Abstract: The single-sector flange bolt joint of a 2 MW wind turbine is investigated, and the bolts (Grade 10.9, M48) in second-layer tower are specifically used for experimental and simulation analyses. The relationship between the clamping force and the external load under different bolt preload conditions is examined, and a mechanical model for calculating force in L-shaped flange bolts with finite stiffness is developed. The bending of bolts and the separation of flange surfaces during tension are also investigated. The simulation and experimental results are presented as follows: First, the incremental clamping force of the tower bolts is nonlinearly related to the external load. The relative width of the external load area on the flange is incorporated into the new mechanical model, demonstrating close agreement between predictions and experimental results. Secondly, a reduction is noted in both the bolt load increment and bending effect under tensile load as the initial preload increases. A positive correlation is observed between the preload and the nonlinear starting point of the load-clamping force curve, with no significant impact on the load factor before the inflection point. Thirdly, the stress distribution in the flange contact area under tension shows that the critical separation load of the eccentric flange is linearly positively correlated with the preload. The minimum design preload should be based on the critical preload that causes initial separation of the bolt hole near the external load side when the flange is subjected to rated external load. Finally, when eccentric joints are subjected to tensile loads below the nonlinear starting point, the significant reduction in the real load factor of the tower bolt compared to that calculated from the relative bolt-flange stiffness is attributed to the non-uniform stress distribution in the flange area, which is beneficial for reducing the fatigue stress amplitude of the bolt.

Key words: wind turbine, bolt, preload, bolt load variation, model