Electromagnetic Acoustic Transducer Research Articles

Development of an array pickup electromagnetic acoustic transducer and application to surface wave visualization

Visualization of surface wave based on signal and imaging processing schemes is an efficient way to enhance the detectability of ultrasonic testing for inspection of surface or near surface small defect in layered structures such as the thermal barrier coating (TBC) system. A full wave field measurement is necessary to get information for visualization of the ultrasonic wave. In this paper, a non-contact, array pickup electromagnetic acoustic transducer (EMAT) with 32 receiving coils and a phased array EMAT system are developed for efficient measurement of full information of surface wave field. Based on the multiple channel EMAT signals at different scanning position, a dynamic visualization algorithm is developed to image the propagation, reflection and scattering wave caused by defect and structure edges. The validities of the array pickup EMAT and the dynamic visualization algorithm are demonstrated by using both simulated and measured surface wave signals for a prototype array EMAT and an aluminum plate with small hole defect.

Development of a phased array flexible Rayleigh-wave electromagnetic acoustic transducer for pipe inspection

In this paper, a novel Phased Array fully Flexible Rayleigh-wave EMAT (PAFR-EMAT) suitable for inspection of curved surface based on flexible coil bias field magnet EMAT is proposed. The PAFR-EMAT consists of a flexible racetrack coil generating long pulsed bias magnetic field instead of the conventional permanent magnets, and flexible meander coils triggered in sequence to induce transient eddy current for exciting enhanced Rayleigh wave of MHz frequency. Based on the Lorentz force mechanism, pulsed electromagnetic fields and the generated ultrasonic waves of the proposed new EMAT are studied through numerical simulation at first. Experiments are conducted then to check the practical performance of the PAFR-EMAT for pipe inspection. Results show that the PAFR-EMAT has good performance in Rayleigh wave generation for inspection of the curved surface of metallic pipe and is capable to detect surface crack of small depth.

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.

Acoustic and Electromagnetic Properties of Civilian Gun Blanks

The non-uniformity of acoustic properties of civilian gun barrel blanks has been studied. Four samples of civilian gun barrel blanks from the same batch, obtained after the first stage of the technological process, were subjected to the study. The blanks are of cylindrical shape with diameter of 34 mm, 210 mm in length and are made of high quality carbon steel 50A. In the research, a control echo-shadow method of multiple reflections was used with the application of an electromagnetic-acoustic transducer and a piezoelectric transducer based on a polyvinyl-dente-fluoride film for excitation and reception of waves. On the basis of the studies carried out using the echo-shadow method of multiple reflections on the samples of civilian gun barrel blanks, the following informative parameters were measured: the speed of the transverse, longitudinal and Rayleigh waves, and the elastic moduli and Poisson's ratio were calculated on their basis. The echo-shadow method of multiple reflections showed high sensitivity of the measured informative parameters of ultrasonic waves to material inhomogeneities in individual sections along the length of barrel blanks, which may be due to violations of machining processes. For transverse and Rayleigh waves, such parameters as the coefficient of detectability and dispersion are calculated, which also indicate the inhomogeneity of properties in individual sections with respect to length of the sample blanks. Using a micrometer, the diameters of the bars were measured at 17 points, and the minimum and maximum values were recorded at each point, which correlates with the results of measuring the wave velocities. The relative values of the electrical conductivity on each bar along the generatrix in 4 positions of the circle (0, 90, 180, and 270 degrees) were measured. Deviations of the obtained electric conductivity values up to 5% being the evidence of local inhomogeneities of electric properties revealing structural inhomogeneity is observed. Value inhomogeneity of coercitive force up to 0.7 A/cm along blank circumference.

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.

Enhancement of Rayleigh Wave Generated by Electromagnetic Acoustic Transducer Based on New Magnetic Configuration

Enhancement of Rayleigh Wave Generated by Electromagnetic Acoustic Transducer Based on New Magnetic Configuration

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.

