Non-contact Electromagnetic Acoustic Transducers Research Articles

Birefringence Technique for Evaluating Thermal Stresses in Railroad Rails

This paper discusses the development of an in situ noncontact electromagnetic acoustic transducer (EMAT) nondestructive evaluation technology to determine rail neutral temperature and estimate rail stress in continuous welded rail (CWR). Stresses develop in CWR due to a lack of expansion joints to accommodate thermal expansion and contraction of the rail when ambient temperatures vary over time. The novelty of the work presented is the usage of ultrasonic birefringence properties using EMATs to estimate thermally induced stresses in rails. EMATs produce polarized shear waves propagating through the rail web in the pulse-echo mode. Experimental tests were performed on machined 136RE and 141RE rail material with applied compressive and tensile stresses to explore the stress-birefringence behavior. Two additional sets of experimental tests were conducted on full-size rail sections with in situ surface conditions to study variations in the in situ birefringence and the acoustic stress constant in different rail materials including 115RE rail, 119RE rail, two different 136RE rails, and 141RE rail. The results show a highly linear relationship between the stresses applied and the measured acoustic birefringence.

Detection of kissing bond type defects and evaluation of the bonding quality in metal/adhesive/composite structures by a wavenumber-frequency insensitive SH mode

This work focuses on the non-destructive evaluation of the bonding quality in different metal/adhesive/composite samples and on the detection of defects simulating a kissing bond, using Shear Horizontal (SH) guided waves. A predictive 2D numerical model, called interphases model and based on finite elements method describing the interface substrate/adhesive by a very thin layer, is developed. Various samples with different adhesion levels obtained by modifying the cohesive and adhesive aspects or including a defect representing a kissing bond are investigated. An experimental study is performed with non-contact ElectroMagnetic Acoustic Transducers (EMAT) in excitation and reception. As the wavenumber-frequency sensitive SH modes to the adhesion quality shown by the numerical predictions are not easy to generate experimentally, the analysis of the behaviour of the SH modes is based here on their amplitude. Results show that it is possible to differentiate between the adhesion levels due to different epoxy conversion, the presence or absence of an adhesion promoter or a pollution by a release agent, as well as the detection of the defect.

Creep-induced Nonlinear Ultrasonic Change in Nickel-based Super alloy, Inconel 718.

We investigated the evolutions of nonlinear acoustic characterization: three-wave mixing, with electromagnetic acoustic resonance (EMAR) throughout the creep life in Nickel-based superalloy, Inconel 718, which has high temperature strength effected by γ″(Ni3Nb) phase at approximately 973 K and is determined that the γ″phase disappearance lead to the creep rupture. EMAR was a combination of the resonant acoustic technique with a non-contact electromagnetic acoustic transducer (EMAT).

Unidirectional emission and detection of Lamb waves based on a powerful and compact coils-only EMAT

This study investigated a compact noncontact electromagnetic acoustic transducer (EMAT) without bulky magnets or separate electromagnets for unidirectional transmission and reception of ultrasonic Lamb waves in 1 mm thick steel plates. The transducer head consists of only two, closely adjacent and elongated flat coils. The powerful sensor technology bridged air gaps up to 1.5 mm. Two phase-adjusted 1 MHz pulses at 1 kV amplitude enhanced the sideways-projected ultrasound field in one direction; others were attenuated by destructive interference. A 5 mm ultrasound wavelength with 1 mm thick steel plate was used for testing; wave field calculations were in good agreement with results. As a possible application, an inaccessible edge of a steel plate was scanned, where the difficulty was increased by an additional edge at a similar distance representing competing interfering reflections. More general progress beyond this specific non-contact application is the demonstration of phase-accurate ultrasound transmission and sensitive echo reception with a compact system of mutually coupling inductors at high power and continuous high-bias currents (1 kA), allowing compact and powerful EMAT phased arrays based on single-layer densely arranged coils.

Examination of the Liquid Volume Inside Metal Tanks Using Noncontact EMATs From Outside.

