Failure modes of bonded wrapped composite joints for steel circular hollow sections in ultimate load experiments

The concept of an innovative bonded joining technology where welding is not required is presented as an alternative to traditional welded connection for steel circular hollow section (CHS). Wrapped composite joints have potential to greatly improve fatigue endurance when applied in multi-membered truss structures, e.g. offshore jackets for wind turbines. This paper focuses on characterization of resistance and understanding of failure modes of wrapped composite joints in static experiments, as the prerequisite for harvesting its potential for high fatigue endurance. Wrapped composite joints at two scales and with two different angles of X-joint geometry are made with GFRP composite material wrapped around steel sections without welding, and tested in 3 monotonic loading cases, tensile, compression and in-plane bending, until failure. Counterpart welded joints are tested at the smaller scale for stiffness, elastic limit and ultimate load comparisons. Two general failure modes of wrapped composite joints, debonding and fracture of the composite material are identified and quantified by surface strain measurements through 3D digital image correlation (DIC) technique. Testing results indicate that wrapped composite joints have 30% to 56% larger stiffness and 3% to 68% larger ultimate load compared to welded counterparts. Debonding and final pull-out of steel brace member from the composite wrap is predominant failure mode in tensile experiments at both scales while cracking of the composite material is the governing failure mode in the bending experiment. In tensile, compressive and bending experiments failure load of wrapped composite joints exceeds the yield resistance of the steel CHS indicating opportunity to optimize the composite wrapping thickness and length.