Shear Wave Electromagnetic Acoustic Transducer Research Articles

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.

High-temperature EMAT with double-coil configuration generates shear and longitudinal wave modes in paramagnetic steel

The non-contact nature of electromagnetic acoustic transducers (EMATs) allows a continuous operation at high temperatures without physical coupling. The existing high-temperature EMATs are mainly shear-wave EMATs, and there are few reports on shear-longitudinal wave or longitudinal-wave EMATs due to the low intensity of the horizontal magnetic field. However, a desirable EMAT design characteristic is the possibility of selecting to generate different acoustic wave modes for various engineering applications. In this paper, a high-temperature EMAT with a double-coil configuration on waveform generation in paramagnetic steel was studied. By adjusting the configuration relationship between the electromagnetic coil and the EMAT eddy-current coil, the selective generation of shear-wave, longitudinal-wave, and shear-longitudinal wave modes was realized. According to quantitative analysis of shear and longitudinal waves generated by the double-coil EMAT, the amplitude ratio of shear and longitudinal waves was about 1 when the diameter of the EMAT eddy-current coil was equal to the inner diameter of the electromagnetic coil. Furthermore, shear-wave mode and shear-longitudinal wave mode high-temperature EMATs were designed and fabricated. The EMAT was placed in a high-temperature environment to continuously measure the paramagnetic steel SUS304. The amplitudes of the shear wave and longitudinal wave at 500 °C decreased by 10.2 times and 3.8 times, respectively, compared with that at 25 °C. The designed EMAT can selectively generate and receive different bulk acoustic wave modes at high temperatures.

Uniaxial stress in-situ measurement using EMAT shear and longitudinal waves: Transducer design and experiments

As a typical stressed state, uniaxial stress widely exists in the components, such as bolt stress, which requires to be tested in-situ regularly. In this article, the uniaxial stress in-situ measurement using the electromagnetic acoustic transducer (EMAT) shear and longitudinal waves is proposed. To improve the EMATs efficiency and meet the measurement resolution constraints based on Lorentz mechanism, the EMAT parameters are optimized through numerical analysis, including the coil size, the permanent magnet size and the distance between magnet and coil. The relationship between the propagation velocity and the stress is analyzed, and a joint model of shear and longitudinal waves for measuring the uniaxial stress is established. Finally, the pre-tightened 7075 aluminum alloy bolt is employed to verify the validity of the designed shear wave EMAT and longitudinal wave EMAT.

Application of chirp pulse compression technique to a high-temperature EMAT with a large lift-off

The interest in applying non-destructive testing to the examination of the flaw and measurement of the thickness in the metals at high temperatures is increasing as the metal processing becomes continuous. Based on this background, we designed a water-cooled shear wave Electromagnetic Acoustic Transducer (EMAT) with a surface cooling technique, and the conversion efficiency could be improved by surface cooling. We used the chirp pulse compression technique to process the raw ultrasonic wave signal without synchronous average. Besides, a large casting slab with coarse grains and a massive forging with high temperatures were chosen as the testing objects. Then, the effect of the lift-off distance on the Signal-to-Noise Ratio (SNR) of ultrasonic signals from the EMATs operated in both the pulse-echo mode and pitch-catch mode was investigated. Subsequently, the ultrasonic signals were de-noised respectively by synchronous averages and pulse compression, and the noise reduction ability was compared. Finally, the EMAT was tested with the forging at different temperatures from 92 °C to 730 °C. The performance of the EMAT with the excitation of a tone-burst signal was also compared with the performance of the EMAT using pulse compression technique. Results show that the lift-off, SNR, range resolution, and inspection efficiency can be largely enhanced with the signal processing of chirp pulse compression to the ultrasonic wave signal with no synchronous averaging. When the forging’s temperature is 730 °C, the SNR of the compressed signal is comparable to the SNR of the signal processed with 64 synchronous averages, and the wave-packet duration can be reduced by 80.5%. The proposed EMAT detection technique is particularly suited for real-time measurement and high-temperature testing.

Creep-induced nonlinear ultrasonic change in an austenitic stainless steel

We investigated the evolutions of two nonlinear acoustic characterizations: resonant frequency shift and three-wave mixing, with electromagnetic acoustic resonance (EMAR) throughout the creep life in an austenitic stainless steel, JIS-SUS 304. EMAR was a combination of the resonant acoustic technique with a non-contact electromagnetic acoustic transducer (EMAT). We used bulk- shear-wave EMAT, which transmits and receives shear wave propagating in thickness direction of a plate specimen. Creep tests with plate specimens were carried out at 973 K, and 100 MPa and interrupted at several time steps. Two nonlinear acoustic parameters showed peas at 40% of creep life. After that they deceased, they increased from 60% of creep life to rupture. We interpreted these phenomena in terms of dislocation recovery, recrystallization, and restructuring and the initiation and growth of creep void, with support from the SEM and TEM observation.

