Identification of the test setup influence on the modal properties of a short wind turbine blade during fatigue test

Modal testing of different test artefacts requires a multitude of decisions to be taken before the test, active experiment and analyses stages of the entire testing campaign. Modal test is often performed only once, and one of its major application is to validate the numerical model. The test-related decisions may impair the quality of the test results and in turn the following correlation and model updating procedure.

The current paper provides a systematic approach for the parametric study of the test setup influence on the blade modal properties. The same wind turbine blade has been configured in four different test setups, including heavy instrumentation of the blade for the fatigue certification tests. The main objective of the research was to provide a comprehensive assessment of the impact of the test setup on the modes.

Experimentally supported evidence has been found that mounting blade on the test block modifies the system structural dynamics. Not only existing global blade modes receive a shift in frequencies, but also new coupled modes may emerge. A new finding of the presented research is an experimental based characterisation of the impact of heavy excitation equipment for fatigue test.

Adding exciters of a mass nearly equal of the blade mass has little influence on the identified natural frequencies of the first two modes, while lower all frequencies from the third mode onwards. The structural damping ratios are also affected, and generally increased. A vibration engineering standard free-free boundary conditions with the impact hammer excitation serves as a baseline model, to which the other test setup are compared.

The estimated modal models of the intact blade will serve as a reference for the modal based damage detection methods application.