Optimization of surface treatment and adhesive selection for bond durability in Ti-15-3 laminates

The promising mechanical performance of a baseline Hybrid Titanium Composit e Laminate (HTCL) inspired an investigation into maximizing the strength an d environmental performance of this new aerospace material. This research f ocused upon finding the strongest and most durably combination of three com mercially-available titanium surface treatments (i.e., Pasa-Jell 107(TM), B oeing's Sol-Gel, and Turco 5578(R)) and two polyimide adhesives (i.e., LaRC (TM)-IAX and FM5(R)) for use in HTCL. The tests employed the cracked-lap sh ear (CLS) specimen geometry for fatigue crack growth measurements and also for fracture toughness analyses of the bonded specimens. The CLS geometry m odels several bonded applications found in the aerospace industry, and it a lso represents the debonding characteristics of a cracked titanium foil in HTCL. The environmental performance of these six material combinations has been e valuated after 5,000 hours of continuous exposure to either a Hot/Wet envir onment that subjected the bonded specimens to 160 degrees F (71 degrees C) with relative humidity in excess of 95%, or to a Hot/Dry environment of 350 degrees F (177 degrees C) with a relative humidity of less than 5%. These two exposure environments utilized in this study are the most aggressive lo ng-term environments that the HTCL is projected to experience while in serv ice. Test results showed that the best combination of the titanium surface treat ments and the polyimide adhesives in the FM5(R) adhesive used in conjunctio n with Boeing's Sol-Gel titanium surface treatment. Though the FM5(R)/Sol-G el system was the strongest of all combinations, its performance dropped to less than 50% of its original strength after exposure to the Hot/Dry envir onment. An important finding is that this bonded system did not significant ly degrade after exposure to the Hot/Wet environment. The only other materi al combination that showed substantial bond strength was the FM5(R)/Pasa-Je ll 107 system, though its strength also dropped to less than 50% of its ori ginal strength after exposure to the Hot/Dry environment.