A fracture mechanics based approach using wedge and double cantilever beam (DCB) specimens is being used to evaluate the durability of a titanium/adhesive system. The adhesive used was a polyimide, developed by NASA Langley Research Center, modified and supplied by Cytec Engineered Materials, Inc., and designated as FM5. Prior to bonding, the adherend surfaces were pretreated by one of two surface pretreatments: phenylethynyl trimethyl silane or chromic acid anodization.
Samples were aged at one of three different temperatures, 150, 177, and 204°C, all of which are well below the (250°C) glass transition temperature (Tg) of the adhesive. Aging was also carried out in one of three different environments, ambient atmospheric air and reduced air pressures of 2 psi air (13.8 KPa) and 0.2 psi air (1.38 KPa), for several months. Samples aged for different times were then tested to evaluate both static and fatigue properties. Results obtained thus far indicate that the greatest loss in strength occurs after aging in air at the highest aging temperature of 204°C. Strain energy release rate G values obtained from the static DCB tests dropped by 10 to 20% after aging for periods of up to 6 months.
Solvent uptake tests were conducted on neat FM5 resin in several common organic solvents followed by tensile tests to evaluate both saturated and residual properties. Strain energy release rate values were also computed from wedge tests of samples immersed in the different solvents for several days. Results showed the FM5 resin to be quite solvent resistant.