The influence of high preloading forces on the behaviour of the bolt in ring flange connection

Ring flange connections with preloaded high-strength bolts are commonly used to connect segments of tubular steel towers supporting wind turbines (WT). The performance of a bolt in a flange connection is crucial for ensuring the structural integrity and reliability of cyclically loaded towers. The behaviour of the ring flange connection is dominantly influenced by the geometry of the connections and accomplished pretension force in bolts, and bolt grade. According to IEC 61400-6, the design pretension force of the bolt is limited to 70% of the yield strength of the bolt. This rule exists in EN1993-3-1-8, and it is imposed to limit the size of plastic strain regions. The plastic strains firstly appear in the threaded part of the bolts, in contact with a nut. In this numerical study, the influence of high preloading forces on the behaviour of ring flange connection is presented. The experimental results and a finite element analysis are compared on a flange segment using M56 bolts, ISO nuts and washers to validate the numerical model. The effect of the preloading on three key aspects: plastic strain distribution, deformation capacity, and a shell(tower)-to-bolt transition function are presented in the paper.