Optimization of the Bias Magnetic Field of Shear Wave EMATs

The main advantage of electromagnetic acoustic transducers (EMATs) over piezoelectric transducers is that no direct contact with the specimen under test is required. Therefore, EMATs can be used to test through coating layers. However, they produce weaker signals, and hence, their design has to be optimized. This paper focuses on the design of a Lorentz force shear wave EMAT and its application in thickness gaging; special emphasis is placed on the optimization of the design elements that correspond to the bias magnetic field of the EMAT. A configuration that consists of several magnets axisymmetrically arranged around a ferromagnetic core with like poles facing the core was found to give the best results. By using this configuration, magnetic flux densities in excess of 3 T were obtained in the surface of a specimen; the maximum value achieved by a single magnet under similar conditions is roughly 1.2 T. If the diameter of an EMAT ultrasonic aperture is 10 mm, the proposed configuration produces signals roughly 20 dB greater than a single magnet, while for a given overall EMAT volume, signals were greater than 3–6 dB. Linear and radial shear wave polarizations were also compared; a higher mode purity and signal intensity were obtained with the linear polarization.

금속 재료의 잠닉손상 평가를 위한 비선형 전자기음향공진 기법에 관한 연구

최근 전통적 초음파기법이 탐상할 수 없는 잠닉손상을 평가할 수 있는 잠재적 기술로서 비선형 초음파기법들에 대한 많은 관심이 있다. 비선형 초음파기법중 하나인 공진주파수 변화를 이용하는 기법은 재료의 탄성영역에서의 이력에 근거한 기술이다. 공진주파수의 변화량이 아주 작기 때문에 정교한 공진주파수 측정장치가 필요하다. 본 연구에서는 비선형 전자기음향공진기법을 적용하였다. 비선형 전자기음향공진기법은 비접촉 EMAT 센서를 사용하기 때문에 재료의 주파수 응답에 영향을 최소화할 수 있다. 3점 굽힘 피로시험을 한 알루미늄판 시편에 횡파 EMAT으로 실험을 실시하였다. 전압을 여러 레벨로 인가하며 공진을 발생시켜 잠닉손상측정에 중요한 요인중 하나인 이력 비선형 파라미터 <TEX>${\alpha}$</TEX>를 공진주파수 변화로부터 산출하였다. 비손상시편과 손상시편에서의 측정된 이력 비선형 인자의 값이 서로 차이가 남을 확인하였다. Recently, much attention has been paid to nonlinear ultrasonic technology as a potential tool to assess hidden damages that cannot be detected by conventional ultrasonic testing. One nonlinear ultrasonic technique is measurement of the resonance frequency shift, which is based on the hysteresis of the material elasticity. Sophisticated measurement of resonance frequency is required, because the change in resonance frequency is usually quite small. In this investigation, the nonlinear electromagnetic acoustic resonance (NEMAR) method was employed. The NEMAR method uses noncontact electromagnetic acoustic transducers (EMATs) in order to minimize the effect of the transducer on the frequency response of the object. Aluminum plate specimens that underwent three point bending fatigue were tested with a shear wave EMAT. The hysteretic nonlinear parameter <TEX>${\alpha}$</TEX>, a key indicator of damage, was calculated from the resonance frequency shift at several levels of input voltage. The hysteretic nonlinear parameter of a damaged sample was compared to that of an intact one, showing a difference in the values.

J041022 非線形超音波法による9Cr鋼溶接継手材のクリープ損傷評価の研究([J041-02]超音波計測・解析法の新展開(2))

In this study, we monitored the change of acoustic nonlinearity with electromagnetic acoustic resonance (EMAR) during creep damage in 9Cr steel welded joint, under lOOMPa, 923K. Resonant frequency shift by nonlinear ultrasound spectroscopy (NRUS) and second and third harmonics were measured as the nonlinear acoustic parameters. These were resonance-base techniques exploiting the significant nonlinear behavior of damaged materials. EMAR was combination to electromagnetic acoustic transducer (EMAT) with resonant acoustic technique. We used axial shear wave EMAT to monitor NRUS and harmonic amplitude of SH wave propagating in the circumferential direction of sample surface. Axial shear wave EMAT was operated with the magnetic strain effect. Furthermore, use of EMAT makes non-contact transduction possibility. NRUS detected damage by using resonant frequency shift. In measurement of resonant frequency shift, as the excitation level increase, resonant frequency was shifted and manifested the elastic nonlinearity. Measurement of harmonics with EMAR, drive of EMAT at half or one third the resonance frequency, the standing wave generate the second or third-harmonics by nonlinear effect. In addition, ultrasonic attenuation measured simultaneously. Measured nonlinear acoustic parameter showed the peak at 25% of creep life. We assumed that these trends were resulted from the increase of void density and the change of dislocation structure.

