On the Durability of Bonded Repairs to Fuselage Structures Subjected to Mechanical and Environmental Conditions

Bonded repair is one of the structural repair technologies that the aircraft industry uses to restore the strength of fuselage structures and increase the life of an aircraft. Bonded repairs are superior to the conventional metallic fastened repairs in that the former yield a higher stiffness and lower stress concentrations and are more aerodynamically and structurally efficient. Bonded repair technology has been used to repair military aircraft for over three decades but has not been certified for repairing commercial aircraft due to the lack of sufficient performance data to support their airworthiness. Experimental work and analytical modeling of bonded repairs have also been limited to those of flat coupon specimen under ambient environmental conditions. In this study, experimental and analytical work on a full-scale curved fuselage panel with various bonded repairs is conducted to investigate their durability and integrity. The experimental work was carried out using the Federal Aviation Administration's Full-Scale Aircraft Structural Test Evaluation and Research fixture. The objectives are to characterize the durability and fatigue performance of boron/epoxy (B/Ep) and aluminum bonded repairs under a simulated service load condition over the design service life of the airplane and to investigate tools for evaluating and monitoring the repair integrity over the life of the part. During all test phases, damage formation and growth of cracks and disbonds were monitored and recorded using a structural health monitoring system and several nondestructive inspection methods. In addition, strains in the vicinity of the repair patches were continuously recording using strain gages and the digital image correlation method. Experimental results indicated that the bonded repairs are effective to reduce the fatigue crack growth rate. A linear elastic fracture mechanics based analytical model is developed to predict the growth rates of fatigue cracks in the curved panel repaired by bonded patches. The unique feature of this model is that, by using a fuselage-dependent geometric correction factor and incorporating the bonded repair theory, this model can be applied to predict the growth of a crack under any type of bonded repair in a given fuselage. The model has been validated by comparing the predicted crack growth rate results with that obtained experimentally.Ph.D., Mechanical Engineering and Mechanics -- Drexel University, 201