Fatigue Design of Wind Turbines - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 712

Figure 12-24. RISO's single-axis resonance test system. The frame holding the rotating
eccentric mass is attached at approximately 2/3-span. Additional masses attached nearer to
the blade tip are designed to shape or “tune” the distribution of bending moments along the
blade axis [Courtesy of the RISO National Laboratories, Denmark]
cyclic forces in the axial direction of the blade, forces that are not present in service. As the
size of the test blade increases, these unwanted axial forces also increase and may ultimately
interfere with the accuracy of the test results.
Dual-Axis Hybrid Resonance Method
The dual-axis hybrid resonance method is one of the latest developments in blade fa-
tigue testing technology [White 2004]. This method is designed to combine the accuracy
advantage of forced-displacement testing with the increased speed and reduced energy of
resonance testing. As shown schematically in Figure 12-25, resonance excitation is provided
by a vertically oscillating mass mounted in a frame attached to the blade. The frame also
carries the hydraulic actuator that drives the mass at the blade's resonant frequency. Because
the energy consumed by blade damping at resonance is relatively small, both the actuator
force and stroke are much reduced, compared to force-displacement bending of the blade in
the flatwise direction. Not shown in Figure 12-25 is the actuator and bell-crank mecha-
nism that apply cyclic edgewise loading. This force-displacement part of the hybrid system
is exactly the same as that shown in Figure 12-23.