This article discusses ultrasound echo examination of the liquid volume inside a metal enclosure using noncontact electromagnetic acoustic transducers (EMATs) from outside the tank. Because only longitudinal sound waves exist in liquids, a novel and powerful EMAT design (kA currents in small coils) with a specifically enhanced transduction of longitudinal ultrasound is presented. Different wall materials (aluminum, steel, and stainless steel) and working frequencies are considered. A particular challenge, in addition to the already weak EMAT signals across an air gap, is the highly reflective interface between the metal wall and liquid because the acoustic impedances of the two media differ considerably. With thicker metal walls (here, 3mm), only either low frequencies < 100 kHz or frequencies close to the corresponding thickness resonance of the wall (here, about 1 MHz) are more suitable. The higher frequencies are preferred, as they show advantages due to a directed beam profile, shorter wavelengths in the liquid, and an overall better pulse fidelity. The simultaneous operation of several and closely neighboring EMATs demonstrates the feasibility of more demanding detection tasks, ultimately leading toward a 3-D localization inside the liquid using a noncontact EMAT array with eight independent elements.

Device and Methods for Measuring of Acoustic Anisotropy and the Residual Stress in the Main Gas Pipelines’ Metal

One of the main conditions of safe operation of gas pipelines is the use of non-destructive diagnostic methods. Particularly important problem is the earlier operational diagnosis of pipes’ material of main gas pipelines based on the evaluation of the stress-strain state, elastic moduli and properties anisotropy by acoustic methods. The aim of the work is to develop methods for acoustic assessment of the stress-strain state, the elastic moduli and the properties anisotropy of pipeline material and to study these characteristics in different sections of main pipelines using a device based on contactlessEMAtransducers.Methods are implemented using specialized equipment (the structurescopeSEMA) and non-contact electromagnetic acoustic transducers. As an object of research, we used fragments – the cuttings of gas pipelines with circumferential welds both after fabrication and removed from service with stress corrosion cracking, including ones with corrosion damage and without visible damage.The method of determining the plane stress-strain state of pipeline elements is based on the phenomenon of acoustoelasticity – the dependence of the propagation velocity of ultrasonic waves on mechanical stresses. The method for determining the elastic characteristics of materials and the anisotropy of their properties is based on the relationship between the velocities of ultrasonic waves and the elastic properties of the medium. Both techniques are implemented by sounding the sample in one section using a longitudinal wave and two shear waves with mutually perpendicular polarization planes coinciding with the main stresses, and measuring their propagation times.It is shown that the stress state distribution both in the circumferential direction and along the generatrix is uneven which is caused by the peculiarities of samples (presence of a welded joint, stress corrosion cracking, long-term operation). The smallest acoustic anisotropy is observed for the Young's modulus. The anisotropy of the properties of most samples is in the range of 12–14 % for shear modulus, 9–10 % for Young’s modulus, 13–15 % for Poisson’s ratio. For samples with stress corrosion cracking a sharp decrease in the anisotropy coefficient is observed which makes it possible to use the indicated characteristics as informative parameters in detecting stress corrosion cracking.A feature of the proposed methods is high accuracy, due to the absence of necessity to determine the material density and precision measurement of its thickness, the measurement error of which is significant by known methods.Keywords:main gas pipeline, elastic moduli, acoustic anisotropy of properties, the biaxial stress-strain state, longitudinal and transversal waves.

Development of Non-contacting Electromagnetic acoustic transducer for Nonlinear three wave interaction method

Development of Non-contacting Electromagnetic acoustic transducer for Nonlinear three wave interaction method

Toward Robotic Inspection of Dry Storage Casks for Spent Nuclear Fuel

Extended dry storage of spent nuclear fuel makes it desirable to assess the structural integrity of the storage canisters. Stress corrosion cracking of the stainless steel canister is a potential degradation mode especially in marine environments. Sensing technologies are being developed with the aim of detecting the presence of chloride-bearing salts on the surface of the canister as well as whether cracks exist. Laser-induced breakdown spectroscopy (LIBS) methods for the detection of Chlorine are presented. In addition, ultrasonic-guided wave detection of crack-like notches oriented either parallel or perpendicular to the shear horizontal wave vector is demonstrated using the pulse-echo mode, which greatly simplifies the robotic delivery of the noncontact electromagnetic acoustic transducers (EMATs). Robotic delivery of both EMATs and the LIBS system is necessary due to the high temperature and radiation environment inside the cask where the measurements need to be made. Furthermore, the space to make the measurements is very constrained and maneuverability is confined by the geometry of the storage cask. In fact, a large portion of the canister surface is inaccessible due to the presence of guide channels on the inside of the cask's overpack, which is strong motivation for using guided waves for crack detection. Among the design requirements for the robotic system are to localize and track where sensor measurements are made to enable return to those locations, to avoid wedging or jamming of the robot, and to tolerate high temperatures and radiation levels.

