Wave Electromagnetic Acoustic Transducers Research Articles

Analytical modelling and analysis of the meander-line coil EMATs

The meander-line coil electromagnetic acoustic transducer (EMAT) is widely used in the field of ultrasonic nondestructive testing due to its convenience to generate specific mode of guided waves. Some design methods of the meander-coil EMATs are developed in the frequency-wavenumber domain while others in the time–space domain. In this paper, a unified theoretical framework is developed by proposing an analytical model from the system perspective. Signal transfers between different physical fields in EMAT excitation, wave propagation and EMAT reception are represented as linear time–space-invariant systems. Taking the Rayleigh wave EMAT detection as an example, the analytical model for the transfer functions of these systems is established. The analytical model is experimentally verified by different Rayleigh wave detection techniques: the conventional EMAT, the spatial pulse compression (SPC) EMAT, temporal-spatial pulse compression (TSPC) EMAT and detection cases employing the same receiving EMAT. From the system perspective, the received signal of EMAT is interpreted as the response of the filter system to the input signal. It is found that the meander-coil EMAT can be regarded as the frequency domain expression during the detection. And the frequency domain expression plays different roles in different techniques.

Underwater quantitative thickness mapping through marine growth for corrosion measurement using shear wave EMAT with high lift-off performance

Underwater inspection is important to ensure the safety, integrity and functionality of underwater structures. Although numerous conventional methods have been adopted for underwater inspection, successful application of most methods relies on the surface condition of the object, which, however, is typically covered by marine growth. Consequently, routine inspection requires thorough cleaning of marine growth, which is time-consuming and costly. Hence a method which can inspect objects without the need for extensive surface cleaning is necessary. Two methods have the potential to achieve this: pulse eddy current (PEC) and electromagnetic acoustic transducer (EMAT). PEC attains a significant lift-off distance, enabling inspection through marine growth. However, it suffers from high sensitivity to environmental conditions and low inspection accuracy due to ‘relative’ property which means its results are interpreted by comparing received signals to reference values. In contrast to PEC, EMAT provides ‘absolute’ measurements, ensuring precise results in the inspection. But it is limited by a small lift-off distance (<2∼3 mm), rendering it unsuitable for underwater applications with marine growth. Therefore, if the lift-off distance can be enhanced to a specific value, this method may offer a superior solution for underwater inspections. In this paper, a quantitative measurement method is proposed through employing a shear wave EMAT with high lift-off performance. A repelling configuration of magnets is introduced to achieve a significantly improved maximum effective lift-off distance of up to 5 mm in both air and seawater conditions with only 400 Vpp applied. This EMAT is then demonstrated to measure thickness through marine growth, showing excellent underwater performance in quantitative thickness mapping for corrosion inspection.

An Adaptive Noise Reduction Method for High Temperature and Low Voltage Electromagnetic Detection Signals Based on SVMD Combined with ICEEMDAN.

In view of the low signal-to-noise ratio (SNR) of shear wave electromagnetic acoustic transducers (EMAT) in the detection of high-temperature equipment, the use of low excitation voltage (LEV) further deteriorates the detection results, resulting in the echo signal containing defects being drowned in noise. For the extraction of the EMAT signal, an adaptive noise reduction method is proposed. Firstly, the minimum envelope entropy is taken as the fitness function for the Harris Hawks Optimizer (HHO), and the optimal successive variational mode decomposition (SVMD) balance parameter is searched by HHO adaptive iteration to decompose LEV EMAT signals at high temperatures. Then the filter is carried out according to the excitation center frequency and correlation coefficient threshold function. Then, improved complete ensemble empirical mode decomposition with adaptive noise (ICEEMDAN) is used to decompose the filtered signal and combine the kurtosis factor to select the appropriate intrinsic mode functions. Finally, the signal is extracted by the Hilbert transform. In order to verify the effectiveness of the method, it is applied to the low-voltage detection of 40Cr from 25 °C to 700 °C. The results show that the method not only suppresses the background noise and clutter noise but also significantly improves the SNR of EMAT signals, and most importantly, it is able to detect and extract the 2 mm small defects from the echo signals. It has great application prospects and value in the LEV detection of high-temperature equipment.

