Investigation of structural bond lines in wind turbine blades by sub-component tests

In the present study, the structural fatigue performance of an adhesive joint, imitating the connection of the shear web to the spar caps of a wind turbine blade, is investigated. An asymmetric three point bending test has been developed from early design steps to manufacturing and testing. The sub-component is supposed to fail in the bondline, behavior consistently validated after a comprehensive testing campaign. Similar failure patterns to the full scale wind turbine blades have been observed in the bond line during the fatigue tests. Tensile cracks propagated transverse to the adhesive longitudinal axis, approximately at 10% of the total lifetime. Their growth was hindered from the spar cap and web laminate. After around 60% of the specimen life, cracks changed their direction resulting in disbonds between the adhesive-adherend interface. Shear stress variations and artificial defects seemed to have a relatively low impact on the sub-component fatigue performance. At the same time it was shown that cover-laminates drastically increase the fatigue performance of the adhesive joint.