Ice monitoring of aluminum conductor steel-reinforced cables using guided waves

Aluminum conductor steel-reinforced (ACSR) cables are commonly used as power transmission conductors. In cold regions, icing of ACSR cables is a common problem. Ice accretion on ACSR cables may cause various problems. Timely ice monitoring is a primary measure for any type of de-icing solution. In this study, a guided wave-based method was proposed to monitor the ice-forming processes on ACSR cables owing to its advantages of high sensitivity and long-range inspection. The dispersion curves were first obtained using the semi-analytical finite element (SAFE) method for the ACSR cross-section geometry, particularly for different ice thicknesses. They could be used to accurately determine the group velocity for each guided wave mode. Subsequently, a series of tests were conducted with different ice thicknesses in the laboratory. Guided waves were excited by an electromagnetic acoustic transducer (EMAT) in ACSR cables. A new method that combined principal component analysis (PCA) and empirical mode decomposition (EMD) was employed to process the original guided wave signals to obtain effective icing indicator. The results obtained by the guided wave-based method were verified using an image-based method. This study indicates that the proposed ultrasonic guided wave combined with the PCA–EMD method is efficient and sensitive for the ice monitoring of ACSR cables.

On improving the efficiency of the pass-through electromagnetic-acoustic transducer of longitudinal waves

The paper theoretically and experimentally investigates the possibility of increasing the efficiency of pass-through electromagnetic-acoustic (EMA) transducers of longitudinal waves when changing the direction of current in a high-frequency inductor and forming a co-directional (counter-directional) switch of the magnetic field of the inductor relative to the polarizing field of constant magnetization. It is shown that the increase in the EMA conversion efficiency due to the electrodynamic mechanism is due to a local increase in the eddy current density along the edges of the aperture of the high-frequency inductor when the magnetic field is turned on counter-directionally. These patterns are most evident for samples with high

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.

INCREASING THE EFFICIENCY OF THE SURFACE-MOUNTED ULTRASONIC ELECTROMAGNETIC-ACOUSTIC TRANSDUCER ON ACCOUNT OF THE MAGNETIC FIELD SOURCE

Model studies were carried out using the COMSOL Multiphysics package, aimed at ensuring the forming of a permanent magnet magnetic field at a considerable distance to a ferromagnetic product from its pole, which is necessary to create efficient portable ultrasonic electromagnetic-acoustic transducers of thickness gauges and testing and diagnostic devices. It is theoretically shown and experimentally confirmed that for portable measuring ultrasonic devices it is expedient to set the height of the permanent magnet at about 60 mm and the cross section of the magnet pole 50x50 mm2. At the same time, with a gap between the magnet pole and the product of about 30 mm, the value of the normal component of the magnetic field induction near the surface of the object is about 0.3...0.4 T, which is sufficient for thickness gauging or diagnostics of ferromagnetic products using the ultrasonic pitch-and-catch method. References 19, figures 8.

Damage imaging of a U-shaped boom using improved periodic permanent magnet electromagnetic acoustic transducers

ABSTRACT In this research, to solve the problem of the ultrasonic guided wave inspection of a U-shaped boom, an improved periodic permanent magnet electromagnetic acoustic transducer (PPM EMAT) is proposed for defect detection for a U-shaped boom. The acoustic field distribution range of a traditional PPM EMAT is approximately ±20°, and the energy distribution is relatively concentrated. By adjusting the tilt angle between adjacent magnets and coils of a traditional PPM EMAT, an improved PPM EMAT is proposed. The acoustic field distribution range of the improved PPM EMAT is approximately ±30°, and the energy distribution in this range is relatively uniform. The simulation and experimental results show that the improved PPM EMAT has superior frequency response characteristics and acoustic field directivity, which is suitable for detecting large and complex structures. Furthermore, based on the traditional PPM EMAT and the improved PPM EMAT array, combined with the reconstruction algorithm for the probabilistic inspection of damage (RAPID), the damage locations of the double-hole defects of a U-shaped boom are studied. It is shown that the imaging results for defect location using the improved PPM EMATs are better than those found using the traditional PPM EMATs.

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.

