Biaxial single frequency fatigue test for wind turbine blades

A test system for testing a wind turbine blade, comprising:

• a test stand configured to receive and support the wind turbine blade during testing,
• at least one first excitation unit configured to be attached to the wind turbine blade at a first longitudinal position, the at least one first excitation unit being configured to actively introduce loadings in the wind turbine blade in a flapwise direction at a first excitation frequency,
• at least one second excitation unit configured to be attached to the wind turbine blade at a second longitudinal position, the at least one second excitation unit being configured to actively introduce loadings in the wind turbine blade in an edgewise direction at a second excitation frequency,
• at least one load controllable unit configured to attached to the wind turbine blade at a third longitudinal position, the at least one load controllable unit being configured to adjust a resonant frequency of the test system in the flapwise direction and/or in the edgewise direction,
• a control unit electrically connected to the first and second excitation units, wherein the control unit is configured to control cyclic loadings introduced in the wind turbine blade by transmitting control signals to the first and second excitation units,
• at least one detector unit positioned at one or more locations on the wind turbine blade and connected to the control unit, wherein the at least one detector unit is configured to detect relative movement or loadings of the wind turbine blade and to transmit at least one output signal to the control unit,
• wherein the control unit is configured to monitor and control an elliptical motion pattern of the wind turbine blade by closed-loop controlling first and second harmonic movements of the wind turbine blade and a phase between the first and second harmonic movement in a closed control loop.