In wind turbine towers, bolted flange connections are subjected to repeated cyclic loading, making fatigue a critical concern for structural integrity. This study experimentally and numerically investigates the fatigue behaviour of M48 bolted L-flange connections subjected to high preload introduced by tension-tightening. The preload initially reached approximately 95% of the bolt yield strength and stabilised at approximately 79% after tensioner removal. A total of 12 full-scale fatigue tests were conducted to quantify the influence of combined axial and bending loading, as well as preload relaxation during cyclic loading. The results show that high preload significantly improves fatigue life, particularly in the endurance-limit region, where existing design standards provide overly conservative predictions. Cyclic loading was found to accelerate preload relaxation, resulting in an increase in the tensile stress range in the bolt. To account for both bending effects and preload loss, an equivalent effective stress range was proposed, which reduced scatter in the fatigue data and provided a more consistent fatigue-damage parameter. In addition, a three-dimensional fracture-mechanics-based fatigue crack growth framework was developed to predict fatigue life. Bolt preload was introduced in the finite element model using a thermo-mechanical approach, and the model was validated against experimental results. Fatigue crack growth parameters were obtained from dedicated tests on specimens extracted from the bolt material. The numerically predicted S-N curves showed good agreement with experimental results and standard-based predictions in the finite-life regime. The proposed methodology improves fatigue-life prediction and supports more reliable design guidelines for preloaded wind turbine flange connections.

Bolted flange connections in wind turbine towers are subjected to cyclic loading, making fatigue a critical concern for their structural integrity. Bolt preload helps mitigate fatigue damage, but actual preload levels often deviate from design values due to uncertainties in the tightening process and geometric imperfections. This study evaluates the fatigue life of bolts L-flange connections under varying preload levels using a numerical fracture mechanics approach. A comprehensive three-dimensional finite element analysis (FEA) is conducted to assess the effects of preload on the stress intensity factor (SIF), crack propagation behaviour, and load transfer function (LTF). Additionally, the influence of thread helix angle, as well as combined axial and bending loads, on SIF and crack front evolution is examined. Experimental validation of the numerically obtained LTF is performed. A methodology for predicting S-N curves is proposed by deriving normalised solutions for LTF and SIF. The results indicate that increasing preload up to 90 % significantly reduces the SIF range, thereby decelerating crack growth and enhancing fatigue life. However, beyond 90 %, the improvement in fatigue life becomes less pronounced. Furthermore, the findings suggest that Eurocode 3 provides conservative fatigue life predictions, as it neglects bending effects, which are less detrimental than axial loading. Notably, even minor preload loss considerably shortens fatigue life, an effect that becomes more pronounced at higher preload levels. This research contributes to the development of predictive fatigue models for the bolted L-flange connection, providing insights into incorporating preload effects into fatigue life assessments.