A Decade Review of the Art of Inspection and Monitoring Technologies for Long-Distance Oil and Gas Pipelines in Permafrost Areas

Long-distance oil and gas pipelines buried in permafrost areas will inevitably encounter typical geological disasters, such as frost heave and thaw settlement and sliding, which easily cause pipeline displacement, bending, or deformation. When there are certain defects in the pipeline, additional complex, external stress will further lead to the failure of the pipeline or weld and can even lead to serious accidents such as pipeline leakage, pipe burst, or fracture. This paper introduces in detail the typical defects and risks of buried pipelines in permafrost areas and summarizes the in-line inspection technologies, off-line inspection technologies, and integrated monitoring systems for pipelines in the pipeline industry. Regarding pipelines in permafrost areas, in-line inspection methods may be employed. These include magnetic flux leakage, electromagnetic eddy current, ultrasonic, IMU, and electromagnetic acoustic transducer inspections. Off-line inspection is also one of the important means of inspecting a pipeline in a permafrost area. Indirect inspection is combined with verification by direct inspection to check and evaluate the integrity of the anticorrosive coating and the effectiveness of the cathodic protection for the pipeline. Meanwhile, considering the external environment of a pipeline in a permafrost area, a monitoring system should be developed and established. This paper discusses and projects the future development of related technologies, which provides reference for the construction and operation of pipelines in permafrost areas.

Optimal design of electromagnetic metamaterial electronic device sensor with specific performance based on multivariate big data fusion

As the basic unit structure of electromagnetic metamaterial, the structure size of open resonant ring directly determines the constitutive parameters of SRR (dielectric constant ε and permeability μ). In order to improve the energy exchange efficiency of the electromagnetic acoustic transducer, the coil backplane of the electromagnetic ultrasonic shear wave sensor was optimized. First, the influence of the thickness of the coil backplane on the ultrasonic signal is studied by the experimental method, and then, the magnetic field distribution of the coil backplane is simulated by the finite element simulation software. Finally, the SNR and lift distance of the shear wave sensor before and after the coil backplane optimization are compared by experiments. In this article, a multi-hypothesis data fusion method in distributed detection system is proposed, which extends the multi-sensor data fusion rules to more general cases. The results show that the optimum thickness of the coil back plate in the electromagnetic ultrasonic transverse wave sensor is 1.5–2.0 mm. Using iron powder with the same length and width as the working area of the coil as the coil backplane can significantly increase the magnetic field strength in the working area of the sensor. Compared with the coil backplane made of non-magnetic materials, the optimized backplane can increase the signal-to-noise ratio of the sensor by about one time and the lift-off distance by about 1 mm.

A Comprehensive Analysis of In-Line Inspection Tools and Technologies for Steel Oil and Gas Pipelines

The transportation of oil and gas through pipelines is an integral aspect of the global energy infrastructure. It is crucial to ensure the safety and integrity of these pipelines, and one way to do so is by utilizing an inspection tool called a smart pig. This paper reviews various smart pigs used in steel oil and gas pipelines and classifies them according to pipeline structure, anomaly-detection capability, working principles, and application areas. The advantages and limitations of each sensor technology that can be used with the smart pig for in-line inspection (ILI) are discussed. In this context, ultrasonic testing (UT), electromagnetic acoustic transducer (EMAT), eddy current (EC), magnetic flux leakage (MFL), and mechanical contact (MC) sensors are investigated. This paper also provides a comprehensive analysis of the development chronology of these sensors in the literature. Additionally, combinations of relevant sensor technologies are compared for their accuracy in sizing anomaly depth, length, and width. In addition to their importance in maintaining the safety and reliability of pipelines, the use of ILI can also have environmental benefits. This study aims to further our understanding of the relationship between ILI and the environment.

Study on the steering capability of a meander-line coil EMAT

Electromagnetic acoustic transducers (EMATs) are well-established as a means of ultrasonic wave generation and reception without the use of a mechanical coupling. When comprising a bias magnetic field and a meander-line coil (MLC), these waves propagate at an angle normal to the emission surface. With the appropriate frequency, the propagation pathway of these ultrasonic waves can be steered to a particular angle. This paper presents the methodology used to find the steering limit of an MLC EMAT and the results from simulations and experimental validations on aluminium. The results show that the maximum shear wave amplitude occurred at around 30°, the steering limit was approximately 50° and the simulations were validated by the experimental set-up to a satisfactory degree.

