Simultaneous Modal Analysis and Fatigue Testing of Wind Turbine Rotor Blades

Kurzfassung

This paper shows a structural health monitoring procedure for rotor blade fatigue testing without stopping the test for inspection. In detail it is demonstrated how modal analysis and fatigue testing of wind turbine rotor blades can be perfomed simultaneously. The proposed method was validated using a 12.6 m rotor blade equipped with 24 accelerometers. An imbalance fatigue exciter was positioned at R = 7.8 m to drive the blade at its first flapwise eigenfrequency of 1.83 Hz. This setup was supplemented by an additional electrodynamic shaker at R = 3.5 m to excite the rotor blade above the first flapwise eigenfrequency. The acceleration data of this combined excitation was then analyzed to determine the modal parameters (eigenfrequencies, damping ratios, and mode shapes). By comparing these results to a reference measurement (taken with the fatigue exciter switched off), it was found that system identification remains possible even despite the dominant sinusoidal excitation of the fatigue exciter. In total, 11 modes were compared in a frequency range of 3 to 50 Hz. The measured eigenfrequencies deviated by no more than 3.7% from the reference, though the damping values showed significantly higher scatter of up to 69%. A comparison of the mode shapes using the MAC matrix reveals a high correlation. Additionally, it was shown which aspects must be considered in such a test setup and which challenges may arise in the system identification. The main advantage of this procedure is that an already time-consuming fatigue test does not need to be interrupted for inspection purposes, and potential damage can be identified through changes in the modal parameters during the ongoing fatigue test.