Complex welds in the joint region reduce the fatigue resistance of structural joints of circular hollow sections with factors that results in increased wall thicknesses of the tubes and overspending of steel in the multi-membered part of the jacket and floating support structures for offshore wind turbines. The wrapped composite joint is a breakthrough technology utilising bonding and fatigue resistant composite material to connect steel tubular members instead of welding. The main technical advantage is that the superior fatigue life of wrapped composite joints. Full-scale wrapped composite joint is tested by cyclic out-of-plane bending loading in resonance-based Cronos testing rig at OCAS N.V. in Belgium. Results reveal exceptional fatigue life of wrapped joints to combined loading when compared to welded joints, showcasing a potential that can radically reduce steel use for offshore structures.

The dominant failure mode was characterized as debonding in the novel non-welded wrapped composite joint made with GFRP composites wrapped around steel sections. Glass fiber composite-steel three-point end notched flexure (3ENF) and four-point end notched flexure (4ENF) specimens were utilized to experimentally investigate mode II fracture behavior of this composite-steel bonded interface. Two new methods were proposed with the help of digital image correlation (DIC) technique to quantify fracture data during the tests: 1) the “shear strain scaling method” to quantify the crack length a; 2) the asymptotic analysis method based on the longitudinal displacement distribution along the height of the specimen at the pre-crack tip to quantify the crack tip opening displacement (CTOD). To numerically simulate the mode II fracture behavior, a four-linear traction-separation law was proposed in the cohesive zone modeling (CZM) where the softening behavior with a plateau was defined by the authors between traditionally considered initiation and fiber bridging behavior. The experimental and numerical approaches were validated mutually through good matches between the test and FEA results. 3ENF test provided good insight into softening behavior while 4ENF contributed to quantification of fiber bridging. These findings contribute to a more comprehensive characterization and understanding of the ductile fracture behavior of bi-material bonded joints, especially in mode II failure scenarios.