Investigation of fatigue life for a medium scale composite wind turbine blade

Abstract

In order to satisfy fatigue requirements in designing a cost effective wind turbine, the wind turbine blade, which is an expensive key component of the wind turbine system, must achieve very long operating life of 20–30 years. In this study, the fatigue life of a medium scale (750 kW) horizontal axis wind turbine system (HAWIS), which has been developed by the present study, was estimated by using the well-known S–N damage equation, the load spectrum and Spera’s empirical formulae in order to confirm more than 20 years operating life. A specific fatigue procedure was proposed with the following three steps. Firstly, from the sample load spectrum data during short period operation, the spectrum data were rearranged as layer numbers, wind speeds, cycles per layer, normalized maximum, minimum, cyclic and average loads, and stress ratios in time order, and then the rearranged data were recorded as cyclic loads per median cyclic load, cycles per layer, cumulative cycles, probability of exceeding, and types of cycles, such as Type I, II and III. Secondly, fatigue loads, such as flapwise and chordwise bending moments were calculated by Spera’s empirical equations with various engineering data of the studying blade for probability of exceeding. Finally, the allowable fatigue strengths were determined from laboratory fatigue property data for the S–N curve of E-glass/epoxy obtained by Mandell, empirical coefficients derived by Goodman diagram with the modified stress ratio and the required design life. In prediction of the fatigue life, it was confirmed that the composite wind turbine blade satisfies the design criteria for the 20 years fatigue life because of sufficient safety margins from the fatigue requirement.