Wrapped composite joints have emerged as a compelling alternative to traditional welding methods for fabricating steel circular hollow section (CHS) joints. These joints are distinguished by their superior performance in ultimate strength and fatigue resistance. This paper presents research on the interfacial properties and fracture mechanisms between fiber-reinforced polymer (FRP) and steel elements within these innovative joints. Given the large-scale dimensions of the wrapped composite joints in practical engineering, the study further explores the impact of size on their interfacial behavior. To this end, FRP–steel interface specimens were fabricated at three different scales. These specimens were subjected to double cantilever beam (DCB) and four-point end notched flexure (4ENF) testing, enabling the analysis of Mode I (opening) and Mode II (in-plane shear) interfacial behaviors. Additionally, finite-element analysis (FEA) was employed to further validate the interfacial properties and fracture characterization. The outcomes from this research provide critical insights into the FRP–steel interface in these innovative joints, which is essential for their accurate modeling and design. This understanding of the interfacial properties is key to the effective implementation and scalability of wrapped composite joints in real-world engineering projects.

Offshore renewable energy sources, such as wind and solar require resilient support structures that are vastly loaded by cyclic loading due to wind and waves. As such, the structures built from steel circular hollow sections are structurally optimal to resist extreme loads but their design is hampered by low fatigue resistance of traditionally welded joints resulting in short lifetime, excessive use of steel material and corrosion problems. Innovative wrapped composite joints connect steel tubes by bonding and replace traditional complex welded joints of tubes by relying on excellent corrosion and fatigue performance of fibre-polymer composite material. Composite joints can reduce amount of steel needed to build supporting structures prone to fatigue up to 50% and can speed up production and assembly of towers supporting wind turbines by factor of 2. In addition, the wrapped composite joints unleash the potential of designing and building corrosion free offshore support structures completely made of composite tubes as structural members, or in combination with steel tubes. This paper presents potential of wrapped composite joints, state of development through experimental testing and numerical modelling, certification of joints and pilot projects in offshore environment. The outlook for further research and development is also given.

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.