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Session: Session 8: NDI, inspections and maintenance 
Session starts: Tuesday 27 June, 13:30
Presentation starts: 14:50
Room: Theatre café: parallel

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Lea Lecointre (The University of Tokyo)
Ryo Higuchi (The University of Tokyo)
Tomohiro Yokozeki (The University of Tokyo)
Masakatsu Mita (Shibaura Institute of Technology)
Shota Tonegawa (Shibaura Institute of Technology)
Naoki Hosoya (Shibaura Institute of Technology)
Shin-ichi Takeda (Japan Aerospace Exploration Agency)

Abstract:
Ultrasonic Testing (UT) is the most common Non-Destructive Testing method used for Composite materials. One of the most promising methods which have been identified in the recent years in order to improve the efficiency of UT is the Lamb Waves Testing. However, Lamb Waves propagation is a complex phenomenon which requires adapted and efficient Signal Processing method. In this study, we propose a method based on the Wavelet Analysis to process experimental Lamb Wave signals propagated into Carbon Fibers Reinforced Plastics (CFRP) samples for detecting artificial defects. Moreover, this experiment uses fully non-contact generation and reception of the Lamb Waves which shows very promising applications for maintenance or complex geometries. The Lamb Waves were generated in several healthy and artificially delaminated CFRP test samples from a Laser Induced Plasma system. The reception of the propagating waves was performed by measurement of the out-of-plane displacement with a Scanning Laser Doppler Vibrometer. Then, a continuous Wavelet Transform was performed on the measured signals, which allowed to store values of Wavelet Transform coefficients (cwt) in a 3D matrix in spatial, time and frequency domains. From this matrix, spatial-time analysis was performed by extraction of 2D images at each frequency sample. The experimental group velocity dispersion curves were calculated and compared with theoretical values. The results shown that the experimental group velocities had a good correlation with theory. Moreover, for 16-ply samples including delamination at the middle depth, group tend to reach the value of 8-ply structure. Finally, a spatial-frequency analysis was performed from the 3D cwt data. Several time samples were extracted for healthy and delaminated plates. The spatial-frequency data for healthy samples were used as baseline for detecting signals in samples containing delamination. The results shown clear and accurate localization of wide delamination from the visible increase of cwt amplitude and frequency bandwidth at the delamination zone. The same process has been performed on samples containing several small delamination. The smaller delamination (Ø10-mm) could not be detected, while Ø30-and Ø50-mm delamination could be detected as groups of defects.