Cause of Fracture on High Strength Bolt for Wind Turbine Blade Connection

The high strength bolts used for blade connection of XE112-2000 wind turbine fractured frequently in service. The causes of fracture on bolt were analyzed by means of macro observation, metallographic examination, mechanical property test and chemical composition analysis. The results show that the fracture mode of bolt was corrosion fatigue fracture. The main reason was that the thread forming process of the bolt was quenching and tempering first and then rolling forming. The outer surface of thread was squeezed, which led to the opposite tensile stress on the top of thread. In addition, due to the influence of work hardening, the bolt produced large residual stress, which increased the possibility of microcracks and surface defects, resulting in the stress corrosion cracks at the center of the top of thread. In the later service, due to fatigue action, stress concentration occured at the crack, which led to rapid crack growth, and finally corrosion fatigue fracture occurred in thread.