Blade Design, Manufacture, and Testing - Small Wind Turbines: Analysis, Design, and Application - page 139

for example, normally require a primer to bond to a gelcoat. Epoxy resin is also
particularly sensitive to ultraviolet degradation and so must be covered if a 20 year
blade life is required. Again there are several choices, including tough two-part
epoxy paints. Leading-edge tape can provide additional protection in the region
where insect and dust impact is greatest 8 or materials like Bladeskyn 9 can be used
to coat the whole blade surface.

7.6 Blade Testing

The blades are the most critical component of a wind turbine in terms of safety,
so it is important to test blades in as many ways as possible. The main tests are:

• ''coupon'' tests of the blade material to determine material properties like
Young's modulus and the yield stress, verify FE models, and justify safety
factors over operational temperature ranges,
• static blade tests, and
• fatigue tests
Coupon tests are specifically required by GL [ 9 ] to establish the temperature-
dependence of the blade material properties. However, only the second of the three
listed tests is required by IEC 61400-2. Figure 7.13 shows a static test of a Hoop
Pine blade of the shape shown in Fig. 7.7 . Clearly Hoop Pine is a very suitable
blade material. In a static test, it is important that the blade be mounted as it is on an
actual turbine. These simple tests measure deflection against load, and provide an
important check of the blade structural modeling such as the FEA in Fig. 7.8 .This
ensures, for example, that a blade bending during high winds does not hit the tower.
Load Case H in the IEC SLM covers this situation as described in Chap. 9 .
Generally small turbines have a larger distance between the tower axis and the
blade attachment point than do large turbines, when expressed as a fraction of R,so
such contact is less likely. Nevertheless it is still necessary to demonstrate
clearance.
Extra information is obtained from static torsional tests and determination of
the blade's natural frequency. With this information it has been demonstrated that
FE models can accurately predict the important operational loads on small wind
turbines, e.g. [ 11 - 13 ].
Using the measurements of Bechly and Clausen [ 11 , 12 ] from an operating,
strain-gauged blade, Epaarachchi and Clausen [ 14 ] developed a fatigue load
procedure to simulate a full life of an operational blade, typically 20 years.
Figure 7.14 shows the 5 kW, 2.5 m long blade from Fig. 7.6 in a fatigue test rig

7

http://www.scottbader.com/composites-products-gelcoats.aspx (accessed 3 Sept 2010).

8

http://www.aircraftspruce.com/catalog/cspages/leadingedgetape.php (accessed 3 Sept 2010).

9

http://www.bladedynamics.com (accessed 4 Sept 2010).