Tham khảo tài liệu 'advances in piezoelectric transducers part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Application of Piezoelectric Transducers in Structural Health Monitoring Techniques 95 In order to simulate the damage a hole of variable diameter is introduced at the plate centre and its diameter was increased from 1 mm to 13 mm. By computing the amplitudes on the 2DFT and STFT analysis as functions of the diameter of the hole the sensitivity of Lamb modes can be analyzed. In fact the application of the dual signal processing approach to the received Lamb wave signals allows us to monitor the damage evolution using the A0 and S0 Lamb mode amplitude variation in two different frequency bands. Moreover it is shown figure 8 that for one of the frequencies a false interpretation can be induced when using only one signal processing technique. It can be noted that both S0 and A0 modes are sensitive to the presence of the hole for both frequencies 400 kHz and 600 kHz but the interaction of the same mode at different frequencies does not give similar results. At 600 kHz and for both S0 and A0 modes see figures 8 a and b the amplitudes of the 2DFT and the STFT drop quasi-continuously according to the hole diameter by keeping close values. Both methods give results in good agreement and demonstrate the validity of their use. In contrast to this in the case of the - 0 STJFT. 2DFT s Ỉ s f 0 . 1 1 0 0 Hole diameter mm SO mode at 400 kHz Fig. 8. STFT and 2DFT amplitude variation as a function of the hole diameter. a S0 mode at 600 kHz b A0 mode at 600 kHz c S0 mode at 400 kHz d A0 mode at 400 kHz. 96 Advances in Piezoelectric Transducers S0 mode at 400 kHz see figure 8 c and for a diameter equal to approximately 9 mm the amplitude of the 2DFT increases whereas the amplitude of the STFT decreases. Although this variation is relatively small it represents typically a problem of false data interpretation. Simultaneously in the case of the A0 mode at 400 kHz see figure 8 d the STFT and the 2DFT amplitudes plotted as functions of the hole diameter show that the amplitudes are .
