Size Effects on Mode I and Mode II Fracture Behavior of FRP–Steel Bonded Interface: Experimental and Numerical Investigation

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.