Effect of mode-mixity on the fracture toughness of Ti-6Al-4V/FM-5 adhesivejoints

Using several different fracture tests, the fracture toughness of a chromic acid anodized titanium (Ti-6A-4V)/polyimide (FM-5) adhesive system was eva luated. Mode I, mode II, and mixed mode (I and II) tests were conducted usi ng double cantilever beam (DCB), end notch flexure (ENF), and mixed mode fl exure (MMF) geometries. Interfacial type failures were observed in the ENF and MMF specimens as a result of the mode II loading inherent in these test s. Pure mode I loading, as is the case with symmetric DCB specimens, result ed in cohesive failures with a fracture energy around 2500 J/m(2) on as-rec eived specimens. The asymmetric DCB specimens had fracture energy values ar ound 2000 J/m(2), the MMF specimens close to 1970 J/m(2), and ENF specimens around 1300 J/m(2). All the above measurements were made on as-bonded (una ged) specimens. Titanium/FM-5 bonds supplied by the Boeing Company were the n aged in one of three different environments for 2 and 6 months respective ly. The environments included: 177 degrees C in air and 2 psia, and 204 deg rees C in air. Following the aging, DCB, ENF, and MMF tests were conducted on the specimens. The results showed that aging in all three environments r esulted in decreases in fracture energy for the above specimen testing conf igurations. The largest drop (20 percent) in fracture toughness was noted i n specimens aged for 6 months in air at 204 degrees C. An unusual finding f rom this study, in contrast to what other researchers have seen on other sy stems, was that increasing mode II loading resulted in significant reductio ns in toughness. Crack path selection and interaction with the woven glass scrim within the bonded specimens may be responsible for the lower mode II fracture energies. From the tests conducted, failure envelopes were develop ed to predict failure energy and type for use in designing structural joint s.