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.

Debonding is one of the most critical failure modes for bonded joints under fatigue loads. Numerical prediction on the fatigue debonding behaviour of bonded interfaces with complex geometry still remains a problem. This paper proposes a numerical methodology based on fracture mechanics to predict crack growth in a complex bi-material interface and illustrates the prediction procedure by a case study on wrapped composite joints. Interface coupon tests provide the fatigue crack growth properties at the composite-to-steel interface used as inputs for finite element (FE) modelling. The FE model is calibrated against fatigue tests of small scale wrapped composite joints with different steel surface roughness subject to different load levels. A sensitivity analysis is conducted to investigate the influence of key modelling parameters. The calibrated model is validated against fatigue tests on upscaled joints. Good agreements are shown between the test and modelling results in terms of crack growth and stiffness degradation, demonstrating the potential of the proposed numerical methodology for predicting fatigue debonding behaviour of complex bi-material interfaces.