Bonded rotor blade joints: Improved test method reduces scale-up risks

Prior to the production of prototypes, adhesive tests have hitherto only been undertaken on coupon specimens. Researchers at Fraunhofer IWES, together with industrial partners, have now developed a subcomponent test as an intermediate step. This provides additional understanding of material behavior on a structure-relevant scale. This more comprehensive approach reduces uncertainty for scale-up process to subcomponent design stage.

The aim of the UpWind project was to develop accurate, verified tools and component concepts for very large wind turbines (8-10 MW), both onshore and offshore. Ever longer rotor blades are being used for multi-megawatt wind turbines. They usually consist of two half-shells, which are bonded together with special adhesive. The loads that act on the bonded joint and the requirement for a service life of 20 years put extreme demands on the bond line. The latter can have a thickness of about 10 millimeters and a length of about 60 meters.

More realistic load distribution
Up until now ca. 15 centimeter long coupon specimens were certified prior to the prototype stage. However, due to production and geometry effects, the load distribution on these specimens differed considerably from the actual load distribution on the rotor blade prototypes.
As part of the EU funded UpWind project, scientists from the Fraunhofer Institute for Wind Energy and Energy System Technology (IWES) investigated whether a so-called beam test can meet these requirements. These activities were

enhanced by parallel industry projects done with Henkel. A “beam in bending” test methodology that was has been developed in collaboration with Henkel was the starting point of the improvement.

“Knowledge of the physical properties of our products under in-service conditions is essential for successful applications,” explains Felix Kleiner, Manager of Adhesive Engineering at Henkel AG & Co. KGaA. “The new test method allows economic evaluation of different adhesives and design variations”. The base model that was used for this was an I-beam – a model which takes into account two bonded seams between spar cap – shear web – spar cap.

Enhanced understanding of material behavior
These tests provided information about the mechanisms of material fatigue and material failure. “In order to investigate the mechanical behavior of the adhesive in a relatively large adhesive volume, the beams were designed to have a critical section”, says Florian Sayer, Team Leader of “Component and Material Testing” at Fraunhofer IWES. This enhanced understanding of material behavior is being utilized in a follow-up research project to scrutinise further options for bond line structuring that will be summed up in a detailed catalogue. Moreover, a simplified numerical beam model for simulating material fatigue at the bonded seam will be developed. The beam test method that was validated in the project is available for interested industrial parties.