Electromagnetic Acoustic Transducer Research Articles

Closed-form shrinkage function based on mixture of Gauss–Laplace distributions for dropping ambient noise

Removing the ambient noise and increasing the signal-to-noise ratio are very important for detecting defects and corrosions of conductive material by using the electromagnetic acoustic transducer. It is still an issue to remove the ambient noise without losing the original signal information. The aim of this paper is to solve the issue by using a new closed-form shrinkage function based on Gauss–Laplace mixture distribution in wavelet domain. First, we prove that Gauss–Laplace mixture distribution is well fitted to the statistical model for wavelet coefficients of noise-free signal of electromagnetic acoustic transducer. As well, we use Gauss–Laplace mixture distribution and Gauss distribution for statistical modeling on the wavelet coefficients of noise-free signal and ambient noise, respectively. Using these distributions, we derive a new closed-form shrinkage function that is an analytical solution of a Bayesian maximum a posteriori estimator. Next, we evaluate the denoising performance of new shrinkage function compared with various shrinkage functions in terms of the improved signal-to-noise ratio, root mean squared error and entropy. The experiment results show that the wavelet denoising method using the proposed shrinkage function effectively removes the ambient noise than the other existing denoising methods for noisy signal of electromagnetic acoustic transducer.

A Method of Rayleigh Wave Combined With Coil Spatial Pulse Compression Technique for Crack Defects Detection

Rayleigh wave methods utilizing electromagnetic acoustic transducers (EMATs) are widely used to detect a crack defect on the metal surface due to the merits of high sensitivity and long detection distance. However, the current EMATs for Rayleigh wave has low energy conversion efficiency and usually produce a wide echo-wave packet, reducing the spatial resolution and signal-to-noise ratio of detection signals. To address the issues, the coil spatial pulse compression technique (CSPCT) is proposed to improve the physical structure of EMATs. First, a comprehensive analysis of the coil design is given to realize spatial phase encoding pulse compression. Second, the influence of the key parameters of EMATs on the pulse compression effect is studied. Finally, the crack detection experiment is conducted on an aluminum plate to verify the performance improvement of the designed EMAT. Compared to existing EMATs, our designed EMATs reduce the width of the main lobes and enhance the amplitude of the echo-wave signals. Results indicated that the spatial resolution was improved by up to 72% with an increase of 0.72 dB in signal-to-noise ratio. In comparison to the existing time-domain pulse compression technique, the CSPCT requires no complex hardware and software tools, which has the merits of low cost and fast response. This work expands the application boundary of EMAT systems.

Enhancing the Lift-Off Performance of EMATs by Applying an Fe3O4 Coating to a Test Specimen

Electromagnetic acoustic transducers (EMATs) are noncontact ultrasonic transducers. The transduction efficiency of a particular EMAT on a given specimen is dependent on the lift-off distance, which is the distance between the EMAT coil and the specimen surface. The transduction efficiency drops dramatically with increased lift-off distance, requiring EMATs to be in close proximity to the specimen, usually within a few millimeters. This article proposes a new EMAT method of applying an Fe3O4 coating to the test specimen and quantitatively studying the enhancement effect of Fe3O4 coating on lift-off distance. To eliminate the interference of the electrical and magnetic properties of the tested specimen, a nonmagnetic and nonconductive glass specimen is selected. The experimental results on a glass substrate coated with Fe3O4 demonstrate the feasibility of EMATs generating and receiving ultrasonic waves through the coating, by a magnetoelastic mechanism. The transduction efficiency of EMATs on an Fe3O4 coating does not increase linearly with the bias static magnetic field, and the maximum measured signal amplitude value occurs at a relatively low flux density of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 0.12$ </tex-math></inline-formula> T. More specifically, it has been shown that the Fe3O4 coating can significantly enhance the lift-off distance of EMATs operating at 4 MHz to 8 mm on coated stainless steel. The performance of the Fe3O4 coating can be optimized, showing considerable potential to expand the application range of EMATs.

