Aluminum plates (type 2024) were subjected to various surface pretreatments and then joined by epoxy resin. The joints were tested for shear strength close to the time of joining, and after various storage periods at 25±5°C and 40% relative humidity. Surfaces of the aluminum plates were examined using X-ray photoelectron spectroscopy, and scanning electron and optical microscopies. Specimens examined shortly after being joined showed a trend of cohesive failure, but those which were stored for some time showed a trend of adhesive failure. It was found that the mean shear strength (MSS) of the specimens treated by a chromic acid solution was lower than that of specimens treated by a phosphoric acid solution. Hardness (Shore type D-2) and structure (by X-ray diffraction) of epoxy control samples were also examined. It was observed that the hardness increases with longer aging time, while changes in the structure were observed after long-term storage (63 months). The failure development with time is discussed and correlated with the MSS and the failure mode.
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Raveh et al. (1991) studied this question.
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