Title:

Fatigue equivalent load cycle method: a general method tocompare the fatigue loading of different load spectrums [ECNC95074]


Author(s):



Published by:

Publication date:

ECN

1995


ECN report number:

Document type:

ECNC95074

ECN publication


Number of pages:

Full text:

18

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Abstract:
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 SN curve on
loglog scale. Different material behaviour may be characterised with
different slopes of the SN curve. A disadvantage of the above method is the
neglecting of the mean level of a load cycle. In case of glasspolyester,
glassepoxy, 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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