Detection of Debonding Area in Aluminium-GFRP Laminates by Electromagnetic Transducer

Electromagnetic Acoustic Transducer (EMAT) is applied to nondestructive testing of aluminum-GFRP laminates. Because of its high specific strength, laminated FRP came to be used in place of metal materials. However, once delamination is introduced by the impact force in the out-of-plane direction, the compressive strength in the in-plane direction is greatly reduced. To improve the tolerance for impact force, fiber metal laminates (FML) have been developed. Since debonding may occur at the interface of FRP and metal sheet, nondestructive detection of the debonding is required as health monitoring of the FML. In this study, GFRP laminates were molded on aluminum sheets, and we measured the distribution of standing wave amplitude in the aluminum sheets using EMAT for shear wave. In the bonded area, the shear wave is partially transmitted to the FRP on reflection at the boundary, which makes the attenuation coefficient larger than that in the debonded area. It was found that in scanning the EMAT, the standing wave amplitude starts to change when the center of the EMAT passes the boundary between bonding and debonding areas. It was also revealed that minimum size of the debonding area that EMAT can detect is comparable to its effective area. For accurate evaluation of the debonding area, resonant frequency measurement is recommended at each point.

Measurement of Plate Thickness Based on the Optimal Unidirectional Generation of Ultrasonic Waves Using Dual Electromagnetic Acoustic Transducer

Unidirectional generation of shear horizontal (SH) ultrasonic guided waves can be achieved using dual-coil electromagnetic acoustic transducers (EMATs), through the interference mechanism when appropriately phased signals are used to drive each EMAT coil. Optimal unidirectional generation requires precise knowledge of the intended wave mode dispersion curve, which in turn, depends on the plate’s thickness. In this paper, we exploit optimal unidirectional generation with a dual-EMAT in order to measure plate thickness. The proposed thickness measurement method uses a dual periodic permanent magnet (PPM) EMAT, acting as transmitter, and two conventional PPM EMAT receivers on opposite sides of the transmitter, in order to receive the forward (enhanced) and backward (weakened) generated SH ultrasonic guided waves. Then, two consecutive frequency sweeps are used to generate the SH0 and SH1 modes, in order to seek the maximum ratio of the forward travelling wave amplitude to the backward travelling wave amplitude. The first sweep determines the optimum operating frequency for the first wave mode, which is a necessary parameter for the next sweep. The second frequency sweep determines the plate thickness. The new method was initially evaluated with a frequency-wavenumber-domain analytical model. The proposed method showed a much higher frequency-specificity, which was assessed by a quality factor figure of merit defined by the ratio of the centre frequency to the -3 dB frequency bandwidth, when compared to that obtained using the conventional method that is based solely on the amplitude of the generated wave, and is therefore less susceptible to error in the thickness estimation. The method was experimentally evaluated with a dual-PPM EMAT on aluminium plates, showing good agreement with simulations and providing more than ten-fold higher specificity than the conventional method, and accurate thickness measurements, with an error lower than 0.8%.

Fatigue state characterisation of steel pipes using ultrasonic shear waves.

The phenomenon of the reduction in the propagation speed of an ultrasonic wave when it travels through a fatigue zone has been well studied in the literature. Additionally, it has been established that shear waves are more severely affected by the presence of such a zone, compared with longitudinal waves. Our study utilises these phenomena to develop a method able to characterise the fatigue state of steel pipes. Initially, the existing theory regarding the increased sensitivity of shear waves to the presence of fatigue is validated through measuring and comparing the change in propagation speed of both longitudinal and bulk shear waves on flat geometries, at different fatigue states. The comparison is achieved with the aid of ultrasonic speed C-scans of both longitudinal and shear waves, with the latter now being obtainable through our implementation of advances in Electromagnetic Acoustic Transducers (EMAT) technology. EMATs have not been traditionally used for producing C-scans, and their ability do to so with adequate repeatability is demonstrated here; we show that shear wave scanning with EMATs now provides a possibility for inspection of fatigue damage on the inner surface of pressure-containing components in the nuclear power industry. We find that the change in ultrasonic wave speed is amplified when shear waves are used, with the magnitude of this amplification agreeing well with the theory. Following the verification of the theory, the use of EMATs allowed us to tailor the shear wave scanning method to pipe geometries, where C-scans with conventional piezoelectric transducers would not have been possible, with the results successfully revealing the presence of fatigue zones.

