Surface acoustic waves, such as Rayleigh or Lamb waves, can be used to characterise the geometry of surface-breaking defects. For accurate measurement of defect length and depth, the in-plane (IP) and out-of-plane (OP) components of the displacement or velocity can be measured and compared. Electromagnetic acoustic transducers (EMATs) offer a simple, non-contact method of doing this, as it is possible to design detection EMATs such that they are sensitive predominantly to either the IP or the OP velocity component. Here we present a new design of differential coil EMAT which measures both components simultaneously, and through simple processing gives the IP and OP velocities. This is compared to previous designs, showing that these components are obtained accurately. The system is used to characterise three artificial surface-breaking defects in aluminium and one thermal fatigue crack in 10 mm thick steel plate under thin coating.

Differential Coil EMAT for Simultaneous Detection of In-Plane and Out-of-Plane Components of Surface Acoustic Waves

The performance degradation of 12CrMoV steel generally occurs under a long-term working period in a high-temperature environment. In order to improve the reliability of a single nondestructive testing method for the performance degradation analysis of 12CrMoV steel, an advanced magneto–acoustic compound inspection method, in which pulsed eddy current (PEC) and ultrasonic waves were obtained from a single electromagnetic acoustic transducer (EMAT) testing signal, was adopted. The magneto–acoustic compound inspection method was carried out on 12CrMoV steel plates heat-treated at different temperatures and times. New parameters, including the slope and peak value of PEC, and the amplitude and attenuation of ultrasonic waves, were introduced to evaluate the performance degradation of 12CrMoV steel. It was found that ultrasonic waves manifested higher sensitivity than eddy current when the microstructure changed significantly. Therefore, the magneto–acoustic compound inspection method is advantageous for evaluating the performance degradation of 12CrMoV steel.

Performance degradation detection of 12CrMoV steel by magneto-acoustic compound inspection method

Shear horizontal (SH) guided waves are widely used in non-destructive evaluation and can be generated by permanent magnet electromagnetic acoustic transducer (PPM EMATs). PPM EMATs generate SH waves that propagate both forwards and backwards. This is often an undesirable characteristic. Unidirectional generation can be achieved by the interference mechanism produced by two independent sources. This, however, is non-feasible for the conventional PPM EMATs due to the arrangement of PPM array and racetrack coil. Recently, we presented a design of an SH EMAT with side-shifted PPM arrays that generates SH waves nominally in a single direction. Due to the shift of the produced wavefronts from each array, that design produced side-lobes in the backward semi-plane. In this paper, we enhance the unidirectional generation of SH waves with side-shifted PPM EMATs by evaluating the effect of the separation between individual arrays as well as the number of rows of magnets in each array. An analytical model is used to calculate the radiation pattern. The amplitude of the backward side-lobes decreases either with increasing the number of magnets rows or decreasing the lateral gap between rows. This behaviour is explained by the amount of wavefront misalignment introduced by each design. Increasing the number of magnets rows also increases the directivity and the energy of the forward generated waves. Twelve different configurations were fabricated, with a reasonably short wavelength of 12 mm, and experimentally evaluated, agreeing with the theoretically calculated radiation pattern. The magnitude of the backward side-lobes was roughly halved when compared with the original design.

Enhancement of the Unidirectional Radiation Pattern of Shear Horizontal Ultrasonic Waves Generated by Side-Shifted Periodic Permanent Magnets Electromagnetic Acoustic Transducers With Multiple Rows of Magnets

In this study, an advanced magnetoacoustic compound inspection method for a high-order Lamb waves mode electromagnetic acoustic transducer (EMAT) was proposed. The proposed EMAT can achieve the excitation and reception of the A 1 mode which is the first-order Lamb waves mode. According to the characteristics of the A 1 mode EMAT testing signal, a signal processing method for obtaining the pulsed eddy current signal by curve fitting was developed to extract the information for the pulsed eddy current. The voltage of the ultrasonic waves was carried on the pulsed eddy current voltage, and it was obtained by removing the pulsed eddy current voltage and extracting the time-domain testing signal of the A 1 mode EMAT. Through A 1 mode EMAT single detection that was used to obtain the pulsed eddy current and A 1 mode, the multi-parameter from the magnetoacoustic compound testing signal of the A 1 mode EMAT could be used to characterize the aluminum plate performance degradation. The detection accuracy of aluminum plate performance degradation was improved by the magnetoacoustic compound inspection compared with a single detection method.

An Advanced Magnetoacoustic Compound Inspection Method of High-Order Mode EMAT

Design and experiment of array Rayleigh wave-EMAT for plane stress measurement

A new design of Window Function modulated electromagnetic acoustic transducers (EMATs) to generate unidirectional Rayleigh waves is proposed. The novel EMATs, Rectangular-Window-Meander-Line-Coil EMATs (RWMLC-EMATs) and Kaiser-Window-Meander-Line-Coil EMATs (KWMLC-EMATs), contain two variable length meander line coils with a spacing of one quarter of Rayleigh waves’ wavelength; the excitation signals for these two coils have a phase difference of 900. The structure of the variable wire length determines the level of the sidelobes suppression; the structure of two meander line coils combined with the excitation signals with a phase difference of 900 determine that Rayleigh waves only propagate along one direction. RWMLC-EMATs and KWMLC-EMATs are analyzed via a wholly analytical method and are validated with experiments, which proves the feasibility of the proposed EMATs structure. In addition, the effect of the wire step and the longest wire length are investigated analytically and experimentally. Results indicate that, wire step mainly affects the suppressed level of sidelobes – a large wire step leads to a small suppressed level of sidelobes. This work can be used for Rayleigh waves EMATs design and optimization to improve their defect detection capability.