Experimental and numerical evaluation of electromagnetic acoustic transducer performance on steel materials

Electromagnetic Acoustic Transducers (EMATs) are an attractive alternative to standard piezoelectric probes in a number of applications thanks to their contactless nature. EMATs do not require any couplant liquid and are able to generate a wide range of wave-modes; however these positive features are partly counterbalanced by a relatively low signal-to-noise ratio and by the dependence of EMAT performance on the material properties of the test object. A wide variety of steel materials is employed in many industrial applications, so it is important to assess the material-dependent behaviour of EMATs when used in the inspection of different types of steel. Experimental data showing the performance of bulk shear wave EMATs on a wide range of steels is presented, showing the typical range of physical properties encountered in practice. A previously validated Finite Element model, including the main transduction mechanisms, the Lorentz force and magnetostriction, is used to evaluate the experimental data. The main conclusion is that the Lorentz force is the dominant transduction effect, regardless of the magnitude and direction of the bias magnetic field. Differently from magnetostriction, the Lorentz force is not significantly sensitive to the typical range of physical properties of steels, as a consequence the same EMAT sensor can be used on different grades of ferritic steel.

Analysis on the relationship between EMAT transmitting efficiency and the electrical conductivity of the coating on material surface

The conductive or ferromagnetic material with coating layer can be tested by electromagnetic acoustic transducer (EMAT) without pre-treatment. The electro-acoustic converting occurred on the surface of the specimens is mainly concentrated within the skin depth of the eddy current. Thus, the electromagnetic property of the material surface has great influence on the efficiency of EMAT. The eddy current densities induced in the coating of specimens and Lorentz force are calculated with fi nite element software ANSYS when EMAT is applied to the 20 steel-based specimens coated with different electrical conductivity materials. With shear wave EMAT, the specimens are tested and the relationship between the amplitude of the first echo wav e signal and electrical conductivity of the coating is analys ed. The theoretical analysis and experimental results show that under the fixed excitation condition, the higher the electrical co nductivity of the coating material, the larger the induced e ddy current density and Lorentz force are; the increase of the electrica l conductivity of the coating is beneficial to the electromag netic acoustic excitation and the acquisition of higher voltage a mplitudes of the echo waves signals.

The measurement of shear and compression waves in curing epoxy adhesives using ultrasonic reflection and transmission techniques simultaneously

Thin epoxy resin adhesive samples were ultrasonically measured during cure using normal incidence radially polarized shear wave electromagnetic acoustic transducers (EMATs). The EMATs used generated predominantly SH shear waves but they also generated/detected compression waves allowing the simultaneous measurement of shear and compression wave propagation through a curing epoxy in a non-contact regime. The source of the compression wave generation has been determined to be the in-plane shear Lorentz force. The adhesive thickness examined in the experiments was approximately 1 mm, which was optimal for experimental measurement using our apparatus: it temporally separated the ultrasonic waves of interest from others present in the pulse whilst remaining thin enough to be representative of a realistic adhesive layer. The rapid cure and standard or ‘longer’ cure epoxy adhesives described in this paper were supplied in a two-part cartridge form. The experiments show that there is a fundamental difference between the way that the elastic moduli develop in the rapid cure and the longer cure time epoxies. The rapid cure epoxy initially developed a shear elastic modulus at a fast rate, which suddenly decreased at approximately the same time that the temperature of the adhesive reached its maximum value during the exothermic reaction. The reflection measurements show that there was significant coupling of the shear wave into the adhesive even in its initial liquid-like state. The ultrasonic shear and compression wave velocities of the epoxy were calculated from the very outset of the cross-linking reaction using the reflection technique. A transmitted compression wave was visible from the outset of cure, and once a transmitted shear wave was detectable it was possible to measure attenuation for both; however, these data are not presented here as they are not key to the paper and they can be found in our previous publications that are referenced in this one.