Design of orthogonal directional shear horizontal electromagnetic acoustic transducer

This research proposed an electromagnetic acoustic transducer (EMAT) for generating and receiving orthogonal directional shear horizontal (ODSH) guided waves in the metal plate-like structure. The proposed EMAT generates the fundamental mode of SH wave in the horizontal and vertical direction in the plane of the test sample. The ODSHEMAT combines a new Chevron-like coil design which generates an eddy current effect in the skin depth of the plate surface, and a periodic permanent magnet that applies a static magnetic field vertically with a periodicity equal to its wavelength. The direction of the resultant Lorentz force acting on the test surface-controlled PPM underneath a specific chevron angle of the coil wire. The constructive interference of propagated waves has been developed in the orthogonal directions, enhancing the ultrasonic signals and improving the signal-to-noise ratio. A finite element simulation has been carried out with a thin aluminum plate, keeping the proposed EMAT at the center of the sample. Further, the simulation results show good agreement with the proposed design.

A Composite Approach of Electromagnetic Acoustic Transducer and Eddy Current for Inner and Outer Corrosion Defects Detection

In the production and service of pipelines, automated electromagnetic detection is widely employed to diagnose potential defects. However, the single inspection method can hardly multiposition detect detection simultaneously due to limited signal features. In this article, a novel composite approach of electromagnetic acoustic transducer (EMAT) and eddy current (EC) is proposed to detect different kinds of defects by combining their technical merits. A comprehensive theoretical analysis of the composite approach is given to derive the modified model of the send-receive mode. The influences of the critical parameters on hybrid signals are discussed and the probe performance is improved by over 65% in magnitude. Then, the time-division multiplexing strategy is utilized to separate hybrid signals. Finally, laboratory experiments are conducted to extract independent features and diagnose potential defects in the steel plate. Results indicated that the proposed approach could achieve a good performance in the detection task of inner and outer defects. Compared with existing detection methods, our proposed approach has significant merits in detectability and efficiency.

A Lorentz Force EMAT Design with Racetrack Coil and Periodic Permanent Magnets for Selective Enhancement of Ultrasonic Lamb Wave Generation

This article proposes an electromagnetic acoustic transducer (EMAT) for selectively improving the purity and amplitude of ultrasonic Lamb waves in non-ferromagnetic plates. The developed EMAT consists of a racetrack coil and a group of periodic permanent magnets (PPMs). Two-dimensional finite element simulations and experiments are implemented to analyze the working mechanism and performance of the PPM EMAT. Thanks to the specific design, the eddy currents increase with increasing wire density and the directions of the magnetic fields and Lorentz forces alternate according to the polarities of the magnet units. Wires laid uniformly beneath the magnets, and the gaps between adjacent magnets generate tangential and normal Lorentz forces, resulting in-plane (IP) and out-of-plane (OP) displacements, respectively. The constructive interference occurs when the wavelength of the generated Lamb wave is twice the spacing of the magnets, leading to large amplitudes of the targeted ultrasonic Lamb waves. Therefore, the PPM EMAT is capable of generating pure symmetric or antisymmetric mode Lamb waves at respective frequencies. The results prove that the developed PPM EMAT can generate pure either S0 or A0 mode Lamb waves at respective frequencies. The increase in wire width and wire density further increases the signal amplitudes. Compared with the case of conventional meander-line-coil (MLC) EMAT, the amplitudes of the A0 and S0 mode Lamb waves of our PPM EMAT are increased to 880% and 328%, respectively.