A novel design of window function modulated meander-line-coils EMATs for unidirectional Rayleigh waves generation and sidelobes suppression

This work presents a new omnidirectional electromagnetic acoustic transducer (EMAT) employing a circumferential periodic permanent magnet array, which was proposed for generating and receiving shear-horizontal (SH) guided waves in an aluminum plate. The novel transducer consisted of 12 axially polarized sector magnets arranged in a circle, and the magnetic field directions of adjacent magnets were opposite. A spiral coil was arranged under the magnet array. A vertical magnetic field generated by the magnets and the circumferential eddy current generated by the spiral coil formed a Lorentz force in a plate acting in the radial direction of this new omnidirectional EMAT. By controlling the sizes of the sector magnets, the omnidirectional SH0 mode (fundamental SH guided wave mode) was generated. To maximize the output of the EMAT at the given excitation frequency, it was proposed to add acrylic spacers between the sector magnets and determine the size of the magnet based on the frequency bandwidth of the excitation signal while also optimizing the omnidirectional sound field. A three-dimensional (3D) finite element model of the EMAT was established using COMSOL Multiphysics to perform numerical simulations, and the effectiveness of the optimization method was verified. The effectiveness was verified by experiments. The simulations and experiments showed that the proposed EMAT could generate omnidirectional SH0 mode guided waves in an aluminum plate.

Development of an Omnidirectional SH 0 Mode Electromagnetic Acoustic Transducer Employing a Circumferential Periodic Permanent Magnet Array

To improve the efficiency and robustness of electromagnetic ultrasonic testing, magneto-acoustic compound inspection for ferromagnetic materials is proposed. Both ultrasonic waves and pulsed eddy current (PEC) could be obtained by the single detection of an electromagnetic acoustic transducer (EMAT), where ultrasonic waves were generated based on the coupling of the magnetostriction effect and the Lorentz force mechanism, when the EMAT was used to detect ferromagnetic materials. In this study, three simulation models of the EMAT were built by coupling different physics fields while keeping the geometry of the models unchanged and the other parameters of the EMAT constant. Transverse wave could be generated by magnetostriction force, and longitudinal wave could be generated by Lorentz force. Based on time-frequency analysis of the simulation results, the energies of ultrasonic waves and PEC were concentrated in different frequency ranges. Through the filtering strategy and by separating the time-domain signal, magnetostriction force ultrasonic wave (MFUW), Lorentz force ultrasonic wave (LFUW), and PEC could be decoupled from the EMAT testing signal. Multiple parameters can be extracted by a single detection of the EMAT, and the ability of the electromagnetic ultrasonic testing can be improved.

Numerical Decoupling Study of EMAT Testing Signal for Ferromagnetic Materials

At present, there are many researches and application fields of Chinese text classification, such as news classification, emotion classification, and spam identification and so on [1]. However, there are relatively few researches on the classification of Chinese abstracts of postgraduate dissertations. The abstract of postgraduate dissertation is a highly summary of the purpose, method, innovation, key and difficult points and results of the whole dissertation’s research. To recognize the abstracts of postgraduate dissertations in the library collection, and then to classify the dissertations in the library collection, is beneficial to the majority of teachers and students to consult the dissertations efficiently, and can better recommend the dissertations to readers according to their research background. Therefore, how to automatically classify postgraduate dissertations in library collection is an urgent problem to deal with. Assume that the abstract of postgraduate dissertation is written strictly according to the writing requirements, therefore, the abstract of the dissertation can represent the classification of the research field for the whole dissertation. Extract the abstracts of the dissertations as characteristic data X, label data Y was made based on the information of the school where the authors of the dissertations work in, based on the postgraduate dissertations collected in the library of the university. The dissertations studied were divided into eight categories, So Y can take integer values from one to eight. After preprocessing, the word vector and classifier were trained by the way of machine learning [2]. The results show that the classifiers can output the classification more accurately when a new abstract is randomly input to the models that has been trained.

Zhilin Huo

Papers

A novel design of window function modulated meander-line-coils EMATs for unidirectional Rayleigh waves generation and sidelobes suppression

Development of an Omnidirectional SH 0 Mode Electromagnetic Acoustic Transducer Employing a Circumferential Periodic Permanent Magnet Array

Performance degradation detection of 12CrMoV steel by magneto-acoustic compound inspection method

Numerical Decoupling Study of EMAT Testing Signal for Ferromagnetic Materials

Research on classifier design of Chinese abstracts of postgraduate dissertations in library collection