Effect of Mode-mixity on the Fracture Toughness of Ti-6al-4v/fm-5 Adhesive Joints

Abstract

Using several different fracture tests, the fracture toughness of a chromic acid anodized titanium (Ti-6A-4V)/polyimide (FM-5) adhesive system was evaluated. Mode I, mode II, and mixed mode (I and II) tests were conducted using double cantilever beam (DCB), end notch flexure (ENF), and mixed mode flexure (MMF) geometries. Interfacial type failures were observed in the ENF and MMF specimens as a result of the mode II loading inherent in these tests. Pure mode I loading, as is the case with symmetric DCB specimens, resulted in cohesive failures with a fracture energy around 2500 J/m2 on as-received specimens. The asymmetric DCB specimens had fracture energy values around 2000 J/m2, the MMF specimens close to 1970 J/m2, and ENF specimens around 1300 J/m2. All the above measurements were made on as-bonded (unaged) specimens. Titanium/FM-5 bonds supplied by the Boeing Company were then aged in one of three different environments for 2 and 6 months respectively. The environments included: 177degC in air and 2 psia, and 204degC in air. Following the aging, DCB, ENF, and MMF tests were conducted on the specimens. The results showed that aging in all three environments resulted in decreases in fracture energy for the above specimen testing configurations. The largest drop (20 percent) in fracture toughness was noted in specimens aged for 6 months in air at 204degC. An unusual finding from this study, in contrast to what other researchers have seen on other systems, was that increasing mode II loading resulted in significant reductions 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 developed to predict failure energy and type for use in designing structural joints.