Effect of manufacturing defects on fatigue life of high strength steel bolts for wind turbines

• Initial cracks in the thread root of bolt have a significant effect on the slope of the S-N curve. • Crack inclination angle has large influence on fatigue performance. • Friction coefficient does not significantly affect mode-I stress intensity factor. • Mode-II stress intensity factor increases with increasing friction coefficient. This work investigates the effect of manufacturing defects induced by thread rolling on the high-cycle fatigue life of M30 class 10.9 stud bolts. High-cycle fatigue tests of two batches of nominally identical bolts show significant differences in fatigue performance between the batches. Scanning electron microscopy characterisation of defects in the form of rolling-induced microcracks in the thread root reveals a clear correlation between the defect size and the fatigue life of the investigated bolts. It is demonstrated that initial cracks present in the stud threads have a considerable effect on the slope of S-N curves. Numerical fatigue analysis shows good agreement with the experimental data. Electron backscatter diffraction is used to establish a characteristic length of microstructurally short defects in the tempered martensite microstructure. The obtained results shed light on the definition of critical manufacturing defect sizes in bolts as an essential parameter for quality control in manufacturing process.