Optimization of Surface Treatment and Adhesive Selection for Bond Durability in Ti-15-3 Laminates

Abstract The promising mechanical performance of a baseline Hybrid Titanium Composite Laminate (HTCL) inspired an investigation into maximizing the strength and environmental performance of this new aerospace material. This research focused upon finding the strongest and most durably combination of three commercially-available titanium surface treatments (i.e., Pasa-Jell 107TM, Boeing's Sol-Gel, and Turco 5578R) and two polyimide adhesives (i.e., LaRCTM-IAX and FM5R) for use in HTCL. The tests employed the cracked-lap shear (CLS) specimen geometry for fatigue crack growth measurements and also for fracture toughness analyses of the bonded specimens. The CLS geometry models several bonded applications found in the aerospace industry, and it also represents the debonding characteristics of a cracked titanium foil in HTCL. The environmental performance of these six material combinations has been evaluated after 5,000 hours of continuous exposure to either a Hot/Wet environment that subjected the bonded specimens to 160°F (71°C) with relative humidity in excess of 95%, or to a Hot/Dry environment of 350°F (177°C) with a relative humidity of less than 5%. These two exposure environments utilized in this study are the most aggressive long-term environments that the HTCL is projected to experience while in service. Test results showed that the best combination of the titanium surface treatments and the polyimide adhesives in the FMR adhesive used in conjunction with Boeing's Sol-Gel titanium surface treatment. Though the FM5R/Sol-Gel system was the strongest of all combinations, its performance dropped to less than 50% of its original strength after exposure to the Hot/Dry environment. An important finding is that this bonded system did not significantly degrade after exposure to the Hot/Wet environment. The only other material combination that showed substantial bond strength was the FM5R/Pasa-Jell 107 system, though its strength also dropped to less than 50% of its original strength after exposure to the Hot/Dry environment.