Degradation of Epoxy-Steel Single Lap Joints Immersed in Water

Exposure to environmental factors, especially moisture, is recognized as the major cause of degradation of adhesive joints. In this work, complementing a previous study on exposure to moisture, single lap joints were subjected to immersion in water, up to five weeks, at room temperature and 50 °C. The material of the adherends was mild steel, and the adhesive was a bi-component
epoxy. The specimens were fabricated using the open-face technique. Mechanical testing at the end of the relevant period of immersion showed an initial loss of ultimate load, after one week at 50 °C or two at room temperature; then, the strength remained practically constant over the remaining time. The loss was more accentuated after immersion at 50 °C, about 70%, than at room temperature, about 30%. Also a reduction in stiffness of the joints was measured, again dramatic (about 70%) after immersion at 50 °C, moderate (about 10%) after room temperature immersion. Optical examination, performed before closing the open-face specimens and after mechanical testing, showed that the major damage mechanism was the formation of blisters filled by liquid at the primary adherend/primary adhesive interface, causing the failure mode to change from cohesive to interfacial.