The wind loads on a rotor blade are variable amplitude, meaning that the cyclic loads continuously change in terms of mean and amplitude (often defined as the distance between peak and mean of the cycle). Nevertheless, material- as well as component or blade testing is performed in 'constant amplitude mode', to simplify machine control and post-processing.
In fatigue tests, different materials may be compared on the basis of their constant amplitude behaviour, by comparing S-N curves. However, in some cases there may be reason to suspect that the behaviour under varying loads is different than the behaviour in constant cyclic loads. For instance, this is the case in steel, where large loads may generate large plastic zones around fatigue crack tips, temporarily hampering further crack development. Thus, the response to such large loads influences the damage that may be incurred by subsequent smaller loads. In that case, variable amplitude load sequences may be used for comparing material behaviour.
By NLR in the Netherlands (see ), a standardised load sequence has been generated, mimicking the realistic load variations in a wind turbine blade. Later, new variations, better reflecting the state-of-the-art, were made in the OPTIMAT  as well as UPWIND  projects, where WMC was involved. The load sequence files, a selection of relevant reports, and an Excel tool for analysing variable amplitude sequences can be found here. These sequences and tools can be used for a comparison of materials or structures in variable amplitude fatigue loading, and for the development and comparison of fatigue life prediction models. They are not intended for design calculations, however.
 Nijssen, Rogier, T. Westphal and E. Stammes., 'Randomising New Wisper - Development and implementation of a method for rainflow equivalent randomization of variable amplitude sequences (UPWIND project)', Knowledge Centre WMC, report WMC-2010-35, Wieringerwerf, the Netherlands, 2010
For inquiries please contact:
Knowledge Centre WMC
1771 MV Wieringerwerf
phone +31(0)227-50 49 49