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REFERENCES 311
Table 5.8 Joint M X and M Y Cycle Distribution
˜M Y (kNm)
˜M X (kNm) 200–300 100–200 0–100 Total No. of M X cycles
100–150 5 5 10
50–100 10 30 40
0–50 50 50
Total No. of M Y cycles 5 15 80
For a circular tower, the stress ranges would have to be computed at several
points around the circumference in order to identify the location (with respect to
the nacelle axis) where the fatigue damage was maximum.
A simpler but potentially cruder approach to the combination of the two fatigue
spectra is to use the ‘Damage Equivalent Load’ method. This involves the calcula-
7
tion of constant amplitude fatigue loadings, M X:Del and M Y:Del , of, say 10 cycles
each, that would respectively produce the same fatigue damages as the M X and M Y
spectra, using the S=N curve appropriate to the fatigue detail under consideration.
If the M X and M Y fluctuations are treated as being in-phase as before, the combined
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
p
‘Damage Equivalent Load’ moment is M 2 þ M 2 .
X:Del Y:Del
References
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