Testing of rotor blades

The bi-axial fatigue loading of wind energy converter rotor blades at various frequencies was tested at Fraunhofer IWES in November 2010. The loading method used to oscillate the blade will be discussed in this paper. More wind energy converter rotor blades will be tested for fatigue life in the future due to the ongoing optimization process within an increasing industry. State-of-the-art tests are made by oscillating the blade separately in flap and edge eingenfrequency for 1 to 5·10e6 cycles. With this method, a constant amplitude test is performed in flap and edge direction separately. Because the eigenfrequencies of the blade cannot be changed, the testing time is set to the load cycle number needed for the test divided by the eigenfrequency for each direction. The test time needed might easily exceed five months. One idea of reducing the testing time but keeping or maybe improving the quality of the test, is to move the blade in a bi-axial oscillation in flap and edge direction at the same time, which should reduce the testing time by a factor of two. The bi-axial blade tests performed in November 2010 at the Fraunhofer IWES within the "Better Blade" project showed that a bi-axial loading of a rotor blade in or near both eigenfrequencies can be performed with two servo hydraulic cylinders directly attached to the blade. It was shown that the movement of the blade is well predictable throughout the test and that there is no measurable disturbance, for example due to higher eigenfrequencies. The different options how to move the blade in a bi-axial way are discussed in this paper. How these loadings affect the material of the different blade structures is not a major topic. Future work will contain the calculation of the damage applied to various blade structures by loading the blade with the different bi-axial movements. Then a comparison with the damage calculated by the numerical turbine simulation should be performed.