Structural Health Monitoring of Wind Turbine Blades - Wind Turbine Control and Monitoring - page 235

Fig. 9.6 The Novelty Index at different rotational speeds under undamaged conditions
The following results show that the effect of this nonlinearity is minor comparing
to the effect of damage.
Figure 9.7 shows the damage index under different damaged levels. The curves
from bottom to top correspond to damage indices in the healthy state and 5, 10, 20,
and 30 % damage magnitudes, respectively. According to this figure, the minor
damage, such as a 5 % stiffness reduction, has a more significant influence on the
damage index. At the same time, the rotational speed has little effect on these
indices. It should be noted that all indices in Fig. 9.7 were obtained from the
structural modal frequencies under known damage levels and rotational speeds.
Figure 9.8 was obtained for unknown damage levels and rotational speeds because
this methodology was designed to address this type of situation. In this case, for a
certain damage level, the modal frequencies with respect to 50 random rotational
speeds were used to calculate the damage indices, which are shown in Fig. 9.8 .
Because the rotational speeds were unknown, the corresponding indices were
plotted from the beginning of the x-axis, and in fact, they were not obtained from 0
to 12.5 rad/s. Finally, Fig. 9.8 shows that the damage indices increase remarkably
with the unknown damage level. The damage detection problem for wind turbine
blades under varying rotational speeds is solved by the methodology presented.
Fig. 9.7 Novelty Index
under different damaged
levels