sin αst

LIST OF ATTACHMENTSSTRUCTURAL ANALYSIS OF BLADES
The first resonance frequency is a flapwise natural bending frequency (about the y-axis),
whereas the others are twisting and edgewise frequencies, so high that they are of no
concern. Figure E.44 shows the first mode shapes for a rotating blade.
180
Figure E.44: First mode shape for rotating blade
The first flapwise natural frequency of the rotating blade exceeds the maximum 3P fre-
quency by more than 14%. There is thus no apparent concern for resonance excitation of
the rotor blade.
To further verify the vibrational stability of the wind turbine, it is necessary to take into
account the dynamic couplings that exist between the interacting components of the
wind turbine, e.g. the tower and the rotor. This is however beyond the scope of this pro-
ject thesis.
E.12 Longevity expectation
The fatigue strength (10 7 ) of the steel shaft has been verified in appendix G and no num-
ber of loads is therefore considered to cause its failure. Other components are considered
replaceable during routine maintenance and therefore do not limit the longevity of the
complete wind turbine. Maintenance may include, but is not limited to:
� Lubrication
� Periodic testing of emergency shutdown/overspeed system
� Replacement of bearings and slip-rings
Since the fatigue properties of the tower are beyond the scope of this project thesis, the
blades are considered the limiting factor when it comes to longevity.
No S-N curves are available for the blade material, which means that it is not immediately
possible to determine the longevity of the blades. An attempt is however made to conser-
vatively estimate their fatigue life. While S-N curves are not available for the used mate-
rial, IEC 61400-2 provides one for birch. Figure E.45 shows the tensile stress range
parallel to the fibres for birch.