fatigue performance of the patched structure. 3 In general, thermal residual stresses in the metallic structure repaired with adhesively bonded composite patches can be reduced by lowering the bonding (curing) temperature. However, lowering the cure temperature of the adhesive can yield adverse effects on the degree of cure of the adhesive and, as a result, yield poorer mechanical propertiesofthe repair.Moreover,lowercure temperatures require longer cure time to achieve, if ever, the same degree of cure with a high cure temperature. In this study, an experimental investigation of various cure cycles for the adhesive was performed to e nd an efe cient cure time and cure temperature combination that can achieve a 100% cure and the desired mechanical properties. Atwo-step bonding cycle was selected toreduce thermalresidual stressesandtostudyitseffectivenessonthefatiguelifeofcomposite patch repairs. Fatigue lives of double-sided patch repairs fabricated with the bonding cycle recommended by the manufacturer (Cytec Industries, Inc. ) of FM73M and by the two-step cycle, respectively, were modeledandcompared.Theexperimental result indicated that lowering thermal residual stresses can be achieved by the two-step