Environmental Engineering Reference
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Fig. 9.25 Photography of the broken blade: a normal view, b close-up view.
static test was then carried out. The actuator footing was given a positive offset of
8 cm, so that it could bend the blade to the failure point while staying within its
20 cm stroke range. During the second half of the static testing, which was carried
out with the same increment-based loading as the first, the load cell reading
reflected a maximum force of 500 N at the loading point before the blade's failure
occurred.
Failure of the turbine blade happened when the resin bond between the top and
bottom layers of glass fiber reinforcement failed due to shear stress. The upper
and lower halves of the blade slipped in opposite directions approximately per-
pendicular to the long edge of the blade. This separation is seen in detail in the
close-up view of Fig. 9.25 . After this failure occurred on the leading and trailing
edges of the blade, it lost its structural integrity and the top and bottom halves
flattened before allowing the blade to fold.
9.4.2 Experimental Results and Discussion
9.4.2.1 Results of Optical Fiber Sensors
Figure 9.26 shows the output of all eight FBG optical strain sensors during the
second part of the testing procedure. Because of the initial load on the blade, each
sensor started this test segment with an initial strain dependent upon its location on
the blade. As described in the procedure section, loading was performed in 33 N
increments which were applied gradually. A delay of several seconds was used in
between increments to allow any signs of failure to be noticed. The short segments
of constant strain are indicative of these delay periods. At approximately 580 s,
failure occurred at 0.74 m from the fixed end of the blade, which is closest to
sensor C. The reading of sensor C at this time shows the highest strain of all the
optical sensors.
Figure 9.27 is a graph confirming the linear relationship between strain at FBG
sensor A and the load applied to the blade.
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