Monitoring fatigue damage accumulation of wind turbine towers using limited number of output-only vibration measurements

Abstract

Fatigue monitoring and remaining fatigue life estimation of structures using output- only vibration measurements has recently garnered increasing attention, producing advances in theoretical, numerical and experimental studies of this phenomenon. The methodology presented in this paper combines methods for estimating stress time histories at the entire body of the structure with fatigue damage accumulation techniques for multiaxial stress state. A novel sequential Bayesian method is employed to estimate both input and state in the modal space and to reconstruct the full-field time-history response in the physical space using output-only vibration measurements. Stress and strain time histories at the finite element level are obtained by using a linear relationship with nodal displacements. Estimated stresses are then used to find the critical plane where the maximum fatigue damage is expected and the shear stress time histories are resolved on this plane. Shear stress cycles are counted by means of the Rainflow Counting Method, and a Modified Wöhler Curve Method is applied to estimate the fatigue damage, whereby normal and shear stress effects are accounted for. This procedure is capable of tackling inherent complexities found in real world applications, such as the multiaxiality of the applied loads and of the resulting stress state. A finite element model of a wind turbine tower was constructed based on reference specifications available from the National Renewable Energy Laboratory and used to illustrate the method presented herein. The results obtained demonstrate the applicability of the methodology as an efficient way to monitor fatigue damage accumulation in the entire body of a steel structure. © 2020 European Association for Structural Dynamics. All rights reserved.