Durability of an off-optimum cured aluminium joint

The durability of an aluminium alloy joint, in which the adherends have been pre-treated with silane coupling agent and strontium chromate primer and bonded with the film adhesive FM-73, was examined for an off-optimum adhesive cure cycle. For bonded repairs a cure cycle of low temperature and longer duration (8 h at 80°C) for FM-73 is employed regularly to reduce thermal stresses. Double cantilever beam testing showed that the improved durability that results from primer application when the adhesive is cured at 120°C does not occur for the 80°C cure. Scanning electron microscopy and X-ray photoelectron spectroscopy of fracture surfaces and cross-sections cut from test specimens indicate differences in fracture paths and interfacial regions for the 120°C and 80°C cure specimens. Changes in the interphase region caused by the off-optimum cure cycle may contribute to an increased susceptibility of the adhesive joint to moisture and consequent bond degradation.