Off-axis loading in rotor blade fatigue tests with elliptical biaxial resonant excitation

In their lifetime of 20-25 years, wind turbine rotor blades are subjected to high-cycle fatigue. Hence, new rotor blade designs need to undergo cyclic fatigue tests to show that they fulfill the reliability requirements. For these tests, a blade prototype is loaded consecutively in its flapwise and lead-lag directions, which does not represent the actual loading in the field very well. This applies especially for the off-axis load directions of the blade, i.e., the load directions between flapwise and lead-lag directions. By combining the two consecutive tests into one biaxial test, these loads can be improved. Exciting both directions at the same time also generates loads in the off-axis directions. This work investigates the loading of a biaxial fatigue test using elliptical biaxial resonant excitation. By changing the phase angle of the elliptical excitation, the loading of the blade can be altered to better represent the operational conditions of the blade.