Influence of Loads and Environmental Conditions on Material Properties over the Service Life of Rotor Blades

This paper presents investigations on the DEBRA-25 wind turbine blades from 1984. The weather condition and the loading of the blade are discussed and compared to modern wind turbine blades. The results of a detailed visual inspection of the blades are described. Furthermore the results of experimental investigations on the moisture content of the load carrying material, the relaxation behaviour of the coupling joints, the natural frequencies of the blade and a full scale static blade test are presented. It is shown that the loading of the DEBRA-25 service blades are comparable to modern wind turbine blades. Although some damage was found by visual inspection, the service blade of the DEBRA25 showed excellent mechanical behaviour in the full scale blade test.