Fatigue Properties and Degradation of Cured Epoxy Adhesives Under Water and Air Environments

In this study, specimens cured with an epoxy adhesive were subjected to fatigue tests, which were conducted under air and water atmospheres at room temperature, because few studies have been conducted on the deformation behavior versus time (number of cycles) of the combined degradation due to moisture and cyclic stress. The epoxy adhesive was cured into plates and then cut into dumbbell-shaped specimens. Micro surface cracks were introduced into the specimen surfaces. The fatigue limit of smooth specimens without cracks in water improved compared to that in air. However, when a pre-crack was introduced at the specimen surface, all specimens fractured from the crack in water and showed the same strength as in air. Fracture toughness showed no significant difference in values between the fatigue tests in air and water. The loss factor, compliance, and creep deformation increased significantly in the fatigue tests in water compared to those for the tests in air. The specimens after testing showed that the C=O peak intensity was the same for immersion in water, fatigue in water, and fatigue in air. Therefore, no change in the chemical structure occurred during any of the loading tests.