ECN publication
Fatigue equivalent load cycle method: a general method tocompare the fatigue loading of different load spectrums [ECN-C--95-074]
Published by: Publication date:
ECN 1995
ECN report number: Document type:
ECN-C--95-074 ECN publication
Number of pages: Full text:
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Fatigue is the main design driver for the calculation of the structuralintegrity of wind turbine components. In the JOULE II project 'Load and Power Measurement Programme on Wind Turbines Operating in Complex Mountainous Regions' and in several other research projects there is a need to compare different fatigue load spectrums on a quantitative basis. A common way to compare two or more fatigue load spectrums is the use of an equivalent load range. The calculation of the equivalent load range is easy to perform. The fatigue behaviour of the material is formulated with a straight S-N curve on log-log scale. Different material behaviour may be characterised with different slopes of the S-N curve. A disadvantage of the above method is the neglecting of the mean level of a load cycle. In case of glass-polyester, glass-epoxy, cast steel, carbon epoxy, or wood laminates the mean level of the cycle effects the fatigue life. This could be avoided by calculating the fatigue stress reserve factor. This factor is defined as the factor by which the prevailing fatigue stress has to be multiplied in order that the calculated fatigue lifetime equals the design lifetime. The disadvantages of the fatigue stress reserve method are the need of detailed cross sectional data, the need of the specific fatigue formulae of the materials, the iterative calculation of the factor, and the fact that the results are not easy to generalize for other materials than considered. In this document an extension to the equivalent load range method is defined. With the extension the mean level of the load cycles is taken into account. The method is easy to apply and fully consistent with the equivalent load range method. In chapter 2 the formulae for calculating the equivalent load range are given. In chapter 3 the formulae for the equivalent load cycle method are given. An example is presented in chapter 4. Some conclusions are given in chapter 5. 2 figs., 3 tabs.

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