Failure mechanisms in adhesively bonded aluminium: an XPS and PEELS study

The morphology and chemistry of the failure of adhesively bonded phosphoric acid anodising (PAA)‐treated aluminium alloy (2014A) has been investigated by XPS and transmission electron microscopy/parallel electron energy‐loss spectroscopy (TEM/PEELS). Cyclic fatigue tests were undertaken using tapered double cantilever beam joints in water. Specimens were prepared for analysis from the crack‐tip region and from areas ahead and behind the crack‐tip (i.e. the unfractured and the fatigue‐fractured and water‐exposed regions). A failure at the crack‐tip is shown by both XPS and energy‐filtered TEM (EFTEM) to be very close to the oxide/adhesive interface, but in the adhesive phase. Behind the crack‐tip, a failure surface devoid of adhesive residues, oxide hydration is observed. An observation made at regions well behind (10–15 mm) the crack‐tip indicates that mechanical damage of the hydrated layer occurs as a result of the cyclic‐fatigue displacement. These results indicate that hydration is a post‐failure event for the system under study in this work.