Design of a Permanent Magnet Electromagnetic Acoustic Transducer (EMAT) for In-Situ Inspection of Metals at high Temperatures

In this proposed research, a permanent magnet longitudinal wave Electromagnetic Acoustic Transducer (EMAT) was designed for In-Situ inspection of metals at high temperatures. A permanent magnet EMAT generates Lorentz force inside metals by the interaction of magnetic fields and eddy currents. As temperature rises, various factors can influence the intensity of the Lorentz force, making it difficult to produce and detect ultrasonic waves. Thus, it is essential to optimize the sensor at ambient temperature before attempting to generate a reliable signal at elevated temperatures. The challenge highlighted has been addressed by optimizing the intensity of static field by varying the parameter like spacing between magnets that are being used. In order to minimize the effects of elevated temperatures, the sensor's stability has been enhanced by the implementation of efficient insulating measures. Furthermore, Finite Element (FE) simulations were conducted under on aluminium sample. These simulation results were then compared to experimental data to validate the efficiency of the designed sensor. Keywords: Permanent magnet, EMAT, Lorentz force, ultrasonic waves, high temperature, static magnetic field intensity.

Magneto-acoustic Fusion Sensing for Surface and Subsurface Defect Differentiation in the Ferromagnetic Material

The conventional periodic permanent magnet (PPM) shear horizontal (SH0) guided wave electromagnetic acoustic transducer (EMAT) has a good detection capability for defects on both the surface and subsurface of the material. However, it is difficult to distinguish between surface and subsurface defects in the material using only the amplitude of the defect echo. In this paper, SH0 guided wave is excited using the SH0 guided wave EMAT with PPM configuration, which is used to achieve the detection and evaluation of defects on the surface and subsurface of ferromagnetic metal; on the other hand, the local magnetic field perturbation (MFP) effect of the PPM configuration on ferromagnetic material is fully exploited to achieve the identification of surface defects by picking up the MFP of the material at the defects. The proposed magneto-acoustic fusion effect sensor has an improved spatial magnetic field at both the mechanical construction and the magnetic sensor compared to the traditional magnetic flux leakage (MFL) detection method. Both simulation and experiment show that the proposed magneto- acoustic fusion effect sensing method based on the PPM configuration is capable of detecting the defects on the surface and subsurface of metal material, while the MFP effect in the fusion sensor can be used to help differentiate between the surface and subsurface defects in the material.

Eddy current array developments for remote deployment on the ITER project

The inspection of welds and components for nuclear applications can be more challenging due to extreme levels of radiation and temperature. A development project in collaboration with the UK Atomic Energy Authority (UKAEA) is being conducted to develop non-destructive testing (NDT) procedures for inspecting the remotely welded joints within the ITER facility. Initially, a feasibility study has been carried out to identify the most suitable non-destructive technique. Eddy current array (ECA), phased array ultrasonics and guided wave electromagnetic acoustic transducers (EMATs) were considered as candidate solutions and, following the feasibility study, ECA was selected to be taken forward for further development. Subsequently, dedicated high- and low-frequency array probes have been developed and tested to detect and size surface and buried defects employing encoder and calibration curves. The research has been extended to characterise defects such as tungsten inclusions using the phase angle. To avoid damage to the electronics in extreme environments, the ECA probe coils were separated from the excitation and data acquisition unit with 60 m coaxial coils to achieve a high signal-to-noise ratio. The results revealed that the ECA technique can successfully be deployed remotely for the detection and sizing of defects down to 3 mm in extreme environments.

Detection mechanism of delamination in thermal barrier coatings of turbine blade using a Rayleigh wave EMAT

Nondestructive testing (NDT) for damage in thermal barrier coatings (TBCs) is significant for the safety guarantee of gas turbine blades. As a new NDT technology, electromagnetic acoustic transducer (EMAT) is widely applied for NDT of conductive structural components due to its advantages of coupling-free and high adaptability. In this paper, numerical simulations are conducted to study the wave propagation and interaction with delamination defects in TBCs inspected with a Rayleigh wave EMAT of the Lorentz force mechanism. Based on the numerical results, the wave structure in TBC, wave conversion at delamination defect, time domain EMAT signals, and its B-scan images are evaluated and the feasibility of Rayleigh wave EMAT to inspect delamination in TBCs was theoretically clarified.

A new chevron electromagnetic acoustic transducer design for generating shear horizontal guided wave

Guided wave electromagnetic acoustic transducers (EMATs) have a significant role in non-destructive testing and structural health monitoring. It can generate horizontally polarized shear waves propagating axially or circumferentially to characterize the shape and orientation of pipeline defects. This work proposes a new Chevron pattern-like coil structure design with a periodic permanent magnet (PPM) EMAT configuration. This specific design of the EMAT coil can generate a bi-directional shear horizontal wave (SH-wave) and reduce the side lobes. An optimal Chevron angle of coil wires assists in generating orthogonal propagatingwaveforms. These phenomena lead to the constructive interference of propagating waves and develop a resulting wave along the horizontal direction. A 3D FEM modeling and simulation have been carried out and validated with experimental results. The proposed EMAT results are compared with the conventional racetrack PPM EMAT model, which shows a significant improvement over conventional EMATs. A prototype of this proposed EMAT has been developed. It can be used to inspect surface defects in applications such as fuel transportation pipelines.

