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NONCONTACT AIR-COUPLED GUIDED WAVE MODE IDENTIFICATION USING WAVELET TRANSFORM Ik-Keun Park1, Yong-Kwon Kim1, Hyun-Mook Kim2, Houng-Kun Ann3, Hong-Jun Kim3, Woo-Sik Shim3, Youn- Ho Cho4, Yong-Sang Cho5 1R. I. of NDE Technology, Seoul National Univ. of Technology, Seoul, Korea 2School of Precision Mechanical Engineering, Hanyang University, Seoul, Korea 3Sae-An Engineering Corporation, Seoul, Korea 4Department of Mechanical Engineering, Pusan University, Pusan, Korea 5Korea Electron Power Corporation, Taejon, Korea Abstract:For efficient NDE of pipes, essential components of power facilities, ultrasonic guided waves were generated and received applying air-coupled transducers and comb ones as non-contact tech. Mode generation and selection were predicted based on theoretical dispersive curve and the element gab of comb transducers. In addition, a receiving angle of the air-born transducers was determined to acquire the predicted modes by theoretical phase velocity of each mode. Theoretical dispersive curve was compared with the results of the time-frequency analysis based on the wavelet transformation and 2D-FFT to identify the characteristics of the received mode. The received modes show a good agreement with the predicted ones.Introduction: There have been many attempts to detect defect existed in plates and shells due to the advantage of guided waves to inspect long range at a fixed position[1-3]. However, a conventional guided wave technology has limitations for field application and automation because of the usage of contact transducers. Hence, the development of a better and faster technology has been demanded. Therefore, recently non-contact based technologies have been of a great concern. They involve EMAT, laser based technology and air-coupled technology[4-9]. Table 1 represents the features of guided wave generation and reception. The Piezo based technology is the most popular one with highest S/N ratio but it has the disadvantage to need for contact between transducer and specimen. This is why a non-contact technique is required. The EMAT, one of non-contact type guided wave techniques has advantage to easily control wavelength, although it has to be placed closed transducer has lack of sensitivity for generation and reception due to high acoustic impedance mismatch between solid specimen and air. Laser based ultrasonic one also has limit in generating and receiving signal over long distance[10]. Consequently, for convenience in automation and enhancement of stability in wave excitation and reception, it is crucial to select an appropriate scheme for wave excitation and reception depending on the material property of specimen and inspection environment. In this study, the feasibility of using the comb transducer combined with air-coupled one is explored. The comb transducer is used to generate a guided wave mode while the air-coupled transducer is employed for reception. The wavelet transformation and the 2D-FFT are also conducted for mode identification, compared to theoretical dispersion curves. Table 1 Type of the generator and detector for of guided wave Guided wave Generator Contact or not
contact closely non-contactnon-contact non-contact Guided Wave Excitation and Reception:[comb transducer] The mode selection and its efficiency rely on the number of PZT elements, the element gap, the element width, frequency and the pressure distribution over each element. Figure 1 and Photo 1 show a schematic diagram of the comb transducer and the picture of the used one, respectively, In Figure 1, the transducer element gap was fixed as 12 mm. The number of elements are five and the center frequency is 1.5MHz.
