Durability of adhesive joints subjected to elevated temperature aging

Fracture toughness parameters for a predictive scheme to estimate the remaining life of a bonded joint are experimentally determined in this paper. To this end, double cantilever beam, end notch flexure and crack lap shear tests were performed to obtain the Mode I, Mode II, and mixed mode strain energy release rates, respectively. Testing was performed at room and elevated temperatures (177 degrees C and -54 degrees C) on Ti-6Al-4V titanium joints bonded with an adhesive (FM(R)x5) based on a polyimide developed at the NASA-Langley Research Center (LaRC(TM)-PETI-5). To enhance the durability of the joints, a Sol Gel chemical pretreatment process was performed on the surface of the titanium adherends. Specimens cut from a bonded sheet were tested in the as-received state, as well as isothermally exposed for 5,000 hours at 177 degrees C, and isothermally exposed to a hot/wet environment (80 degrees C, 90%+ relative humidity). In all loading cases except Mode I, the threshold strain energy release rates were reduced as a result of 5,000 hours of aging and elevated temperature testing. Specimens tested in Mode II or Mixed Mode I & II appear to be more susceptible to degradation by environmental exposures and elevated temperature testing than by Mode I testing alone.