Hideki Soejima

Delamination detection in composite laminates using dispersion change based on mode conversion of Lamb waves

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 A1 mode changed, depending on the delamination length owing to the mode conversion between the A1 mode and the S0 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.

Demonstration of detectability of SHM system with FBG/PZT hybrid system in composite wing box structure

We have developed a novel damage monitoring system that can monitor the integrity of composite structures in aircrafts. In this system, fiber Bragg grating (FBG) sensors are used as sensors and piezoelectric transducers (PZT) are used as the generators of elastic waves that propagate in the structure to be inspected. The damage monitoring system can detect the structural integrity by the change in elastic waves that are detected by the FBG sensor and arrayed waveguide grating (AWG)-type filter. We confirmed that the structure health monitoring (SHM) system was able to monitor the damage initiation and propagation by a change in the waveform of the elastic waves in coupon specimens and structural element specimens. In this study, we demonstrate the detectability of the damage monitoring system by using a subcomponent test specimen that simulates an actual aircraft wing box structure composed of carbon fiber reinforced plastics (CFRPs). The FBG sensors and PZTs are bonded to the surfaces of hat-shaped stringers by an adhesive. Damages such as de-bonding and delamination are introduced in the bonded sections of the skin and stringers by impact. Damage monitoring and diagnosis are carried out by the SHM system under ambient conditions. We successfully verify the detectability of our system.