The development of shear and compression elastic moduli in curing epoxy adhesives measured using non-contact ultrasonic transducers

Thin epoxy resin adhesive samples were ultrasonically measured during cure using normal incidence radially polarized shear wave electromagnetic acoustic transducers (EMATs). Although the EMATs used predominantly generated shear waves (SH) they were designed so that they also generated/detected compression waves allowing the simultaneous measurement of shear and compression wave propagation through a curing polymer in a non-contact regime. The adhesive thickness examined in the experiments was approximately 1 mm, which was optimal for experimental measurement using our apparatus. The epoxy resin systems (rapid cure and a standard cure) described in this paper were supplied in a two-part cartridge form, mixed by injection through a mixing nozzle. Five different samples have been investigated at this stage, and there appears to be a fundamental difference in the way that the elastic moduli develop in the rapid cure and longer cure time epoxies. This can possibly be explained in terms of the reaction kinetics and the development of the structure on a microscopic scale. The rapid cure systems initially develop a shear elastic modulus at a faster rate which suddenly decreases at approximately the same time that the temperature of the adhesive reaches its maximum value during the exothermic reaction. This is consistent with an initial rapid reaction rate which is then greatly reduced as the remaining un-reacted polymer chains require a finite time to move into a position where they can react and join the network.

Considerations for the ultrasonic inspection of metal-adhesive bonds using EMATs

This paper describes some results on the ultrasonic analysis of adhesively bonded aluminium plates using normal incidence radially polarised shear wave ElectroMagnetic Acoustic Transducers (EMATs). The EMAT system presented here has undergone significant development work and improvements compared to previously published examples.(9,19) The electronic ‘deadtime’ of the system has been reduced to approximately 1 microsecond and the frequency range over which measurements can be taken has been extended. The capability of the technique to detect gross and subtle defects is demonstrated.

Ultrasonic diffraction from a transducer with arbitrary geometry and strength distribution

The exact solution to the Helmholtz equation with a Dirichlet boundary condition is obtained to study three-dimensional ultrasonic diffraction phenomena and derive the numerical data of amplitude loss and the phase shift for correcting induced errors. Calculation is made for near-field diffraction, for the rectangular transducers, and for the transducers with strength distribution on the radiating area. In the near field, where the wavelength and the propagation distance are comparable with each other, the longitudinal and shear waves undergo different diffraction. For transducers having a noncircular shape and a strength distribution on the area, both the amplitude loss and the phase shift experience different tendencies from the classical work on the circular piston source. Use of diffraction data specific to each measurement condition is then necessary to correct the errors. The calculated results are verified for pulse-echo measurements using a shear-wave electromagnetic acoustic transducer.

Ultrasonic attenuation and grain-size evaluation using electromagnetic acoustic resonance

Electromagnetic acoustic resonance is applied for determining the frequency dependence of the ultrasonic attenuation and the average grain size of low-carbon steels. Use of a noncontacting electromagnetic acoustic transducer (EMAT) makes it possible to isolate the attenuation within the plate specimens. The method relies on the Lorentz force mechanism to couple the EMAT to the specimen surfaces and then eliminates the other losses, which may otherwise occur with the contacting piezoelectric transducers. The measurement is independent of the EMAT used, the specimen thickness, the surface condition, the lift-off, etc., and is stable because of the noncontacting nature. First, the resonant frequencies are measured, to the accuracy of 10 Hz, by sweeping the operating frequency and obtaining the amplitude spectrum over a band in the 0.5- 20-MHz range. The ringing signals are excited and received by a shear wave EMAT and then processed with a superheterodyne receiver. Second, the attenuation coefficient as a function of the resonant frequency is determined. At each resonant frequency, the output signal rings down exponentially with time and the attenuation coefficient is obtained from the time constant by fitting an exponential decay to them. After correcting for the diffraction effect, the average grain size is obtained from the fourth-power term in the frequency dependence. The final results are favorably compared with the average of the three-dimensional grain-size distribution of steels.

Directional characteristics of electromagnetic acoustic transducers : Ultrasonics, vol.26, no.1, pp. 5–13 (Jan. 1988)

Abstract The directivity patterns of a wide range of electromagnetic acoustic transducers (EMATs) have been determined experimentally using a second, spiral coil EMAT as the detector around a cylindrical surface. A qualitative comparison to theory is presented, together with a discussion of the generation mechanisms involved. In addition, the use of a dipolar shear wave EMAT detector with a pulsed laser source is presented and some applications are discussed.

Directional characteristics of electromagnetic acoustic transducers

The directivity patterns of a wide range of electromagnetic acoustic transducers (EMATs) have been determined experimentally using a second, spiral coil EMAT as the detector around a cylindrical surface. A qualitative comparison to theory is presented, together with a discussion of the generation mechanisms involved. In addition, the use of a dipolar shear wave EMAT detector with a pulsed laser source is presented and some applications are discussed.