A New Design to Rayleigh Wave EMAT Based on Spatial Pulse Compression.

The main disadvantage of the electromagnetic acoustic transducer (EMAT) is low energy-conversion efficiency and low signal-to-noise ratio (SNR). This problem can be improved by pulse compression technology in the time domain. In this paper, a new coil structure with unequal spacing was proposed for a Rayleigh wave EMAT (RW-EMAT) to replace the conventional meander line coil with equal spacing, which allows the signal to be compressed in the spatial domain. Linear and nonlinear wavelength modulations were analyzed to design the unequal spacing coil. Based on this, the performance of the new coil structure was analyzed by the autocorrelation function. Finite element simulation and experiments proved the feasibility of the spatial pulse compression coil. The experimental results show that the received signal amplitude is increased by 2.3~2.6 times, the signal with a width of 20 μs could be compressed into a δ-like pulse of less than 0.25 μs and the SNR is increased by 7.1-10.1 dB. These indicate that the proposed new RW-EMAT can effectively enhance the strength, time resolution and SNR of the received signal.

Debonding Detection in Aluminum/Rigid Polyurethane Foam Composite Plates Using A0 Mode LAMB Wave EMATs

Aluminum/rigid polyurethane foam composite plates (ARCPs) are widely used for thermal insulation. The interface debonding generated during manufacturing degrades the thermal insulation performance of an ARCP. In this study, the debonding of an ARCP, a composite plate with a porous and damped layer of rigid polyurethane foam (RPUF), was detected using A0 mode Lamb wave electromagnetic acoustic transducers (EMATs). The low energy transmission coefficient at the interface caused by the large acoustic impedance difference between aluminum and RPUF made the detection difficult. Based on these structural characteristics, an A0 mode Lamb wave with large out-of-plane displacement was used to detect the debonding. EMATs are preferred for generating A0 mode Lamb waves due to their advantages of being noncontact, not requiring a coupling agent, and providing convenient detection. A finite element simulation model considering the damping of the RPUF layer, the damping of the PU film at the interface, and the bonding stiffness of the interface was established. The simulation results indicated that the Lamb wave energy in the aluminum plate transmits into the RPUF layer in small amounts. However, the transmitted energy rapidly attenuated and was not reflected into the aluminum plate, as the RPUF layer was thick and highly damped. Therefore, energy attenuation was evident and could be used to characterize the debonding. An approximately linear relationship between the amplitude of the received signals and the debonding length was obtained. Experiments were performed on an ARCP using EMATs, and the experimental results were in good agreement with the simulation results.

Application of Pulse Compression Technique in High-Temperature Carbon Steel Forgings Crack Detection with Angled SV-Wave EMATs.

In order to solve the difficulty in localization and poor signal-to-noise ratio (SNR) of the angled shear vertical wave (SV wave) electromagnetic acoustic transducer (EMAT) in cracks detection of high-temperature carbon steel forgings, a finite element (FE) model of the angled SV wave EMAT detection process was established, and the influence of specimen temperature on the EMAT excitation, propagation, and reception processes was analyzed. A high-temperature resistant angled SV wave EMAT was designed to detect carbon steel from 20 °C to 500 °C, and the influence law of the angled SV wave at different temperatures was analyzed. Then a circuit-field coupled FE model of angled SV wave EMAT in the carbon steel detection process based on the Barker code pulse compression technique was established, and the effects of the Barker code element length, impedance matching method, and matching component parameters on the pulse compression effect were analyzed. In addition, the noise suppression effect and the SNR of the crack-reflected wave in the tone-burst excitation method and the Barker code pulse compression technique were compared. The results show that the amplitude of the block-corner reflected wave decreases from 556 mV to 195 mV, and the SNR decreases from 34.9 dB to 23.5 dB when the specimen temperature increases from 20 °C to 500 °C. When the temperature is 500 °C, the SNR of the crack-reflected wave obtained by the Barker code pulse compression technique can be improved by 9.2 dB compared to the tone-burst excitation method with 16 synchronous averages. The study can provide technical and theoretical guidance for online crack detection for high-temperature carbon steel forgings.

