In this paper a vibration-based approach to identify a crack in a wind turbine blade is described and demonstrated\ud numerically and experimentally.\ud Operational modal analysis has been performed before and after a buckling test, and vibration data, gathered from some\ud accelerometers placed along the blade, was used to monitor the integrity of the structure, since the modal parameters\ud are directly influenced by the physical properties of the structures. Additionally a numerical prediction has been done\ud both with a full-scale model and with a one-dimensional model. The results show that this approach is able to estimate\ud successfully the presence of damage and a good numerical and experimental correlation has been found. Finally, some\ud considerations regarding the rotation of the blade in the undamaged and damaged cases have been done

This paper presents selected results and aspects of the multidisciplinary and interdisciplinary research oriented for the experimental and numerical study of the structural dynamics of a bend-twist coupled full scale section of a wind turbine blade structure. The main goal of the conducted research is to validate finite element model of the modified wind turbine blade section mounted in the flexible support structure accordingly to the experimental results. Bend-twist coupling was implemented by adding angled unidirectional layers on the suction and pressure side of the blade. Dynamic test and simulations were performed on a section of a full scale wind turbine blade provided by Vestas Wind Systems A/S. The numerical results are compared to the experimental measurements and the discrepancies are assessed by natural frequency difference and modal assurance criterion. Based on sensitivity analysis, set of model parameters was selected for the model updating process. Design of experiment and response surface method was implemented to find values of model parameters yielding results closest to the experimental. The updated finite element model is producing results more consistent with the measurement outcomes.

The ability to identify the dynamic properties of offshore wind turbines allows validating and updating numerical tools, which can be used to enhance the design. At the same time, these dynamic parameters can serve as a basis to continuously monitor the integrity of the machine. However, modal identification of turbines in operating conditions still poses some major issues, in particular in removing the rotor harmonic components, which are polluting the measured signals. This paper will present and discuss some recent developments for removing harmonic components from operational wind turbine data. The possibility to track the evolution of specific modes is compared against classical techniques such as Time Synchronous Averaging and Cepstrum, which show limitations due to rotational speed fluctuations, amplitude modulation of the harmonic components and the interaction between the harmonics and the aerodynamic loads. The methodologies are firstly presented and then applied to real data of an offshore wind turbine installed in the Belgian North Sea. The ability to identify more accurately the modal parameters will allow improving the correlation with the varying environmental conditions and provide additional input data to validate numerical models.

As the vibro-acoustic requirements of modern products become more stringent, the need for robust identification methods increases proportionally. Sometimes the identification of a component is greatly complicated by the presence of a supporting structure that cannot be removed during testing. This is where substructure decoupling finds its main applications. However, despite some recent advances in substructure decoupling, the number of successful applications has so far been limited. The main reason for this is the poor conditioning of the problem that tends to amplify noise and other measurement errors.

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