Relative Ability of Wedge-Coupled Piezoelectric and Meander Coil EMAT Probes to Generate Single-Mode Lamb Waves

Abstract

Ultrasonic guided waves are used extensively when checking for defects in petrochemical and other industries and are mostly generated using piezoelectric transducers on an angled wedge or electromagnetic acoustic transducers (EMATs) in different configurations. Low-frequency inspection allows for long-distance propagation, but it is best suited for detecting relatively large defects, while at higher frequencies, the presence of multiple wave modes limit defect detectability, so achieving practical single Lamb mode excitation via careful transduction is very beneficial. This paper investigates the relative ability of angled piezoelectric and meander coil EMAT probes to produce single-mode transduction in the medium (~1-5 MHz-mm) and high (>5 MHz-mm) frequency-thickness regions of the dispersion curves. The nature of each transducer is studied analytically by simulating the corresponding surface forces, followed by the use of a Fourier transform in time and space (2-D fast Fourier transform) to highlight the excitation region in the wavenumber-frequency space. With angled wedge excitation there is a linear relationship between the excitation frequency and the wavenumber which means that the excitation tends to track typical dispersion curves, allowing for easier pure mode generation. In contrast, the EMAT controls frequency and wavenumber separately which makes it more difficult to generate a pure mode when dispersion curves are close together; however, by narrowing the frequency bandwidth via a large number of cycles in the excitation signal, pure mode generation via an EMAT was shown to be possible even in areas of closely spaced modes. As example cases, analytical results, backed up by experiments, showed that signals dominated by the A0 mode at 1.5 MHz-mm and also the A1 mode at 18 MHz-mm can be generated with both angled piezoelectric and EMAT probes.

Interaction of SH guided waves with wall thinning

“…In laboratory tests, notches and machined wall thinning sections are commonly used as a first step approximation of a corrosion type defect [10][11][12][13][14][15][16]. According to the working frequency, two regimes of operation are commonly defined, namely low and high frequency-thickness regimes [15][16][17]. In the latter, the operating frequency range encompasses more than one mode.…”

Multi-feature Fusion and Damage Identification of Large Generator Stator Insulation Based on Lamb Wave Detection and SVM Method

“…By extracting frequency components and amplitude characteristics of lamb wave signal in frequency domain, which can be used for identifying the damage of stator insulation. Based on frequency domain analysis theory, the Fast Fourier transform (FFT) [18,19] and power spectral density (PSD) [20] analysis are used to extract the frequency domain features of Lamb wave signals in this paper. FFT algorithm is developed from Discrete Fourier Transform (DFT).…”

Excitation of Single-Mode Shear-Horizontal Guided Waves and Evaluation of Their Sensitivity to Very Shallow Crack-Like Defects

“…As mentioned above, the frequency and wavenumber bandwidths are dictated by the nature of the transducer design, so it is necessary to investigate how to obtain the optimal setup for pure SH1 mode excitation in the desired frequency-thickness product. Here a simple 2-D analytical model, implemented in the MATLAB software, was employed as it can capture the essential physics while enabling the rapid study of a large number of probe configurations [24][25]. In order to predict the wavenumber and frequency bandwidths of the excitation force, the surface force generated by the PPM EMAT was simulated by an array of in-plane surface point forces followed by the use of a Fourier transform in space and time (2-D FFT).…”

“…The polarization and the spacing between the magnets determine the nature of the Lorentz force induced on the waveguide surface which in turn dictates the wavelength (λ) and hence the wavenumber bandwidth of the probe. It is noteworthy that due to the unique design of PPM EMATs, compared to meander coil EMATs [25], the excitation wavelength is dictated by the magnet arrangement/size rather than the coil which means the performance of this type of transducer is limited to the strength of the field generated by the magnets. This can be problematic when using PPM EMATs for higher frequency wave excitation (due to small wavelength, the magnet array must be shrunk which results in lower energy transmission).…”