We have developed a damage detection system that generates ultrasonic waves with a
piezo-ceramic actuator and receives them in a fiber Bragg grating (FBG) sensor. In this
research, this system was applied to evaluate the debonding progress in carbon fiber
reinforced plastic (CFRP) bonded structures. First, small-diameter FBG sensors were
embedded in adhesive layers of a double-lap-type coupon specimen consisting of CFRP
quasi-isotropic laminates bonded with epoxy adhesive films. Then, an ultrasonic wave at
300 kHz was propagated through the debonded region, and the wavelet transform was
applied to the received waveform. The obtained results showed clear differences depending
on the debonding length. Hence, a new damage index was proposed using the
difference in the distribution of the wavelet transform coefficient. The damage index
increased with an increase in the debonded area. Furthermore, this system was
applied to a skin/stringer structural element of airplanes made of CFRP laminates.
In this case, a correlation coefficient was also calculated from the results of the
wavelet transform. As a result, the damage index increased and the correlation
coefficient decreased with an increase in the debonded area. Hence the length
of the debonding between the skin and the stringer could be easily evaluated.
We have been studying optical sensing technologies that use fiber Bragg gratings (FBGs) for health monitoring of aircraft structures made of carbon fiber reinforced plastic (CFRP) composite materials. The sensing system is composed of a piezoelectric transducer (PZT) actuator, which generates an elastic wave of several hundred kHz, and FBG sensors that receive the elastic wave. When some damage occurs in the composite materials, the elastic wave that propagates through those materials changes. Therefore the damage can be detected by analyzing the elastic waveform to be received by FBG sensors. For detecting this wave, we developed a high-speed optical wavelength interrogator for FBG sensors, and FBG sensor modules that can be embedded in the composite materials. In this interrogator, we employed an arrayed waveguide grating (AWG) as an optical filter that can convert the wavelength shift of the FBG sensors into optical power change. Using this interrogator and FBG sensor modules, we detected elastic waves of 300 kHz in frequency. We determined the required characteristics of FBG sensor both through simulation and experiments for improving the sensitivity of this health monitoring system.
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