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.

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.

Investigation of Methods for Data Communication and Power Delivery Through Metals

When a sensor is located within a metallic housing, conveying data back to a controller or data logger is not trivial. To maintain structural integrity, or simply for ease of installation, it is often not desirable to break the wall of the vessel, preventing any hard-wired solution. The conducting nature of the structure prevents the effective use of radio communications due to skin effect. Applications involving sealed containers also have a requirement for power delivery, as the periodic changing of batteries is impractical. In this paper, a number of systems are presented for through-metal communications and power delivery. A novel noncontact electromagnetic acoustic transducer communication system is demonstrated, which is capable of data rates in excess of 1 Mb/s. Furthermore, a power transfer efficiency of approximately 4% is shown to be achievable through 20-mm-thick stainless steel using inductive coupling. These novel solutions are critically compared to previous systems based on piezoelectric transducers.

Flexural vibrations of a rectangular plate for the lower normal modes

Theoretical and experimental results for flexural waves of a rectangular plate with free ends are obtained. Both the natural frequencies and mode shapes are analyzed for the lower normal modes. To take into account the boundary conditions, a plane wave expansion method is used to solve the thin plate theory also known as the 2D Kirchhoff–Love equation. The excitation and detection of the normal modes of the out-of-plane waves are performed using non-contact electromagnetic-acoustic transducers. We conclude that this experimental technique is highly reliable due to the good agreement between theory and experiment.

Microstructural and Non-Destructive Evaluation of Creep Damage in Martensitic Stainless Steel

We studied the evolution of microstructure in a Martensitic Stainless Steel (JIS-SUS403) during creep by monitoring ultrasonic attenuation. After a series of creep samples with various strains under a tensile stress of 120MPa at 873K was obtained, small samples were removed from the creep samples and free vibration resonance frequencies and attenuation coefficients were measured with electromagnetic acoustic resonance (EMAR). EMAR is a combination of the resonant acoustic technique with a non-contact electromagnetic acoustic transducer (EMAT). We measured changes in ultrasonic attenuation of the small creep sample by free vibration resonance with EMAR through a magnetostrictive mechanism. The attenuation measurement is inherently free from any energy loss, resulting in pure attenuation in a metal sample. Furthermore, the evolution of microstructure was observed with electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The result from the small samples showed the same trend as our previous result from larger sample. We propose a non-destructive method using EMAR to evaluate creep damage in small specimens sampled from structural metals in-service.

Creep-Induced Microstructural Changes and Acoustic Characterization in a Cr–Mo–V Steel

We studied the evolution of microstructure in a Cr–Mo–V steel (JIS-SNB16) during creep by monitoring ultrasonic attenuation. After obtaining a series of creep samples with various strains under a tensile stress of 25 MPa at 923 K, we removed small samples from the creep samples and measured free vibration resonance frequencies and attenuation coefficients with electromagnetic acoustic resonance (EMAR). EMAR is a combination of the resonant acoustic technique with a non-contact electromagnetic acoustic transducer (EMAT). The attenuation measurement is inherently free from any energy loss, resulting in pure attenuation in a metal sample. Furthermore, we observed the evolution of microstructure with electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The result from the small samples shows the same trend as our previous result from larger sample. We propose a non-destructive method using EMAR to evaluate creep damage in small specimens sampled from structural metals in-service.

1502 クリープ損傷を受けたCr-Mo-V鋼のミクロ損傷解析と内部摩擦の変化(S10-1 非破壊評価とモニタリングI,21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

We studied the evolution of microstructure in a Cr-Mo-V steel (JIS-SNB16) during creep by monitoring internal friction. After obtaining a series of creep samples with various strains under a tensile stress of 25 and 35 MPa at 923K, we removed small samples from the creep samples and measured free vibration resonance frequencies and internal friction with electromagnetic acoustic resonance (EMAR). EMAR is a combination of the resonant acoustic technique with a non-contact electromagnetic acoustic transducer (EMAT). The measurement of internal friction is inherently free from any energy loss, resulting in pure attenuation in a metal sample. Furthermore, we observed the evolution of microstructure with electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The result from the small samples shows the same trend as our previous result from larger sample. We propose a non-destructive method using EMAR to evaluate creep damage in small specimens sampled from structural metals in-service.

