Effects of preload deficiency on fatigue demands of wind turbine tower bolts

ABSTRACT This paper investigates the effect of bolt preload level on fatigue damage in eccentrically loaded bolts of ring flange connections, which are commonly utilized in wind turbine towers. Aerodynamic simulations of the turbine were conducted, and resulting forces were applied to a detailed finite element model of the maximally loaded segment of the bolted connection. Fatigue analyses were performed on the results of the finite element analyses. The study results indicate that a reduction in bolt preload led to an increase in ranges of stress fluctuations, which in turn resulted in increased fatigue damage. The effect of preload loss on stress ranges and fatigue damage were observed more clearly and significantly after the preload dropped to values around 25% of the design preload. Moreover, the simulation results showed that the bending stresses have a major role in fatigue demands in the bolt, and therefore should be included in fatigue damage calculations. However, the contribution of the bolt bending stresses to fatigue demands is more significant in higher preloads, and decreases as the preload is reduced.