Damage Analysis of the Blade to the Rotor Hub Connection in the Wind Turbine

The paper analyzes possible causes of bolts fracture in the connection of the blade to the hub of the wind turbine rotor. This failure has been growth shortly after the wind turbine was started, a few days after the storm. During the storm, electric power was turned off in the whole surrounding area for many hours, therefore it was impossible to control the device. After an initial analysis of failure modes in other wind turbines, it was found, that the bolts failure in the analysed case may be regarded as quite rare. Due to the fact that a significant part of bolts was broken brittle (flat fracture surfaces and lack of permanent elongation), it was decided to analyse the structure of the material in the direction of stress corrosion and hydrogen embrittlement. Among others, microscope analysis of fracture surfaces after their proper cleaning from corrosion products was carried out. The broken bolts were made as undersized, what is mainly used in fasteners intended for hot-dip galvanizing. On the basis of the purchase documentation analysis, it was presumed that the bolts were not galvanized by their manufacturer, which is obliged to perform the routine inspection in production process preventing the introduction of the hydrogenated bolts into the market. Besides the bolts examination, the scenarios of connection loads in extreme situations, in particular the impact of wind with high energy in the most unfavourable direction relative to the blade and nacelle, as well as the force resulting from the impact of the rotating blade on the tower that preceded the collapse of the whole structure, were considered. Conclusions from the performed tests and calculations by use of two calculation models of the connection were presented.