Available Technology

Dual-Axis Resonance Testing of Wind Turbine Blades

Wind turbine blades must undergo strength and fatigue testing in order to be rated and marketed appropriately. Presently, wind turbine blades are fatigue-tested in the flapwise direction and in the edgewise direction independently. This testing involves placing the blades through 1 to 10 million or more load or fatigue cycles, which may take 3 to 12 months or more to complete for each tested direction. There is a need for blade testing techniques that are less expensive to use and require less time to complete while still providing accurate fatigue testing results. NREL scientists have designed a dual-axis resonant testing apparatus that can perform fatigue-testing in the flapwise and edgewise directions concurrently which cuts down the time and expense necessary to carry out these tests. The invention, by allowing for performing both tests simultaneously, significantly reduces the time required for blade fatigue testing and reduces associated costs such as the amount of energy while increasing test accuracy and freeing up facilities for additional testing. Additionally, with independent flap and edge fatigue tests, it had typically only been possible to properly fatigue the blade at two locations (e.g., one location for each blade test). However, by combining the flap and edge loading as part of a single fatigue test, this dual-axis resonant testing apparatus allows simulation of realistic loading conditions at multiple locations around the perimeter and span of the blade.
A dual-axis resonant test system is provided for concurrently testing wind turbine blades and other elongate test articles in two directions such as the flapwise and edgewise directions of a blade. The test system includes an oscillating or actuator assembly that is mounted to the blade at a distance from the root, which is semi-rigidly mounted to a test stand or fixture. The cantilevered blade then is fatigue tested by operating the oscillating assembly to impart a forcing function in the flapwise direction and a forcing function in the edgewise direction to cause the blade to concurrently oscillate principally in both the flap and edge directions. Concurrently, the flapwise actuator and the edgewise actuator impose an external force in the flap and edge directions respectively. Further, this can be executed with a controller-provided amplitude at or near the resonant frequency of the blade in both directions simultaneously. The controller may process feedback signals from sensors mounted on the blade to determine loads or bending moments on the blade in the flap and edge directions, store this data, and also provide adaptive displacement signals with amplitudes to the flapwise and edgewise actuators to maintain the applied loads within a desired load or test envelope (e.g., the dual-axis testing may involve a load that varies over time or between cycles but that is controlled to fall within an acceptable envelope). The dual-axis resonant test method can be applied to any length of turbine blade, but, more particularly, an apparatus is provided for fatigue testing elongate test articles including wind turbine blades such as larger blades over 40 meters or the like.
Quicker fatigue testing -Reduced testing costs -More realistic testing conditions
Internal Laboratory Ref #: 
NREL 07-20
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