Shear horizontal wave transducers for structural health monitoring and nondestructive testing: A review.

A variable-frequency bidirectional shear horizontal (SH) wave transducer based on dual face-shear (d24) piezoelectric wafers.

Lamb waves have the characteristics of long propagation distance, and they can be used to realize rapid and wide-scale nondestructive testing of plates. The detection sensitivity can be improved by using a high-order Lamb wave mode with a short wavelength. However, Lamb waves have multimode characteristics. The number of Lamb wave modes greatly increases at high frequencies, and it is difficult to obtain a pure single high-order Lamb wave mode with electromagnetic acoustic transducers (EMATs); this increases the difficulty of signal processing and analysis. In this study, an obliquely incident EMAT based on the inclined static magnetic field was designed to solve this problem. The mode selectivity of the proposed obliquely incident EMAT is superior to that of the traditional EMAT. A new parameter is proposed to improve the mode selectivity of EMATs: the polarization angle of the permanent magnet. The incident angle of incident ultrasonic waves is controlled by selecting the polarization angle of permanent magnet. A rectangular permanent magnet is obliquely polarized to provide a biased inclined static magnetic field. The obliquely incident Lorentz force is formed through the coupling between the eddy current and the inclined static magnetic field by an obliquely polarized permanent magnet. The obliquely incident Lorentz force is the source of the obliquely incident ultrasonic wave. Simulation and experimental results show that the developed obliquely incident EMAT can reduce the amplitude of the unwanted modes and enhance the mode selectivity. It was also verified that the developed obliquely incident EMAT had better frequency-response characteristics and sound-field directivity.

Obliquely incident EMAT for high-order Lamb wave mode generation based on inclined static magnetic field

We propose a new frequency-mixed point-focusing shear horizontal (SH) guided-wave electromagnetic acoustic transducer (EMAT) in this work to obtain the defect positions and plate thickness simultaneously and accurately. Compared with other guided-wave detection methods, it is not required to measure the plate thickness in advance because we can easily obtain it during the test. We use the variational mode decomposition method to decompose the received frequency-mixed defect signal into subsignals with different center frequencies and to remove the noise. Furthermore, we use the continuous wavelet transform to analyze these subsignals using the time–frequency method and to obtain the time-of-flight information of the guided wave under different frequencies and modes. Therefore, we can obtain accurate defect positions and plate thicknesses via the new transducer and signal processing methods while improving the signal intensities. In the identification of defect types, we first constructed a database set containing three types of defects of different sizes using data enhancement methods. Then, the dense network, convolutional neural network, recurrent neural network, and newly proposed deep GFresNet are studied to analyze the defect classification performance of each structure. The results show that the proposed GFresNet has very good defect identification accuracy, which is about 95% along any depth of the defects, and that it can automatically extract high-level information without sophisticated feature engineering.

Development of Frequency-Mixed Point-Focusing Shear Horizontal Guided-Wave EMAT for Defect Inspection Using Deep Neural Network

As an important branch of non-destructive testing, electromagnetic ultrasonic testing is widely used in various industries because of its high detection efficiency and the lack of a need for the test sample to have a coupling agent or a surface pretreatment. As a core component of electromagnetic ultrasonic testing, electromagnetic acoustic transducers (EMATs) have received extensive attention in recent years. However, the mode selectivity of EMATs is reduced because of the non-uniformity of the magnetic field of traditional permanent magnets and the multimode phenomenon of guided waves. To solve this problem, an omni-directional magnetic-concentrator-type electromagnetic acoustic transducer (OD-MC-EMAT) is proposed herein. Which was added to the magnetic concentrator on a traditional EMAT to guide and concentrate the magnetic field of traditional permanent magnets, thereby improving the defect inspection ability of the EMAT. The proposed OD-MC-EMAT consisted of a cylindrical permanent magnet, an omni-directional magnetic concentrator, and a multi-cluster of circular meander coils, and it exhibited better mode selectivity than a traditional EMAT. A finite element simulation and an experiment proved that the mode selectivity of the transducer improved after adding the magnetic concentrator, and experiments to study the transducer characteristics and defect detection were also carried out. Finally, the configuration of the OD-MC-EMAT was optimized through an orthogonal experiment. The influence of each parameter on the transducing efficiency of the proposed OD-MC-EMAT was studied, and the optimal parameter combination was confirmed.

Development of an omni-directional magnetic-concentrator-type electromagnetic acoustic transducer

Due to the simple wave structure and low-attenuation and non-dispersive properties, the lowest-order shear horizontal mode SH0 can realize the efficient large-scale damage detection in plate structures and shows a great application potential in non-destructive testing and structural health monitoring. In order to excite SH0 mode in aluminum plates, a direction-tunable shear-horizontal mode array magnetostrictive patch transducer (DT-SHMA-MPT) is developed based on the magnetostrictive effect. The proposed DT-SHMA-MPT consists of a transducer sleeve, four identical planar fan-shaped meander coils, four identical fan-shaped NdFeB permanent magnets, and a circular nickel patch. Four identical planar fan-shaped meander coils are connected in parallel to form a meander coil array. These planar fan-shaped meander coils are double-layer structures and made of flexible printed circuit (FPC). The experimental results show that the developed DT-SHMA-MPT can effectively excite and receive a single SH0 mode in aluminum plates. For the DT-SHMA-MPT with n planar fan-shaped meander coils and n fan-shaped permanent magnets (n = 4, 3, 2), its frequency response characteristics are tested at the center frequencies of 775 kHz, 780 kHz, and 783 kHz, which are close to the predicted value. Furthermore, the effects of the number of fan-shaped meander coils and corresponding permanent magnets on the directivity of the transducer are also investigated. The acoustic field distribution or the directivity of the transducer directly depends on the number of fan-shaped meander coils and permanent magnets. Finally, the direction tuning of proposed transducer is investigated. The ability of excitation and reception of this transducer can be controlled by tuning the directional arrangement of fan-shaped meander coils and permanent magnets.

A direction-tunable shear horizontal mode array magnetostrictive patch transducer

Permanent magnet is widely used as one of the three elements of electromagnetic acoustic transducer (magnet, coil, tested specimen). However, due to the immutability and non-uniformity of the permanent magnet magnetic field, the electromagnetic acoustic transducer (EMAT) will produce other unwanted modes, which will affect the purity of the targeted mode in signals. Therefore, the multi-mode problem of EMAT is still an important issue to be solved. In order to solve this problem, a new type of directional magnetic-concentrator-type electromagnetic acoustic transducer (D-MC-EMAT) is proposed to generate a single S0 mode in plates. The magnetic concentrator is added to the traditional EMAT to guide and concentrate the magnetic field generated by the magnet and remove the unwanted A0 mode. Both finite element simulation and experiments proved that the mode selectivity of proposed D-MC-EMAT improved by adding the magnetic concentrator, and transducer characteristics and defect inspection experiments were carried out. Finally, the structure of D-MC-EMAT was optimized by an orthogonal experiment and the optimal parameter combination was confirmed.

Aili Li

Papers

A direction-tunable shear horizontal mode array magnetostrictive patch transducer

Development of an omni-directional magnetic-concentrator-type electromagnetic acoustic transducer

Development of a directional magnetic-concentrator-type electromagnetic acoustic transducer for ultrasonic guided wave inspection

Development of a shear horizontal wave electromagnetic acoustic transducer with periodic grating coil

Development of a mode-tuning magnetic-concentrator-type electromagnetic acoustic transducer.