Structural degradation of a large composite wind turbine blade in a full-scale fatigue test

Wind turbine blades are expected to sustain a high number of loading cycles typically up to a magnitude of 1,000 million during their targeted service lifetime of 20-25 years. Structural properties of composite blades degrade with the time. Although substantial studies, such as [1,2], have been carried out at a coupon level to characterize fatigue degradation of composite materials, there is no much study focusing on fatigue degradation of rotor blades at a fullscale structural level. Do structural properties of composite blades degrade in a similar manner to what has been observed in material tests at a coupon level? What might be the concerns one should take into account when predicting residual structural properties of rotor blades? To answer, at least to a partial extent, these questions, this study conducts a full-scale fatigue test on a 47m composite rotor blade according to IEC 61400-23 (ed. 2014). A conventional single-axis mass resonance excitation (rotating mass) method is used as it is now still widely used for blade certification. The blade is tested in a flap-wise bending direction with the suction side primarily under compressive stress and pressure side under tensile stress, see Fig. 1. The applied loads are increased to reduce the number of cycles to 2.0 million cycles. Bending stiffness of the blade is measured at different span-wise sections during the fatigue test in order to measure its possible degradation. Natural frequencies and damping ratios are measured both before and after fatigue test. Post-fatigue damage of the blade is examined throughout the blade. It is found that the blade exhibited different stiffness degradation patterns at different cross sections. As shown in Fig. 2, the bending stiffness of the blade from 0 to 19 m did not show obvious degradation during fatigue test. However, the bending stiffness of the blade from 0 to 28 m and that from 0 to 39.5 m showed very similar degradation pattern to composite materials, which is fast at the early stage and slow at the following stage. In addition, it is noted that the overall stiffness degradation is shown to be not significant.