New approach for the detection of defects in the core support structure of SFRs using EMAT based on a Halbach magnet

This paper investigated a new method based on a combination of electromagnetic and ultrasonic sensors for inspecting the core support structure of the reactor vessel in sodium-cooled fast reactors from a position located outside of the vessel. Using an electromagnetic acoustic transducer (EMAT) with a Halbach magnet, the feasibility of the detection of the defects in the reactor core support structure was investigated via experimental measurements using a reduced half-scale mock-up model in a sodium-free environment. The amplitude of the signal from various small slits (5 %tw-20 %tw) was enhanced using a combination of EMAT geometry, Halbach magnet configuration, and signal processing methodology in view of increasing the signal-to-noise ratio when the EMAT continuously scanned the area of interest for defects deep inside the metallic structure.

Development of Electromagnetic Acoustic Transducer System for Coin Classification

In this work, a method for identifying counterfeit coins based on an electromagnetic acoustic transducer (EMAT) to detect the difference in the coin's natural acoustic frequency response is presented. In the experimental system, the acoustic oscillation induced by a pulsed magnetic field is received by a microphone and recorded by an oscilloscope. The natural acoustic frequency of the coin is resolved by the fast Fourier transform (FFT) method on the computer. It is found that the natural frequencies of the possible counterfeit coins deviate significantly from the standard ranges of 16.9 to 17.4 kHz for the authentic 50 New Taiwan Dollar (NTD) coins. The observed natural frequencies of the coin are consistent with the values predicted by analytical estimation. We also built a prototype EMAT coin classification system to detect the natural acoustic frequency by direct frequency counting using a microcontroller. The prototype system demonstrates that a counterfeit coin can be identified by its natural frequency in less than 30 ms using the EMAT method. The proposed technique can be applied to the vending machine to improve the accuracy in discriminating between authentic and counterfeit coins.

Selective simultaneous generation of distinct unidirectional wave modes in different directions using dual-array transducer

Shear horizontal (SH) waves are widely used in the non-destructive evaluation and structural health monitoring fields. Conventional periodic permanent magnet electromagnetic acoustic transducers (PPM EMAT) can generate a single SH wave mode, which propagates in both forward and backward directions. Generation in a single direction is usually preferred to ease signal interpretation and increase inspection reliability. Recently, we have proved that a single, either fundamental or high-order, SH wave mode can be unidirectionally generated using a dual PPM EMAT, with carefully designed driving signals. Distinct SH modes present different applicability due to their particular response to features in the structure, such as defects. Hence, unidirectional generation of two different SH modes simultaneously with a dual-array transducer is investigated in this paper. The selectivity of wave modes and the selectivity of the propagation direction are studied by combining pure wave mode excitability with the superposition of optimal excitation signals for unidirectional generation, addressed in the frequency–wavenumber domain. Excitation signals for optimal and simultaneous generation of the SH0 and SH1 modes propagating in the same direction or in opposite directions have been designed. Experiments on an aluminium plate have demonstrated that the two wave modes can be effectively simultaneously generated with only one dual-array transducer and the propagation direction of each wave mode can be flexibly controlled using the designed excitation signals.

Nonsingular Integral Terminal Sliding Mode Control for Resonant Frequency Tracking of Electromagnetic Acoustic Transducers (EMATs) Based on Fixed-Time Strategy.

Resonant frequency tracking control of electromagnetic acoustic transducers (EMATs) remains a challenge in terms of drifting working frequency and reduced conversion efficiency caused by working environment changes. This paper presents a fixed-time nonsingular integral terminal sliding mode (FT-NITSM) control strategy for resonant frequency tracking of EMATs to realize precise and high robustness resonant frequency tracking performance. Specifically, a FT-NITSM control method with fast convergence feature is developed and a resonant frequency tracking controller for EMATs is further designed to improve the convergence speed and tracking accuracy. Fixed time stability of the proposed frequency tracking control system is proved through Lyapunov function analysis. Moreover, numerical simulations demonstrate that the FT-NITSM control strategy can ensure precise tracking of the system's operating frequency to its natural resonant frequency in less than 3 s with a tracking error of less than 0.01 × 104 Hz. With the maximum overshoot variation between -20 and 20 and error range in -5 and 5° at the steady state, the FT-NITSM control strategy can ensure the control system impedance angle θ being consistent and eventually bounded. This study provides a toolbox for the resonant frequency tracking control and performance improvement of EMATs.

