Sub-component versus full wind turbine blade structure: Influence of manufacture-induced thermal residual stresses on crack initiation in adhesive joints

This work investigates the root causes for tunneling crack initiations in the trailing-edge adhesive joint that were observed during the manufacture and in field on the fleet of more than 100 blades of a 2 MW turbine type. The highest crack frequency was observed at around 50% blade length. To identify the root cause, a trailing-edge sub-component of the blade type was manufactured from the main mold under comparable process conditions as used during the full blade manufacture. Surprisingly, no cracks were initiated during the sub-component manufacture which was contrary to the observations made for the full blade. A linear thermal stress analysis by means of a comparable FE blade model revealed that the full blade was subjected to a higher residual stress level than the sub-component which was in line with the observations. This research shows that sub-components do not necessarily represent all aspects of the structural behaviour of a full blade. To this end, the design of sub-components for design model validation should be conducted with caution