A Peak Frequency Shift Method for Guided Wave Thickness Measurement and Its Realization by Different Transducer Techniques

Ultrasonic guided wave inspection is much more efficient than traditional point-bypoint examination. Guided waves can propagate over long distances from a single position, and therefore is a good method of evaluating thickness degradation, especially for large-area structures such as pipes and vessels. In this paper, guided wave thickness measurement potential is studied utilizing a peak frequency approach. Because guided wave velocities are functions of the product of the frequency and the structure thickness, the dispersion curve of phase velocity vs. frequency will shift as the thickness changes, and the mode excitation frequency, which is called peak frequency, will also shift. The relationship between peak frequency shifts and thickness changes can be used for guided wave thickness measurement. Both Lamb waves and shear horizontal (SH) guided waves were studied. Experiments were carried out on plates with different thicknesses. Phase velocity dispersion curves and their changes with the thicknesses variations were calculated theoretically. Peak frequency shifting information was acquired experimentally by signal analysis for establishing an algorithm of calculating thicknesses from peak frequency shifts. Different guided wave transducer techniques, such as piezoelectric and electromagnetic acoustic transducer (EMAT) technique, were studied and compared for realization of the thickness measurement method. Introduction: Ultrasonic guided waves can propagate over a long distance from a single point, and therefore present a fast and efficient NDE method for large area structures, such as pipes, rails, vessels, and aircraft [1-2]. When structures age, some thickness degradation or loss may occur due to various field conditions such as corrosion and erosion. Thickness measurement or monitoring is becoming an important aspect of structural health monitoring. Traditional point-bypoint thickness gauges utilizing bulk waves are inefficient, for example, for the inspection cases with limited access, and also can easily miss some critical points in large area structures. Guided wave thickness measurement gives an estimate of the thickness value over the wave propagation distance, and therefore realizes a fast and reliable inspection, even for objects with limited access. Guided wave thickness measurement has been studied by some investigators. But most concentrate on the studies of Lamb wave group velocity or phase velocity. For example, Pei et al. [3] proposed a method utilizing dry contact transducers to detect the thickness variations by the changes in the velocity of the Lamb wave A0 mode, and then studied tomographic reconstruction of thickness variations using Lamb wave velocities [4]. Moreno et al. [5] evaluated the possibility of thickness measurement for composites by studying the phase velocity of the Lamb wave A0 mode at a low frequency range, followed by a study of the viscoelastic effect [6]. Hayashi et al. [7] studied the group velocity method using the A0 mode of laser generated Lamb waves, in which group velocities were estimated experimentally by the wavelet transform. Jenot et al. [8] presented a corrosion thickness gauging method by measuring the group velocity of the Lamb wave S0 mode. Other studies can be found from the work of Gao et al. [9] and Sun et al. [10]. However, the velocity method usually works at low frequencies for lower order modes, but it is sometimes difficult to measure the velocity for a certain mode because of the superposition of different modes. In this study, a different method considering of a peak frequency shift is presented for both Lamb wave and SH wave thickness measurement of a plate. Some of the ideas presented in this paper were taken from [11] and [12]. Experiments were carried out on various thickness stainless steel plates. Based on an analysis of the acquired wave signals, thickness