Environmental Engineering Reference
In-Depth Information
Fig. 9.17 Initial strain
amplitude along the blade. a
Upper surface, b lower
surface
First, the blade was carried out statist test. An 80 N force, which is approxi-
mately 15 % of the bearing capacity, was loaded on the top of the blade (115 cm
from the blade root). The stain distribution under this fixed loading was obtained by
the DPP-BOTDA sensing system. The test result was used as the baseline for
damage detection. Then the blade underwent fatigue excitation from undamaged
state to failure. The fatigue loading system was paused after fixed fatigue cycle for
blade inspection and the blade was carried out the same statist test as the first step.
At first, the fatigue loading system was paused every 4 h (*40,000 cycles). After
28 h (*260,000 cycles), a multitude of gel-coat cracks and some larger visible
cracks were identified on the blade. The resonance frequency decreased to 2.2 Hz,
as the stiffness degradation and accumulated cracks reduced the local flexural
stiffness of the blade. Then the fatigue loading system was paused every 2 h
(*20,000 cycles) to carefully study the signal changes of DPP-BOTDA system
near the failure moment. Comparisons of the stain distribution and specific response
of the DPP-BOTDA at different fatigue cycle count were used as a feature to track
the progression of structural change over the course of the fatigue test.
9.3.3 Test Results and Discussions
9.3.3.1 Blade Failure Scenario and Mechanism
The blade was failed after 310,000 cycles (34 h) and the fatigue damage, i.e.,
adhesive joint failure between skins at the leading edge, was visually identified at
75 cm from the root, which was also the high strain region of the blade. The blade