Design and Manufacture of an Optimised Side-Shifted PPM EMAT Array for Use in Mobile Robotic Localisation.

Guided wave Electro Magnetic Acoustic Transducers (EMATs) offer an elegant method for structural inspection and localisation relative to geometric features, such as welds. This paper presents a Lorentz force EMAT construction framework, where a numerical model has been developed for optimising Printed Circuit Board (PCB) coil parameters as well as a methodology for optimising magnet array parameters to a user's needs. This framework was validated experimentally to show its effectiveness through comparison to an industry built EMAT. The framework was then used to design and manufacture a Side-Shifted Unidirectional Periodic Permanent Magnet (PPM) EMAT for use on a mobile robotic system, which uses guided waves for ranging to build internal maps of a given subject, identifying welded sections, defects and other structural elements. The unidirectional transducer setup was shown to operate in simulation and was then manufactured to compare to the bidirectional transmitter and two-receiver configurations on a localisation system. The unidirectional setup was shown to have clear benefits over the bidirectional setup for mapping an unknown environment using guided waves as there were no dead spots of mapping where signal direction could not be interpreted. Additionally, overall package size was significantly reduced, which in turn allows more measurements to be taken within confined spaces and increases robotic crawler mobility.

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.

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.

Design method of unidirectional wideband SH guided wave phased array magnetostrictive patch transducer

The existing meander coil type SH guided wave electromagnetic acoustic transducer (EMAT) has a fixed center frequency and narrow bandwidth, which cannot quantify the defect size by continuous frequency scanning. Therefore, this study proposes an array of wideband unidirectional SH guided wave magnetostrictive patch transducer and its control method for excitation of unidirectional SH guided waves at random frequencies over a wide frequency range. A narrow magnetostrictive patch and a linear coil are used for each array element to increase the bandwidth of a single element. By controlling the excitation delay and initial phase of each array element separately, the direction of the SH guided wave can be controlled, and its wavelength can be changed without replacing the coil. In addition, a mathematical model of the array sources of finite width is developed to excite unidirectional SH guided waves at different frequencies. Based on this model, the effects of the number, width and distance of the array element on the frequency response characteristics of the transducer array are analyzed. The experiments show that: The proposed transducer array can excite unidirectional SH guided waves of random frequency over a wide frequency range. The amplitude of the guided wave signal on the suppressed side is almost 0, while the signal on the non-suppressed side is significantly increased, leading to a significant improvement in the energy conversion efficiency. The frequency range can be selected by designing the width and distance of array elements, and the experimentally measured frequency response characteristics of the transducer array are highly consistent with the trend of the mathematical model results.

Microcrack Defect Quantification Using a Focusing High-Order SH Guided Wave EMAT: The Physics-Informed Deep Neural Network GuwNet

It is challenging to apply deep learning in professional fields that lack big data support, especially in industrial structure health assessments using ultrasonic guided wave nondestructive testing (NDT) method. To solve this problem, one feasible solution is to introduce the concept of NDT physics into a deep neural network to compensate for the network's poor predictive abilities when trained on small datasets. Therefore, we propose a physics-informed deep neural network, named GuwNet, based on a unidirectional oblique-focusing (UOF) high-frequency, high-order shear horizontal guided wave electromagnetic acoustic transducer (EMAT) to quantify microcrack defects more accurately. First, the designed focused-transmission omnidirectional-reception UOF-EMAT can produce pure high-frequency, high-order guided waves. Through a circumferential arrangement of multiple receiving transducers, the maximum amount of information can be obtained regarding the reflected waves of the defect. This method solves the inherent problems of an EMAT (i.e., low energy conversion efficiency) and achieves effective detection of microcrack defects. Second, we study the quantification principle of microcrack defects suitable for UOF-EMAT, and propose a deep neural network using physical knowledge regarding this theory. We rationally design the network structure based on the quantitative principles and logic obtained from this article. In addition, feedback and feedforward loss functions suitable for evaluating different forms of variables are proposed to integrate the physical concepts of ultrasonic guided wave testing into the neural network training. Finally, we verify the performance of the proposed GuwNet based on the UOF-EMAT. Compared with traditional nonphysics-informed methods, the length, depth, and direction of the quantification errors are reduced to 0.127 mm, 0.279% <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d_t</i> , and 1.843°, respectively, and the average quantification error is reduced by more than 80%.

