Fatigue crack growth behavior of aerospace and ship-grade aluminum repaired with composite patches in a corrosive environment

Investigation of the performance for a composite repair patch to prolong the service life of pre-cracked 7050, 7075, 5083 and 6061 aluminum plates under fatigue conditions in a corrosive environment was conducted. Both insulated graphite-epoxy and boron-epoxy composite patches were evaluated for the effects of corrosion fatigue. The repair patches consisted of unidirectional plies (laminae) oriented in the loading direction. The improvement of service life and the effect that a corrosive environment had on crack propagation rates for the repaired aluminum plates were examined. The bond durability between the aluminum plates and the boron patch were also assessed. As expected, the introduction of salt water during testing greatly increased crack growth rates and can be quantified for comparison. The graphite patch consistently showed positive results compared to the boron patch for 6061 aluminum samples. The same system on 5083 aluminum had mixed results. Examination of potential 5083 sensitization due to elevated cure cycles is discussed. The boron-epoxy repairs showed a positive life improvement in both lab air and salt water exposed environments for 7050 and 7085 aluminums while the graphite-epoxy repair accelerated crack growth rates in the salt water environment.