Effect of Temperature and Humidity on Mechanical and Fatigue Behaviors of Adhesively Bonded Patch Repairs to Aluminum Fuselage Structure
Authors:   , Neel Ryan James , Awerbuch Jonathan , Tan Tein-Min 
Published: September 2023

In a collaborative effort, the Federal Aviation Administration (FAA) and The Boeing Company investigated the structural robustness and damage-resistance capabilities of adhesively bonded repair patches through the testing and analysis of metallic B727 fuselage panels using the FAA's Full-Scale Aircraft Structural Test Evaluation and Research (FASTER) laboratory. The program objectives were to characterize the durability and fatigue performance of adhesively bonded boron/epoxy (B/Ep) and aluminum (Al) repair patches subjected to simulated service load (SL) conditions over the typical design service goal (DSG) of an airplane (60,000 cycles for a B727) and to investigate tools used to evaluate and monitor the integrity of the repair patches over the life of the part. This thesis summarizes the results of the fourth and final fuselage panel test conducted in this program. The objective was to determine the effects that environmental conditions (i.e., temperature and humidity) have on the mechanical and fatigue behavior of adhesively bonded repair patches installed on an Aluminum fuselage structure. Hot-wet (165°F and 85% humidity), cold-dry (-25°F), and ambient environmental conditions were considered. B/Ep and Al doublers were used to patch active, through-thickness, center-bay cracks in the skin of a fuselage panel removed from a retired Boeing 727 aircraft. Repair patches included those that were properly designed, as well as those that were intentionally made deficient through under design and the insertion of disbonds to encourage damage growth. This allowed for the generation of data for analysis verification and assessment of non-destructive inspection (NDI) methods used to monitor repair integrity. Efforts were focused on assessing residual strains that developed during the curing cycle of patch installation and the four-week period of hot-wet conditioning that occurred immediately after repair patch installation (i.e., prior to load application) for select repair patches. Following the completion of this period, the fuselage panel was subjected to fatigue testing under cold-dry environmental conditions up to the typical DSG of 60,000 cycles. For the B/Ep repair patches, high thermal residual strains developed during curing as a result of mismatches in the thermal properties of the repair patch and fuselage skin. Throughout the fatigue test, the magnitude of notch-tip strains increased as temperature decreased. Consequently, the fatigue crack growth increased as the temperature decreased for the B/Ep repairs. Results for the Al repairs indicated limited effect of environment on the notch-tip mechanical strains and the fatigue crack growth behavior under hot-wet or cold-dry conditions. Because both the repair and the fuselage skin are made of Al, residual strains due to thermal mismatch were limited. Multi-Site-Damage (MSD) formed in the panel lap-joint area outside the test section during fatigue cycling resulting in crack link-up to a 20-inch long crack after the completion of 92,834 cycles of fatigue. Due to stress redistribution, data analysis was terminated at that point. Data from this program will be used to assess tools and methods for evaluating and monitoring the repair integrity.