Application of pulse compression technique in metal materials cracks detection with LF-EMATs

ABSTRACT To address the problem of low signal-to-noise ratio (SNR) and poor spatial resolution of cracks detection of the angled SV wave electromagnetic acoustic transducer (EMAT), pulse compression technique (PCT) and line-focusing technique are introduced into the angled SV wave EMAT. A finite element (FE) model of the line-focusing (LF) EMAT detection process based on the Barker code excitation was established, and the effects of the design parameters of magnet and meander-line coil (MLC) on the main lobe amplitude and width of the defect echo after pulse compression were analysed. The optimal combination of LF-EMAT design parameters and excitation parameters was obtained, and it was compared with the traditional angled SV wave EMAT and the tone-burst excitation method by experimental measurements. The results shows that compared with angled SV wave EMAT with tone-burst excitation, LF-EMAT can improve the SNR by 14.9 dB with no synchronous average and be processed by PCT. The SNR from LF-EMAT with PCT is10 dB at 1.5 mm lift-off, while the SNR of the angled SV wave EMAT with tone-burst excitation is less than 0 dB at 1 mm lift-off. When the Barker code PCT is applied to the LF-EMAT detection, we can achieve the purpose of increasing EMAT lift-off and reducing the synchronous averages to meet the demand of online, rapid, and high-temperature detection.

Investigating the Influence of Design Frequency on the Ultrasonic Signal of PFSV-EMATs

The point-focusing shear vertical wave electromagnetic acoustic transducer (PFSV-EMAT) with a concentric meander line (CML) coil has been developed to detect and size micro defection concealed within a certain depth of materials. It is proved to be sensitive to the design frequency, which is associated with the spacing of CML coil. In this work, the relationship between the design frequency and characteristic of PFSV-EMATs, such as the particle displacement and focal offset, is studied quantitatively. First, the configuration and working principle of PFSV-EMATs are introduced comprehensively. Then, a finite element model is established to simulate the incentive and propagation process of ultrasonic waves. In addition, the relationship between magnet-to-coil radius ratio and the signal amplitude generated by the PFSV-EMATs is explored to ensure the consistency of transducer parameters at different frequencies. Furthermore, under the optimal magnet-to-coil radius ratio, the influence of design frequency on the characteristic of PFSV-EMATs is studied. Finally, the experimental results show good agreement with the simulation results and indicate that the signal amplitude and point-focusing performance of PFSV-EMATs employing CML coils could be improved by carefully choosing the design frequency.

Optimization Design of Surface Wave Electromagnetic Acoustic Transducers Based on Simulation Analysis and Orthogonal Test Method

The energy conversion of electromagnetic acoustic transducers (EMATs) is typically lower, which seriously restricts the application of EMATs in the field of non-destructive testing and evaluation. In this work, parameters of surface wave EMATs, including structural parameters and electrical parameters, are investigated using the orthogonal test method to improve the transducer’s energy conversion efficiency. Based on the established finite element 2-D model of EMATs, the amplitude of the displacement components at the observation point of a plate is the optimization objective to be maximized with five parameters pertaining to the magnets, meander-line coils, and excitation signal as design variables. Results show that the signal amplitude of EMATs is 3.48 times on in-plane and 3.49 times on out-of-plane, respectively, compared with the original model. Furthermore, a new material (amorphous nanocrystalline material of type 1K107) is applied to optimize the magnetic circuit of EMATs and enhance the eddy current in an aluminum plate to increase the signal amplitude. Finally, the signal amplitudes obtained from the three types of models, that is, the original one, the optimization one after an orthogonal test, and the optimization one with the addition of magnetic concentrators, are analyzed and compared, indicating that the signal amplitude, compared with the original one, is 6.02 times on in-plane and 6.20 times on out-of-plane, respectively.

The Change of Non-linear Ultrasonic Characterizations in Ti-Al Alloy during Creep

We investigated the evolutions of two nonlinear acoustic characterizations: resonant frequency shift and high harmonic components, with electromagnetic acoustic resonance (EMAR) throughout the creep life in the advanced heat-resistant Ti-Al alloy, Ti-43Al-5V-4Nb. EMAR is a contactless resonant method with an electromagnetic acoustic transducer (EMAT). This method enables not only to measure exact ultrasonic attenuation of measured sample but also to eliminate nonlinear acoustic effect between the sample and transducer. We used the axial SH wave EMAT, which transmits and receives axially polarized shear wave along a cylindrical surface of a circular rod for non-ferromagnetic materials. Two nonlinear ultrasonic properties and attenuation coefficients increases significantly at the beginning of creep life and then gradually decreases to rupture.