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.