Observation of lamb wave mode conversion on an aluminum plate

Observation of Lamb Wave Mode Conversion on an Aluminum PlateHyeon Jae Shin, Sung-Jin Song
School of Mechanical Engineering and Safety and Structural Integrity Research Center
Sungkyunkwan University
300 Chunchun-dong, Changan-gu,
Suwon, 440-746
Republic of Korea
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Abstract
Lamb wave mode conversions were experimentally studied to utilize the mode conversioncharacteristics for the nondestructive testing. Aluminum plates with various depths of EDM notches were used for the experiments. The numerical results of Lamb wave dispersion in the plate were given. Short time Fourier transform was employed to obtain the dispersive patterns of the group velocity of the received signals, which represented experimental results of Lamb wave dispersion. Then, the modeidentifications were performed by the comparison of the dispersive patterns obtained by numerically and experimentally. For the excitation and reception of the Lamb waves, angle beam transducers with tone burst signals were used. Mode conversion was observed in both pitch-catch and pulse-echo techniques. Mode conversions occurred in forms of phased velocity shift and frequency shift. It showed thepossibility of defect detection by using Lamb wave pitch-catch techniques. In addition, by analyzing the characteristics of mode conversion, it may be possible to size defects with Lamb waves.
Keywords: Lamb waves, mode conversion, short time Fourier transform, mode identification, detection, pitch-catch
Introduction
Lamb waves are known as an efficient means for the nondestructive testing of largearea without moving sensors. The capability of Lamb waves to detect and locate defects has been demonstrated in plates and tubing [1-4]. Up to date, the existence of multi modes that are dispersive has been understood and the mode selection concepts were studied for the optimal defect detection [5-7]. Also the mode excitation concepts for the selected mode generation were studied [8-10]. For theidentification of dispersive and/or superposed Lamb waves, time-frequency analysis using wavelet transforms [11] and short time Fourier transforms [12] were introduced.
In Lamb wave scattering, it is understood that the received modes are distorted from the incident modes. One of the reasons for the distortion is dispersive nature of the impinged Lamb waves. The other reasonable cause of thedistortion is the mode conversion that makes energy redistribution among multi modes. It is not only interesting but also valuable to observe the mode conversion to enhance the capability of the Lamb waves in nondestructive testing.
In this study, the mode conversion was observed when Lamb waves were reflected from and transmitted through various sizes of EDM notches in an aluminum plate of 1 mmthick. To identify the Lamb wave modes, the received signals were analyzed in the time-frequency domain that was obtained by short time Fourier transforms. The possibility of the defect detection by using the Lamb wave pitch-catch techniques is discussed. The defect sizing possibility by observing the Lamb wave mode conversion in both the pitch-catch and pulse-echo techniques is also discussed.Experimental Conditions and Dispersion Curves
An aluminum plate of 1 mm thick was used for the observation of Lamb wave mode conversion. EDM notches of 5%, 20%, 35%, 50%, 65%, and 80% of through wall depth were machined in the aluminum plate. In the pitch-catch transducer set up, the notches were located at the center of the sending and receiving transducers spaced out 30 cm apart. In the pulse-echoexperiments, the notches were at 15 cm away from the transducer.
A variable angle beam transducer was used, so that the phase velocity of Lamb waves in both generation and reception could be controlled. The angle was tuned for 20 degrees that corresponded to the phase velocity of 7.9 km/s. Frequency of excited signals was controlled by a tone burst system. In this experiment, four cycles of...