Fatigue of composites for wind turbines

The size of wind turbine rotors has increased in the past decade from 40 m to more than 120 m diameter. The resulting mass of about 18 tons per rotor blade causes high bending moments at the inner part of a blade due to the gravitational loads. More than 108 load cycles will happen in the prospected lifetime of 20 years of a turbine. During this time the rotor blades are exposed to various hostile conditions such as extreme temperatures, humidity, rain, hail impact, snow, ice, solar radiation, lightning and salinity. In order to withstand these external conditions without diminishing the safety a sound knowledge of the fatigue behaviour of the material and structural properties is needed. To meet the upcoming requirements the paper will highlight some fatigue and lifetime aspects on wind turbine rotor blades made of composite materials. This includes an historical part in connection with glider technology, the presentation of relevant S–N curves not only for the 0°-orientated fibres representing the spar cap but also for ±45°-lay-ups in shear web and shell, the influence of fibre content and architecture, of environmental effects, a view on lifetime prediction on structural elements as well as on present and future work.