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various NREL development agreements. At the NWTC, engineers can perform static, fa-
tigue, and modal testing of blades at least 45 m in length.
Methods of Blade Fatigue Testing
Each of the four facilities listed above has independently developed methods for per-
forming blade tests. Over time, some original test methods have been replaced by newer
techniques, and several of these test facilities now use similar equipment and procedures
[White 2004]. There are three general methods used currently at national laboratories to
test the fatigue endurance of full-scale wind turbine blades. These may be referred to as (1)
the
dual-axis forced-displacement method
, (2) the
single-axis resonance method
, and (3) the
newly-developed
dual-axis hybrid resonance method
. In all three methods, the root end of
the test blade is attached to a very stiff, vertical load frame or
strongback
, and the blade is
cantilevered outward.
Dual-Axis Forced-Displacement Method
The dual-axis forced-displacement loading system, currently used at NREL and other
research facilities, employs a servo-hydraulic system with dual actuators to load the blade
cyclically and simultaneously in both the flatwise (
flap
) and edgewise (
chordwise
, or
lead-
lag
) directions [Hughes and Musial 1999, Larwood
et al.
2001, Hughes and Musial 2002].
Cyclic loads are applied at frequencies well below the blade's flatwise first (
fundamental
)
natural frequency. The dual-axis fatigue test system at NREL is shown in Figure 12-23. The
Figure 12-23. NREL's dual-axis forced-displacement test system
[Courtesy of the Na-
tional Renewable Energy Laboratory]
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