Hydrothermal degradation behaviour of bonded joints using open-faced specimens

Moisture ingress into adhesive joints is known to reduce bond strength. The strength loss due to moisture is attributed to two primary causes. The first is the degradation of the adhesive layer, which progresses relatively slowly. The second is the reduction in the interfacial strength between the adhesive and the adherend, which is more difficult to predict. The degradation behaviour of interfacial strength varies depending on the adherend type. Therefore, the effect of water penetration on three open-faced steel and magnesium alloy specimens bonded with one-component epoxy adhesives was investigated. The open-faced specimen had no adherend on one side, and water penetrated at a high speed from the open side. The saturated water absorption state was reached faster in this specimen, which allowed the rapid confirmation of degradation after water absorption. To reduce test time, the specimens were examined in hot water at 87 °C. Initially, cohesive failure in adhesive layers occurred for the steel specimens and the strength decreased with time. Prolonged immersion resulted in progressive corrosion, and the fractures within the corrosion layer began to mix. For magnesium alloy plates, immersion in hot water caused immediate adhesion failure and, simultaneously, a significant reduction in the adhesion strength. When closed-faced specimens (i.e. normal specimens) were used instead of open-faced specimens, a sequential transition was observed from cohesion to adhesion failure with immersion from the edge.