Development of a linear array electromagnetic acoustic transducer for shear horizontal guided wave inspection

Ultrasonic guided wave screenings have proven to be fast and reliable in detecting various types of defects in plate-like structures. Today, low frequency ultrasonic guided waves are routinely used to screen long sections of pipelines. For many years, ultrasonic guided waves at frequencies beyond the cutoff of the first high-order mode attracted interest in the research community due to the plurality of modes that can propagate. During operation at a frequency beyond the cutoff of the first high-order mode, the transduction mechanism becomes crucial for selectively exciting and detecting a single high-order mode or a group of high-order modes. Ultrasonic comb transducers allow to selectively excite and detect high-order ultrasonic guided waves at a desired wavelength. Linear array transducers are even more flexible, and allow virtually full control in the frequency wavenumber space. High-order shear horizontal (SH) modes have multiple potential applications including, for example, remote thickness gauging and crack monitoring. However, these modes are notoriously difficult to excite and detect using conventional piezoelectric transducers. Electromagnetic acoustic transducers (EMAT) make the excitation and detection of SH modes relatively simple. Periodic permanent magnet (PPM) EMAT can be used to selectively excite and detect high-order SH modes based on a desired wavelength. However, in some applications, it may be necessary to excite and detect high-order SH modes with more control in the frequency wavenumber space. In this paper, a novel EMAT linear array is proposed using a configuration similar to a PPM EMAT but with individual coils wrapped around the magnets. The performance of the EMAT linear array in transmission and reception is demonstrated in simulations and experimentally on a 9.53 mm steel plate. The results show a successful transmission and detection of SH0 to SH4 for a 300 to 800 kHz frequency range.

Excitation of Mechanical Resonances in the Stationary Ring of a Mechanical Seal by a Continuously Operated Electromagnetic Acoustic Transducer.

Acoustic/ultrasonic testing is now a common method in the field of nondestructive testing for detecting material defects or monitoring ongoing mechanical changes in a structure during operation. In many applications, piezoelectric transducers are used to generate mechanical waves inside the specimen. Their actual operating frequency is highly dependent on the dimensions of the transducer. Larger dimensions of the piezoelectric transducer allow for a lower operating frequency. However, these dimensions limit the use of piezoelectric transducers in certain applications where the size of the transducer is restricted due to limited installation space and when low-frequency excitation is required. One application that places these requirements on the transducer is the monitoring of mechanical seals. Here, the transducer must be mounted on the stationary ring of the seal. In this paper, a continuously operated electromagnetic acoustic transducer (EMAT) is presented as an alternative to piezoelectric transducers as a transmitter. The advantage of a EMAT is that it meets the requirements of limited sensor size (sensor area < 10 × 6 mm) and can excite mechanical waves with frequencies below 10 kHz. A structural analysis of the stationary ring shows that the first two mechanical resonances occur around 4 and 5.5 kHz. An experimental study meterologically demonstrates the ability of the EMAT to excite these first two mechanical resonances of the ring. A comparative simulation agrees well with the measurement.

Calibration-free SH guided wave analysis for screening of wall thickness in steel with varying properties

Guided shear horizontal (SH) ultrasonic wavemodes show promise for fast screening of wall thinning and other defects, with miniaturised electromagnetic acoustic transducers (EMATs) offering the potential for automated robotic inspection. However, the use of guided waves for full defect characterisation is strongly affected by mode conversions, changes in lift-off from the sample, variations in material properties and other effects, and these are not typically considered when reporting laboratory results. We show that these factors can have a significant effect on the data as one moves towards industrial implementation, but that a combination of different analysis methods on the SH0 and SH1 wavemodes can offer a high reliability of detection, and mitigate some of the issues with changes in experimental conditions and mode conversions. For inspection of 10 mm thick steel plates, the proposed processing of the signals is shown to reliably detect 40 mm diameter flat bottomed holes with 5%–50% wall loss. A rough, approximately 20 × 20 mm square defect with up to 20% wall loss is also detected.