Proof of concept: Elliptical biaxial rotor blade fatigue test with resonant excitation

Rotor blades of wind turbines are subjected to high-cycle fatigue during their lifetime of 20-25 years. To show that a blade design fulfils the normative requirements, the test campaign usually consists of two consecutive cyclic tests, i.e., in the flapwise and lead-lag directions. The fatigue test excites the blade close to its corresponding natural frequency. Combining the two uniaxial tests into one biaxial test excites the blade in both directions simultaneously. Using the same excitation frequency for both directions results in the blade cross-sections describing an elliptical deflection path. To realize such a test while still exciting close to resonance, the natural frequencies for the two directions need to be equalized with the aid of decoupled masses and stiffness elements. This approach reduces the testing time, and induces a more realistic loading which is comparable with field conditions while keeping the energy consumption of the hydraulic actuation low. This work describes the concept of the elliptical biaxial rotor blade fatigue test with resonant excitation using a commercial blade design. To this end, the design model, which uses both a transient and a harmonic simulation, is validated with the experimental results of the test. The simulation model and the experiment agree well with each other in terms of displacements and loads along the blade.