Durability and Damage Tolerance of Bonded Repairs to Metallic Fuselage Structure

The durability and damage tolerance of boron/epoxy and aluminum bonded repairs to a metallic fuselage structure was characterized under simulated flight load conditions up to one typical design service goal of 60,000 cycles. Bonded repairs to two damage scenarios were considered, namely, a mid-bay through-the-thickness crack and a lap joint scribe. Several methods were used to monitor and record damage formation and growth of cracks and disbonds, including visual inspections, eddy current, and flash thermography. There were no indications of damage development in the form of crack growth or disbonding for the lap joint scribe line repair patches. For the mid-bay repair patches, no disbonding occurred; however, slow crack growth was measured. Test and analysis results revealed that the installation of the mid-bay repairs resulted in eccentric loading and inward deformation of the mid-bay region, resulting in higher tensile strains on the inner skin surface and compressive strain on the outer patch surface. All repairs were effective in containing damage during the post-fatigue residual strength test to the damage tolerance requirements in Title 14 Code of Federal Regulations 25.571.