Study of the Influence of the Backplate Position on EMAT Thickness-Measurement Signals.

Nondestructive testing (NDT) is an essential method for assessing structural integrity in the oil and gas industry. Electromagnetic acoustic transducers (EMATs) have been extensively used to detect the wall-thickness reduction of plate-like structures, because they do not require direct contact. The pulse intervals of echoes are used to calculate the remnant thickness of structures. If the width of a single pulse is too large, multiple pulses will be superimposed, making it more difficult to extract the pulse interval. Thus, the width of a single pulse affects the resolution of measurements. This paper investigates the impacts of the backplate position on the pulse width and amplitude of thickness-measurement signals, using EMATs. By means of impedance modeling and measurement, it can be shown that the output impedance of the receiving coil is strongly influenced by the coil-backplate gap. With the increment in the coil-backplate gap, the signal amplitude and damping coefficient increase, while the self-resonant frequency decreases. By means of signal measurements on the specimen, it is shown that the pulse width and the signal amplitude can be significantly influenced by the backplate position. By reducing the coil-backplate gap, the pulse width can be reduced by over 80%, and by increasing the gap, the signal amplitude can be increased by over 300%. These research results can be used to optimize EMAT design, thereby suppressing the superposition of pulse echoes.

Extending the Incidence Angle of Shear Vertical Wave Electromagnetic Acoustic Transducer with Horizontal Magnetization.

Angled shear vertical (SV) waves have been successfully employed in the non-destructive testing of welds, pipes, and railways. Non-contact meander-line coil electromagnetic acoustic transducers (EMAT) have many benefits in generating angled SV waves. The most important benefit is that the incidence angle of an SV wave can be controlled by the excitation frequency. However, the incidence angle of a traditional SV-wave EMAT is reported to be under 45 degrees in many cases. In this work, such cases are tested, and the problems of the received signal at large incidence angles are found to be due to wave interference and small signal amplitudes. An equivalent finite element (FE) model is established to analyze the problem, and the main reason is found to be the head wave. An alternative configuration of angled SV-wave EMAT with horizontal magnetization is proposed to reduce the influence of the head wave. Finally, the results from simulations and experiments show that the proposed EMAT has a larger signal amplitude and significantly reduced interference in large-incidence angle scenarios. Moreover, an incidence angle of an SV wave of up to 60 degrees can be achieved, which will help improve the performance and capability of nondestructive testing.

Pressure monitoring of Special Nuclear Material containment

All Magnox Nuclear Reactors are now in the process of decommissioning and nearly all Magnox spent fuel has been reprocessed, producing uranium and plutonium and small volumes of highly radioactive waste. The recovered plutonium is stored in specialist containers in secure facilities while the government reaches a decision on possible re-use or disposal, following the completion of technical studies. There is potential for the containment to become pressurised over time due to alpha decay and radiolysis of the plutonium, and highlights the need for continuous monitoring to assess the integrity of the containment. This paper presents a new technique of interpreting the internal pressure of the containment by understanding the change in the vibrational response of the outer containment wall for different internal pressures. Electromagnetic acoustic transducers have been used to excite the outer containment wall in a non-contact and non-destructive manner, and capture the changes in the resonant frequency response of the system for different internal pressures.

