Up-tower repair of pultruded carbon spar cap planks in wind turbine blades

Modern wind turbine blades implement carbon fiber-reinforced polymer (CFRP) planks in their load carrying structures, i. e., the spar caps. These CFRP planks are pre-manufactured in a pultrusion process and are stacked into the blade structure. The pultrusion process ensures a high quality with respect to fiber alignment, fiber volume fraction, and low manufacturing defects. All these metrics can be manufactured with a high reproducibility leading to outstanding mechanical properties. CFRP structures can be damaged during operation, e.g., due to voids, undulations, defects, dielectric breakdowns, or flashovers because of a lightning strike [1]. Conventional repair techniques by infusion of dry fiber patches cannot achieve the properties of the original CFRP planks. Therefore, pre-cured and chamfered CFRP plank patches are bonded into the recess. In case innermost CFRP planks are damaged in the inboard blade region, a repair from the blade's inside is required, which requires grinding off the damaged CFRP planks and cutting away a window from the shear web. An up-tower repair is generally preferred over a down-tower repair at site or in the workshop when considering the longer downtime and higher efforts for logistics as well as mounting. In an up-tower repair situation the temporarily weakened structure needs to withstand extreme wind situations, in particular offshore. To this end, this research investigates the feasibility of an up-tower repair concept by means of (i) simulations and (ii) practical considerations. Therefore, a detailed finite element model of a commercial rotor blade, which was designed for a 7MW offshore turbine, was subjected to a maintenance design load situation. The blade loads were determined by aeroelastic simulations. The weakened blade structure was investigated towards permissible strains and buckling for conducting the repair.