Influence of a wind turbine service life on the mechanical properties of the material and the blade

This paper presents results out of investigations of the DEBRA-25 wind turbine blades. Almost unique in the history of modern wind energy, these blades were in operation for 18years next to a weather station and were investigated afterward. Therefore, the loads experienced in the operational life could be post-processed accurately with the measured data of the weather station and the turbine. The blades are made of materials that are similar with today's wind turbines. Furthermore, intensive laboratory tests and free field tests have been carried out, and all load assumptions and data and results are still available today. The results include experimental investigations on the moisture content of the load-carrying material, static and fatigue behavior of the material, the relaxation of the coupling joints, the natural frequencies of the blade and a full scale static blade test. It is shown that the structural performance of the DEBRA-25 service blades is comparable with modern wind turbine blades. Although some damage was found by visual inspection, the service blade of the DEBRA-25 showed excellent mechanical behavior in the full scale blade test. Only small changes of the edgewise eigenfrequencies were detected. The pre-tensioning forces of the IKEA bolts, where the two blade parts are connected, were measured and were still adequate.