Development of a Longitudinal Wave EMAT With High Conversion Efficiency

Until now, most of the bulk-wave electromagnetic acoustic transducers (EMATs) are developed for shear wave (S-wave) testing. This article presents the development of new longitudinal wave (L-wave) EMATs with high conversion efficiency by using a new magnetic configuration and a meander-line coil. The new magnetic configuration consists of several rectangular magnets arranged face to face to generate a strong horizontal magnetic field in periodical distribution under the magnets. By using the meander-line coil under the magnets, a strong L-wave can be generated and detected. The new EMATs have been characterized and optimized using numerical simulation methods. Simulation results show that the flux density of the horizontal magnetic field of the new EMATs can be increased by a factor of ~4.7, compared with that of the conventional U-type magnet. So, the conversion efficiency for L-wave generation and detection can be significantly improved. Finally, experimental measurement and comparison with the current L-wave EMATs with the U-type magnet have also been carried out. The experimental results have confirmed that the signal amplitude of the new L-wave EMATs is increased by a factor of ~8.3.

Application of Adaptive Filtering Based on Variational Mode Decomposition for High-Temperature Electromagnetic Acoustic Transducer Denoising

In high-temperature environments, the signal-to-noise ratio (SNR) of the signal measured by electromagnetic acoustic transducers (EMAT) is low, and the signal characteristics are difficult to extract, which greatly affects their application in practical industry. Aiming at this problem, this paper proposes the least mean square adaptive filtering interpolation denoising method based on variational modal decomposition (AFIV). Firstly, the high-temperature EMAT signal was decomposed by variational modal decomposition (VMD). Then the high-frequency and low-frequency noises in the signal were filtered according to the excitation center frequency. Following the wavelet threshold denoising (WTD) for the noise component after VMD decomposition was carried out. Afterward, the noise component and signal component were connected by an adaptive filtering process to achieve further noise reduction. Finally, cubic spline interpolation was used to smooth the noise reduction curve and obtain the time information. To verify the effectiveness of the proposed method, it was applied to two kinds of ultrasonic signals from 25 to 700 °C. Compared with VMD, WTD, and empirical mode decomposition denoising, the SNR was increased by 2 times. The results show that this method can better extract the effective information of echo signals and realize the online thickness measurement at high temperature.

Analysis of the Effect of Magnetic Field on Solidification of Stainless Steel in Laser Surface Processing and Additive Manufacturing

The problem of surface processing and microstructure refinement in stainless steels in laser surface processing in the presence of an external magnetic field has been studied experimentally and theoretically. The effect of both alternating and permanent magnetic fields is discussed. The experimental part includes microstructure assessment of a thin stainless plate annealed by a quasi-continuous laser in the presence of an electromagnetic acoustic transducer. Complementary analytical calculus and numerical simulations of complex transport phenomena in the melting zone are performed. Based on the received data, the effect of the electromagnetic field on the molten zone under laser melting conditions is evaluated and quantified. The obtained results are relevant to laser surface hardening and additive manufacturing.

Variational Wavelet Ensemble Empirical (VWEE) Denoising Method for Electromagnetic Ultrasonic Signal in High-Temperature Environment with Low-Voltage Excitation

Low excitation voltage for an electromagnetic acoustic transducer (EMAT) is necessary for the petrochemical equipment and facilities inspection, which work at high-temperatures, to avoid potential explosion. However, low excitation voltage causes low signal-to-noise ratio (SNR) signals that are difficult to extract features, especially in a high-temperature environment, which causes high noise. In this study, a denoising method called the variational wavelet ensemble empirical (VWEE) method was proposed by combining the advantages of the variational modal decomposition (VMD), wavelet threshold (WT) denoising, and ensemble empirical mode decomposition (EEMD) methods. To validate the VWEE method, EMAT signals obtained in the temperature range of 25 to 700 °C were analyzed. The results show that, compared with VMD, WT and empirical mode decomposition denoising methods, the SNR of proposed method is improved more than two times. The VWEE method dramatically improved the SNR of a high-temperature EMAT signal and enhanced the accuracy of defect echos extraction.