The effects of temperature and saltwater aging on the mode II fatigue crack growth behavior of composite-steel bonded joints

Abstract

Hybrid structures built with composite and steel emerge across industries (offshore, shipbuilding, bridges, etc.) due to benefits of weight optimization, fatigue and environmental resistance. Particularly, the wrapped composite joints emerge as a new method to connect steel circular hollow sections for application in supporting structures of offshore wind turbines. The implementation of this technology requires predicting the long-term performance of the bi-material interface under operational conditions of loading and environment. This work addresses the effects of temperature and saltwater aging on the fatigue crack growth behavior of the composite-steel bonded joint under mode II loading conditions. Fatigue tests were performed using a 4-point end-notched flexure (4ENF) set up with digital image correlation (DIC). A numerically based method was applied to calculate the strain energy release rate (SERR) accounting for friction effects, geometrical and material non-linearities. The consistency of the manufacturing process was evaluated by tests performed in room conditions (21 °C). The mode II fatigue behavior of the composite-steel bonded joints remained between an upper and a lower bound of the Paris curves, characterized by composite delamination and adhesive failure, respectively. Then, the effect of temperature was assessed by experiments in −10 °C and 70 °C. Short-term temperature changes showed a significant effect on the fatigue resistance of the bonded joint, followed by changes in the failure mode. Finally, a decrease in performance was observed as a consequence of the long-term aging of specimens in saltwater for up to 549 days.