Page 430 - Wind Energy Handbook

404 COMPONENT DESIGN

In deriving the fatigue damage plots in Figure 7.15, a mid-range value of 4 has
been adopted, resulting in total damages of 0.96 and 0.17 for the pitch and stall
regulated machines respectively. However, if, the minimum value of 2.7 were
adopted, corresponding to ó 0k ¼ ó ck and ª mu ¼ ª mf , the fatigue damages would rise
by a factor of 50 for m ¼ 10.
The other important material property governing the criticality of fatigue loading
is, of course, the slope index of the log–log S–N curve, m, which affects the value of
the notional one cycle equivalent load, ó eq(n¼1) . With the high values applicable to
wood laminates, fatigue is much less likely to govern.

Other sources of variability

There are a number of other sources of variability in fatigue damage calculations,
apart from uncertainty about the material properties themselves, some of which are
detailed below.

(1) IEC 61400-1 provides two alternative stochastic turbulence models – those due
to Von Karman and Kaimal. The Von Karman model is isotropic, whereas in the
Kaimal model, which is more realistic in this respect, the standard deviations of
lateral and vertical turbulences are 80 percent and 50 percent of the longitudinal
turbulence respectively. In the case of stall-regulated machines, where wind
misalignment at high wind speeds is often the main source of fatigue damage,
the choice of turbulence model could clearly have a decisive effect.

(2) When the fatigue assessment is based on simulations of limited duration
(typically 300 to 600 s), the damage is often dominated by a few extreme cycles,
which are subject to significant statistical variation from one simulation to
another. Accordingly several simulations at a given mean wind speed are
necessary to obtain an accurate result (Thomsen, 1998).

(3) In allowing for the reduction in fatigue strength due to mean stress. For
example, in Equation (7.7), the mean stress can either be calculated over each
stress range obtained by rainflow cycle counting or over the length of the
simulation.

Fatigue due to gravity loading

In-plane fatigue loads arise from gravity loading and fluctuations in the in-plane
aerodynamic loadings, but gravity loadings dominate for machines large enough to
be grid connected, rendering the loading calculation relatively straightforward.
In order to compare the approximately constant amplitude in-plane fatigue
loading with the spectrum of out-of-plane fatigue loading, it is convenient to
express both as equivalent loads at a specified number of cycles, n eq . Often the 1 Hz
equivalent load is used, in which case the number of cycles, n eq , is equal to the
number of seconds in the machine lifetime during which the machine operates. For