Fig. 1 Parameters of the comb transducer Photo. 1 A comb transducer Figure 2 is the dispersion curves obtained from the material properties of Table 1. With the wave length, possible guided wave modes are generated at each cross point on the corresponding diagonal line as shown in Figure 2. Varying frequency and element gap, further mode selection is also possible. Cp⋅= ⋅ ∆ =λ(1)f SfTable 1 Material properties and dimensions of the sample tube Materials
Outer Diameter
Thickness Longitudinal velocity Transverse velocity 20000 17500 15000 12500 10000 7500 5000 2500 00.0 0.5 1.0 1.5 2.0 2.5 3.0 6000 5500 5000 4500 4000 3500 3000 2500 2000 1500 1000 500 00.0 0.5 1.0 1.5 2.0 2.5 3.0 · · Fig. 2 The relationship between the wavelength and sound velocities Fig. 3 Consideration of leaky guided wave and oblique angle of air-coupled transducer [Air-Coupled Transducer] The receiving angle of air-coupled is determined in terms of the phase velocity of a desired mode and the wave velocity of the air, based on the following Snell's law. sinθ(2) Ca is the wave velocity of the air and Cp is the phase velocity of the generated mode.= - 1 a Cp CTable 2 show the calculated receiving angle and the corresponding phase velocity at the cross points of Figure 2 for wavelength. The wave velocity of the air is 340 m/sec at 23~ room temperature. The frequency bandwidth of the air-coupled transducer is 0.2~2.25MHz. Photo. 2 A scene of experimental setup and the adjusting zig. Table 2 Selected modes and oblique angles of leaky guided waves into air 12mm Wavelength
Mode Cp [m/sec] θ [°] Digital Oscilloscope (Lecroy 9374M) Pentium IV PC (MATLAB) Ultrasonic Pulser/ Receiver (Parametric 5800) Charge Amplifier 100V Bias (Cooknell Inc.) comb transducer Air- couple transducer Fig. 4 The setup of experimental system Experimental Details: Figure 4 represents a schematic diagram of experimental setup. The ultrasonic pulse/receiver is equipped with the comb transducer to generate guided wave mode in 2 mm thickness stainless pipe. The distance from the middle point of the comb transducer to air-coupled one is set as 1000 mm and the air- coupled receiver was placed with the gap of 30 mm from the outer surface of the pipe. The amplitude change of the air-coupled receiver was negligible with increase of the gap between transducer and specimen up to 50 mm and it remarkably decayed beyond this point. However, it was possible to receive the signal of guided wave modes leaked through the air even at the gap of 100 mm. The scanning system for the air-coupled transducer was built up to selectively receive a guided wave mode. The received signal form the air-coupled transducer was magnified by the charge amplifier of 100 V bias. High and Low pass filters were set as 100 kHz and 5 MHz respectively to remove mechanical and acoustic noise. The received guided wave signal was installed through the signal averaging scheme with 1000 sampling data and the averaged signal is used for the time-frequency analysis. It is necessary to acquire enough number of data at a different position moving the air-coupled transducer with uniform increment in the longitudinal direction of pipe to obtain a reasonable resolution in 2D-FFT image. The 100 data set was collected every 1 mm increment in the scanning path from 500 mm to 599 mm span between the comb transducer and the air- coupled receiver. The gap between the specimen and air-coupled transducer is maintained as 70 mm. (a) θ = 9° (b) θ = 3.7° (c) θ = 20° Fig. 5 Waveform of predicted L(0,1) and L(0,2) mode with λ=12mm Results: Figure 5 is the waveform received by air-coupled transducer. Each waveform was obtained depending on the corresponding receiving angle calculated for the mode of Table 2. Figure 5(a) is the result for the receiving angle of 9 degree, which is associated with L(0,1) mode. As shown in the figure, the single L(0,1) mode dominantly appears and it is observed from the waveform that the mode is very dispersive. In addition, the group velocity of the mode increases with frequency. The time-frequency analysis can help to enhance the accuracy of predicting guided wave mode variation on dispersion curves. Figure 6(a) shows the result of the time-frequency analysis by wavelet transformation of Figure 5(a). As predicted, Figure 5(a) turns out to be L(0,1) mode. The profile of the L(0,1) time- frequency analysis appears broadly on the group velocity dispersion curve over the frequency range of air-coupled transducer, 0.2~2.25 MHz. Figure 5(b) is the waveform of L(0,2) received at 3.7 degree. It was observed that a new mode occurs at 250 µsec and the mode is corresponding to L(0,2) mode from the group velocity dispersion curves. The L(0,1) mode was consistently received when receiving angle was change. Figure 5(c) is the waveform of L(0,1) mode at 20 degree receiving angle. Increasing receiving angle, the mode with faster group velocity disappears and the other mode with slower group velocity becomes dominant. This results in the shrink of the time- frequency analysis image for L(0,1) from Figure 6(a) to Figure 6(c). 