Fatigue Design of Wind Turbines - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 711

hydraulic actuator for applying flatwise loads is oriented vertically below the outboard end
of the blade. Edgewise loads are applied by a horizontal bar loaded through a bell-crank
mechanism by a vertical actuator, as seen at the far right in the figure.
The primary advantage of this system is that its biaxial loading can be independently
adjusted in the flatwise and edgewise directions for accurate simulation of the actual operat-
ing loads on a blade. While the dual-axis forced-displacement technique is the most accurate
method currently used to test wind turbine blades, it also has several drawbacks that become
significant as capacity requirements increase to match the increasing size of modern utility-
scale wind turbines:
-- Ever-larger forces and displacements are required from the hydraulic actuators.
-- New actuators must be designed and built each time a larger blade is to be tested.
-- Hydraulic pumping requirements increase as actuator size increases.
-- Substantial equipment costs are incurred with each capacity increase.
-- Increased hydraulic fluid flow requirements create significant operating energy costs.
-- Blade tip sections are normally removed, in order to avoid unwanted vibration modes and
to fit larger blades into the existing facility, and therefore are not included in the test.
Flatwise actuators have the larger requirements for both displacement and force. Edge-
wise actuators have smaller displacement requirements because of the higher blade stiffness
in the edgewise direction.
Single-Axis Resonance Method
In the single-axis resonance method, currently used at the RISO laboratory, an electric
motor is mounted on the blade and spins an eccentric mass in order to excite blade reso-
nance in either the flatwise or edgewise direction. Whereas the dual-axis forced-displace-
ment method permits applying flatwise and edgewise loads simultaneously, the single-axis
resonance method is restricted to applying these two cyclic load components in separate test
periods, effectively doubling the test duration.
Figure 12-24 shows a single-axis resonance blade fatigue test in progress at the RISO
facility. In this test, fatigue loads are being applied in the flatwise direction. By attaching
additional masses to the test blade, it is possible to adjust the bending moment distribution
along the blade length for increased accuracy in matching operating loads. These added
masses lower the fundamental natural frequency of the blade, and therefore the rate of test-
ing. In general, the added masses lower the natural frequency by 25 percent to 30 percent
[White 2004]. However, in spite of testing separately in flatwise and edgewise directions and
the lower cyclic frequency with auxiliary masses, the test cycle frequency remains substan-
tially higher than that of the forced-displacement method. Thus, increased speed of testing is
a major advantage of the single-axis resonance method.
By taking advantage of the displacement magnification that occurs naturally when cyclic
loads are applied at frequencies near resonance, the force required to achieve the required
bending moments is substantially lower for this method. This results in much lower energy
consumption during testing. Because the testing is performed at the blade's natural fre-
quency, the blade bends into a natural mode shape . This makes it possible to test the entire
blade without introducing unwanted vibration modes, so the tip section does not need to be
removed before testing.
Compared to force-displacement testing, resonance excitation presents several advan-
tages for fatigue testing of larger wind turbine blades, including lower testing costs, faster
results, and the ability to test the entire blade. However, there are limitations to this method.
One of these limitations is the fact that the rotating eccentric mass also applies horizontal