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.

The presented research aims to examine local fatigue failure modes of Glass Fiber-polymer Composite (a.k.a. GFRP), particularly how the bending of the Web-to-Flange Junctions (WFJ) and tensile loading of the material affects the composite's fatigue resistance. By conducting experimental analyses, the study investigates GFRP's efficacy both as a material and as a component element, drawing parallels with Eurocode standards for steel and concrete. The coupon tests under static and fatigue loading conditions provided an empirical data on the Ultimate Limit State (ULS) and fatigue resistance of the GFRP composites, including a better understanding of the effects of variation in the facing’s thicknesses in the web-core composite on the fatigue response. Subsequent three-point bending tests on the WFJ provided the insights into the strength of WFJ with the first indications on the governing failure mechanisms in the WFJ.