6000 6000 5500 5500 5000 5000 4000 4000 4500 4500 3500 3500 3000 3000 2500 2500 2000 2000 1000 1500 1500 1000 500 500 00.0 1.0 2.0 3 .0 00.0 1.0 2.0 3 .0 · · · · (a) θ = 9° (b) θ = 3.7° 5500 6000 4500 5000 4000 3000 3500 2500 2000 1000 1500 500 00.0 1.0 2.0 3.0 · · (c) θ = 15° Fig. 6 The Group velocity dispersion curve and time-frequency contour plot by wavelet transform -20 1 2 3 4 5 6 x 1 0 4 2 3.5 ude [V 3 ] 1 0 A m -1 2.5 t pl i ] y [M H z 2 Tim e [se c] equenc F r 2 1.5 1 [V] 1 t i ude pl 0 A m -1 0.5 -20 1 2 3 4 5 6 x 1 0 00 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Tim e [se c] Wavenumber [1/m] Fig. 7 The waveform at regularly spaced points Fig. 8 k-f image by 2D-FFT Figure 7 represents the guided wave signals collected for 2D-FFT. Figure 7(a) is a schematic diagram showing the scanning process with air-coupled transducer. Figure 7(b) shows the RF waveform at 500 mm and 599 mm propagation distance, respectively. Figure 8 is the comparison of the wave number versus frequency image between theoretical data and 2D-FFT one. Those two data show a good agreement with each other and the waveform of Figure 7(a) is proven to be L(0,1). Conclusion: The combination of comb transducer with air-coupled one was employed to propose an efficient generation/reception mechanism for guided wave. It was possible to selectively receive a mode generated from the comb transducer by the air-coupled transducer. The results of wavelet transform and 2D-FFT are consistent with theoretical data representing a promising feasibility to identify a guided wave mode in further applications. References: [1]J. L. Rose and Y. H. Cho, "Ultrasonic Guided Wave Inspection Potential in the Power Generation Field", Safety & NDT' 95, pp. 101-115, (1995) [2]H. J. Shin and J. L. Rose, "Guided Wave Tuning Principles for Defect Detection in Tubing", Journal of Nondestructive Evaluation, Vol. 17, No. 1, pp. 27-36, (1998) [3] A. E. Bahrawy, "Stopbands and Passbands for Symmetric Rayleigh-Lamb modes in a plate with corrugated surfaces", J. Sound Vibration, Vol. 170, No. 2, pp. 145-160, (1994) [4] B. Djordjevic, "Advanced Ultrasonic Probes for scanning of Large Structure", Proc. Ultrasonic International, Vienna, Austria, (1993) [5] D. A. Oursler and J. W. Wagner, "Narrow-band hybrid pulsed laser/EMAT system for noncontact ultrasonic inspection using angled shear waves", Material Evaluation, Vol. 53, pp. 593-597, (1995) [6] S. G. Pierce, B. Culshaw, W. R. Philp, F. Lecuyer and R. Farlow, "Broadband Lamb Wave Measurements in Aluminum and Carbone/grass Fiber Reinforced Composite Materials using Non-contacting Laser Generation and Detection", Ultrasonics, Vol. 35, pp. 105-114, (1997) [7] D. A. Hutchins, W. M. D. Wright, G. Hayward and A. Gachagan, "Air-coupled Piezoelectric Detection of Laser-generated Ultrasound", IEEE Trans. Ultrason. Ferroelec. Freq. Control, Vol. 41, pp. 796-805, (1994) [8] J. R. Park, K. Y. Jhang and K. C. Kim, "Analysis of the Characteristics of Laser-Generated Ultrasonic Waves Detected by PZT Transducer", Jounal of the Korean Society of Mechanical Engineers A, Vol. 23, No. 9, pp. 1590- 1596, (1999) [9] K. C. Kim, H. Yamawaki, K. Y. Jhang, "Detection of Laser Generated Ultrasonic Wave Using Michelson Interferometer", Journal of the Korean Society for Nondestructive Testing, Vol. 20, No. 9, pp. 27-32, (2000) [10] W. M. D. Wright, D. W. Schindel and D. A. Hutchins, "Studies of Laser-generated Ultrasound using a Micromachined Silicon Electrostatic Transducer in Air", J. Acoust. Soc. Am., Vol. 95, pp. 2567-2575, (1994) [11] S. Pelts, J. L. Rose and Y. Cho, "A Comb Transducer for Guided Wave Mode Control", Review of Progress in Quantitative NDE, Vol. 18, pp. 1029-1036, (1998) [12] T. Hayashi, K. Kawashima, "Mode Extraction from Multi-modes of Lamb Waves", Review of Progress in Quantitative NDE, Vol. 21, pp. 219-224, (2002) |
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