Environmental Engineering Reference
In-Depth Information
where e
ð
z
Þ
is the axial strain along the optical fiber, t
ð
z
Þ
is temperature and t
r
is the
reference temperature. C
s
and C
t
are the proportionality constants of strain and
temperature. m
B
ðÞ
is the initial Brillouin frequency in a strain-free state at the
reference temperature t
r
. Since the Brillouin frequency shift depends on both
the strain and temperature applied to the optical fiber, the Brillouin frequency shift
due to strain change could not be easily distinguished from that caused by tem-
perature change. This study used the unstrained part of the optic fiber that attached
to the surface of the blade to compensate the frequency shifts due to the tem-
perature change. The strain distribution is calculated from the following equation:
e
ðÞ¼
m
B
ðÞ
m
B
z
ðÞ
C
s
ð
9
:
16
Þ
where m
B
z
ðÞ
is the Brillouin frequency shift at reference position z
0
, i.e., z
0
is the
unstrained part of the optic fiber that attached to the surface of the blade.
9.3.2 Fatigue Damage Detection Test
9.3.2.1 Experimental Setup
The experimental setup of DPP-BOTDA is shown in Fig.
9.13
. A narrow linewidth
2 kHz fiber laser operating at 1550 nm is used as the light source. A 3-dB coupler
is used to split the output light into two parts, providing the pump and the probe
waves, respectively. An electro-optic modulator (EOM1) with high extinction ratio
(ER [ 40 dB) is used to generate the optical pulse, which is then amplified by an
erbium-doped fiber amplifier before being launched into the sensing fiber.
The microwave generator outputs microwave signals to modulate the light through
EOM2, and the first-order lower sideband is chosen as the probe wave by a narrow
bandwidth fiber Bragg grating. A 7-m polarization-maintaining fiber (C
s
¼
0
:
0483MHz
=
le), whose BFS is 10.845 GHz at room temperature (20 C), is used
as the sensing fiber to remove the polarization fading and improve the effective
signal-to-noise ratio (SNR). Two polarization controllers are used to ensure that
the pump and probe waves are launched into the same principal axis of the sensing
fiber. A 3.5 GHz bandwidth detector with a transition time (10-90 %) of 115 ps,
which thus can resolve the cm-order variation in strain or temperature, is used to
detect the Brillouin signal. The sampling rate of the digitizer is set at 10 GHz s-1,
which corresponds to 1 cm/point in the optical fiber. The double pulse pair used is
39.5 and 41.5 ns and the pulse-width difference is 2 ns, so the special resolution of
the DPP-BOTDA system is 20 cm.
A 1.25 m prototype glass fiber reinforced plastic (GFRP) composite wind
turbine blade was used in the fatigue test. The blade was mounted horizontally in a
flap direction with its root fixed and the low-pressure blade surface facing down.
An electric motor was mounted on the blade tap at the free end. Fatigue loading
