Abstract. The full-scale fatigue test of rotor blades is an important and complex part of the development of new wind turbines. It is often done for certification according to the current IEC (2014) and DNV GL AS (2015) standards. Typically, a new blade design is tested by separate uniaxial fatigue tests in both main directions of the blade, i.e. flapwise and lead lag. These tests are time-consuming and rather expensive due to the high number of load cycles required, up to 5 million. Therefore, it is important to run the test as efficiently as possible. During fatigue testing, the rotor blade is excited at or near its resonant frequency. The trend for new rotor blade designs is toward longer blades, leading to a significant drop in their natural frequencies and a corresponding increase in test time. To reduce the total test time, a novel test method aims to combine the two consecutive uniaxial fatigue tests into one biaxial test. The biaxial test excites the blade in both directions at the same time and at the same frequency, resulting in an elliptical deflection path of the blade axis. Using elliptical loading, the counting of damage equivalent load cycles is simplified in comparison to biaxial tests with multiple frequencies. In addition, the maximum loads in both main directions remain separated, while off-axis loading is introduced. To achieve such a test, specific load elements need to be arranged so as to equalize the natural frequencies of the test setup for both test directions. This is accomplished by adding stiffness or inertial effects in a specific direction. This work describes a new method to design suitable test setups. A parameterized finite element (FE) model of the test with beam elements for the blade represents the test setup. A harmonic analysis on the FE model can identify the load distribution and the test conditions of a specific test setup within seconds. An optimization algorithm that varies parameters of the model and searches for the optimal setup is then applied to the analysis. This approach allows the efficient determination of a test setup, suited to the predefined requirements. The method is validated by applying it to three different test scenarios for a modern rotor blade: (a) state-of-the-art uniaxial setups, (b) uniaxial setups including springs and (c) a biaxial setup. In conclusion, the resulting setups are evaluated in terms of test quality and efficiency.

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.

Abstract. The full-scale fatigue test of rotor blades is an important and complex part of the development of new wind turbines. It is often done for certification according to the current IEC (2014) and DNV GL AS (2015) standards. Typically, a new blade design is tested by separate uniaxial fatigue tests in both main directions of the blade, i.e. flapwise and lead-lag. These tests are time consuming and rather expensive due to a high number of required load cycles, up to 5 million. Therefore, it is important to run the test as efficiently as possible. During fatigue testing, the rotor blade is excited at or near its resonant frequency. The trend for new rotor blade designs is toward longer blades, leading to a significant drop in their natural frequencies, and a corresponding increase in test time. In order to reduce the total test time, a novel test method aims to combine the two consecutive uniaxial fatigue tests into one biaxial test. The biaxial test excites the blade in both directions at the same time and at the same frequency, resulting in an elliptical deflection path of the blade axis. Using elliptical loading, counting of damage equivalent load cycles is simplified in comparison to biaxial tests with multiple frequencies. In addition, the maximum loads in both main directions remain separated, while off axis loading is introduced. To achieve such a test, specific load elements need to be arranged to equalize the natural frequencies of the test setup for both test directions. This is accomplished by adding stiffness or inertial effects in a specific direction.

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.