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.@en

A new technology of composite wrapped joints emerges as a promising solution for improving the longterm performance of connections between circular hollow sections (CHS). The use of composite materials is shown to improve the fatigue life of structures. However, a major challenge to the implementation of this technology is to ensure the long-term performance of the composite materials subjected to the operational conditions. This work aims to evaluate the effects of temperature changes on the fatigue delamination of a glass fiber reinforced polymer (GFRP) for application in offshore structures. Specimens were manufactured by hand lay-up and series of experiments are performed to access delamination fatigue crack growth behavior in a range of operational temperatures: -10, 21 and 70 °C. Displacement controlled end-notched flexure (ENF) tests were applied to measure the fatigue behavior together with a digital image correlation (DIC) system to monitor the displacements during the tests. Results show that the delamination fatigue performance of the composite material is not significantly affected by the range of tested temperatures. The fracture behavior also remained unchanged. Standard ENF test method has limited range of crack growth to evaluate the fatigue behavior of composite laminates.@en

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.

@en