Influence of bolt strength in L-type flange joints for wind turbine support structures: An experimental investigation

Offshore wind turbine support structures rely on bolted ring-flange joints, where bolt strength and joint deformation critically influence structural reliability. This study experimentally investigates the tensile performance of Grade 12.9 and Grade 10.9 high-strength bolts using a full-scale L -type flange joint configuration representative of a 5 MW wind turbine. Results show that the S1–1(Gr.12.9) specimen achieved a 4.7 % higher ultimate load-carrying capacity and approximately 20 % lower flange gap opening, reflecting greater joint stiffness, while the S2–2(Gr.10.9) specimen exhibited higher strain and greater ductility, with more pronounced flange deformation. Shell-strain measurements indicate localized early yielding in the upper shell for S1–1(Gr.12.9) due to stiffer axial load transfer, whereas all other shell regions experienced higher deformation under S2–2(Gr.10.9). The S1–1(Gr.12.9) configuration also showed elastic tension–compression reversals in the external shell, a behavior not observed in S2–2(Gr.10.9). These findings highlight a clear strength–ductility trade-off between the two bolt grades and provide quantitative guidance for selecting bolt grades in offshore wind turbine flange-joint applications. • Experimental study of L-type flange joints with Gr. 12.9 and Gr. 10.9 bolts under axial tension. • Gr. 12.9 bolts increased ultimate strength by 4.7 % and reduced flange opening by 20 %. • Gr. 10.9 bolts showed higher ductility, lowering stiffness but increasing deformation risk. • Results provide quantitative guidance on bolt-grade trade-offs for offshore wind turbine structures.