A new ultrasonic propagation system has been constructed using macrofiber composite (MFC) actuators and fiber Bragg grating (FBG) sensors. The MFCs and FBGs can be integrated into composite laminates because of their small size and high fracture strain. The developed system can send and receive broadband Lamb waves. In this research, this system was used to detect delamination damage in composite laminates. First, the multiple modes of Lamb waves in a carbon-fiber-reinforced plastic (CFRP) quasi-isotropic laminate were identified by transmitting and receiving the symmetric and antisymmetric modes separately. Then, the mode conversions at both tips of a delamination were investigated through an experiment and a two-dimensional finite element analysis (FEA). A new delamination detection method was proposed on the basis of the mode conversions, and experiments were carried out on laminates with an artificial delamination. When antisymmetric modes were excited, the frequency dispersion of the received A 1 mode changed, depending on the delamination length owing to the mode conversion between the A 1 mode and the S 0 mode. This phenomenon was confirmed through the FEA and these results prove that this new method is effective in detecting a delamination in CFRP laminates.
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.
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