Ultrasonic Lamb wave tomography using scanned EMATs and wavelet processing

A continuous wavelet transform has been used to analyse multi-mode Lamb wave propagation. This was applied to experimental data from non-contact electromagnetic acoustic transducers (EMATs) in thin metal plates. The technique was able to identify the presence of s 0, a 0, a 1, s 1 and s 2 modes in an aluminium plate from a single waveform, and hence to separate the modes. The technique has been applied to tomographic reconstruction of artificial defects in plates from individual modes to illustrate the usefulness of the approach.

Ultrasonic attenuation and microstructural evolution throughout tension–compression fatigue of a low-carbon steel

We have studied the microstructural evolution in a wrought low-carbon steel (ASTM-A105), containing 0.21 mass% C and subjected to tension–compression cyclic loading, through in situ monitoring of the attenuation and velocity of a surface shear-wave with the electromagnetic acoustic resonance (EMAR) technique. This technique is a combination of the resonant technique and a non-contacting electromagnetic acoustic transducer (EMAT). The EMAT operates with a magnetostrictive mechanism and it is the key to establishing a non-contacting monitoring of microstructural change in a material's surface region with high sensitivity. The attenuation coefficient is sensitive to the accumulated fatigue damage, showing two peaks around 2% and 90% of life. This novel phenomenon is interpreted in terms of dislocation mobility change and dislocation rearrangement. Transmission electron microscope (TEM) observation has supported this view. This technique has a potential to assess damage and predict the fatigue life of steels.

Inspection of drinks cans using non-contact electromagnetic acoustic transducers

Electromagnetic acoustic transducers (EMATs) have been used to transmit ultrasonic signals across liquid-filled aluminium beverage containers. The work has demonstrated a non-contact method for both determining the location of liquid boundaries, and detecting the presence of foreign bodies using fan-beam tomographic imaging. Pulse compression signal processing, followed by acoustic mode extraction, has been used in a new technique, whereby unwanted guided waves that travelled around the container wall can be identified and removed from the image reconstruction process. Such waves would normally complicate the identification of the signals require for image formation that propagated through the liquid. The result is a method that has been used to provide images of simulated foreign bodies, by scanning EMATs around outer surface of aluminium drinks cans. The approach has also demonstrated the successful measurement of fill level.

Coating thickness and elastic modulus measurement using ultrasonic bulk wave resonance

Measurement of the resonant through thickness ultrasonic modes of a homogeneous plate using a fast Fourier transform of the temporal data can be used to calculate plate thickness very accurately. We describe an extension of this principle to two-layer systems, examining a thin coating on a substrate of known properties. The resonant behavior of these systems is predicted and we explain how this approach is used to measure coating thickness and elastic modulus. Noncontact electromagnetic acoustic transducers are used for ultrasonic measurement, as they do not significantly affect the resonant response of the system, unlike alternative contact transducers.

2821 電磁超音波共鳴法による低炭素鋼の疲労過程のモニタリング(S37-1 非破壊評価とモニタリング(1),S37 非破壊評価とモニタリング)

We have studied the microstructural evolution in a wrought low-carbon steel (ASTM-A105), containing 0.21 mass% C and subjected to tension-compression cyclic loading, through in-situ monitoring of the attenuation and velocity of a surface shear-wave with the electromagnetic-acoustic-resonance (EMAR) technique. This technique is a combination of the resonant technique and a non-contacting electromagnetic acoustic transducer (EMAT). The EMAT operates with a magnetostrictive mechanism and it is the key to establishing a non-contacting monitoring of microstructural change in a material's surface region with high sensitivity. The attenuation coefficient is sensitive to the accumulated fatigue damage, showing two peaks around 2% and 90% of life. This novel phenomenon is interpreted in terms of dislocation mobility change and dislocation rearrangement. Transmission electron microscope (TEM) observation has supported this view. This technique has a potential to assess damage and predict the fatigue life of steels.