Permanent Magnet Electromagnetic Acoustic Transducers Research Articles

A new diamond-shaped periodic permanent magnet electromagnetic acoustic transducer for side-lobe suppression of shear horizontal ultrasonic waves

Periodic permanent magnet electromagnetic acoustic transducers (PPM EMATs) effectively generate shear horizontal (SH) ultrasonic guided waves in metallic media. Depending on their construction parameters, PPM EMATs can generate ultrasonic waves towards unwanted directions, due to the presence of side lobes in their radiation pattern. In this paper, a new PPM-like EMAT is proposed with the objective of reducing the side-lobe level in the radiation pattern of PPM EMATs. This was achieved by altering the magnets’ dimension in the PPM array following a diamond-like shape, reducing their width as one gets further from the centre of the transducer. The radiation behaviour of the EMAT was evaluated by means of an apodization factor, defined as the aperture reduction ratio. The side-lobe level showed a maximum reduction for an apodization factor of approximately 2. Four different configurations of the new PPM-like EMAT design and two reference conventional PPM EMATs were fabricated and experimentally evaluated on an aluminium plate. Experimental radiation patterns were compared to analytical model results showing good agreement. Relative to the conventional PPM EMAT, the new apodized diamond-shaped PPM EMAT presented a side lobe suppression of up to approximately 8 dB.

Internal and External Pipe Defect Characterization via High-Frequency Lamb Waves Generated by Unidirectional EMAT

Periodic permanent magnet(PPM) electromagnetic acoustic transducers (EMATs) are commonly employed for axial defect inspection in pipelines. However, the lowest-order shear horizontal waves (SH0) guided waves have difficulties in distinctly differentiating internal and external defects. To enhance the signal-to-noise ratio and resolution, a unidirectional electromagnetic acoustic transducer (EMAT) based on Circumferential Lamb waves (CLamb waves) is developed. Through structural parameter optimization and excitation frequency adjustment, high-amplitude and low-dispersion CLamb waves are successfully generated in the high-frequency-thickness product region of the dispersion curve. Finite element simulations and experimental validation confirm the capability of this EMAT in exciting CLamb waves for the detection of crack-like defects. Experimental results demonstrate that the excitation efficiency of the CLamb EMAT exceeds that of the periodic permanent magnet electromagnetic acoustic transducer by more than tenfold. The defect reflection signal of the CLamb EMAT exhibits higher resolution and more significant amplitude compared to the PPM EMAT. The integration of this method with SH0 mode detection allows for the inspection of both internal and external defects in pipelines, offering a new avenue for EMAT applications in pipeline inspection.

On the use of a linear array EMAT for remote thickness gauging using the reflected modes on a steel pipe

Guided wave nondestructive testing refers to a set of fast and reliable techniques used to scan structures such as plates or pipes over medium to long range distances. The structural filtering method uses high-order modes and their cutoff frequency-thickness products to estimate the thickness along an inspection line. A corroded region, which can be approximated by a local loss of thickness, will act as a filter on the propagating modes. Because of their cutoff frequency-thickness product, some modes will not be able to propagate and will be either reflected or converted. Shear horizontal guided waves are the main candidate for structural filtering due to the simplicity of interpretation of their dispersion curves and their low sensitivity to non-viscous surface fluid loading. Electromagnetic acoustic transducers (EMAT) provide fast and efficient SH wave transduction but the most common designs, namely, periodic and permanent magnet EMAT is a mono element probe allowing modes to be excited only around a fixed wavelength. Recent work has shown that a linear array EMAT can be combined with a phase velocity excitation to allow almost complete control of the emitted waves in the frequency wavenumber space. In this paper, the linear array EMAT developed is used to estimate the minimum thickness of a defect machined on a 10.2 mm thick steel pipe with an external diameter of 323.8 mm. The measurements and finite element simulations carried out enabled us to estimate the thickness in several positions, with a maximum error of 0.4 mm and locate the defect with an error of 10 mm over a propagation distance of 600 mm.

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.

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

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

Optimization of a Lorentz forces EMAT for the reconstruction of the circumferential thickness profile of a steel pipe using high order shear horizontal modes

Corrosion is a chemical reaction affecting a wide range of materials. Safety-critical structures in the oil, gas, petrochemical, and many other industries need a fast, reliable screening technique that can be performed without operational disruption. Partially accessible structures such as pipes at supports or under a layer of insulation are particularly challenging. In this respect, high-order shear horizontal (SH) guided wave modes can be used in medium-to long-range thickness gauging thanks to their cutoff frequency-thickness product. As the wave propagates, a reduction in the thickness behaves like a low-pass filter for high-order modes. It has been demonstrated that periodic and permanent magnet electromagnetic acoustic transducers (PPM EMAT) can be used for transmission and reception. This paper presents a comparative study between different PPM EMAT configurations on a 323.8 mm diameter, 10.2 mm thick steel pipe. Finite element (FE) simulations and experimental measurements are used to compare the effectiveness of the different configurations to measure the thickness in four intervals: less than 5.4 mm, between 5.4 and 7 mm, between 7 and 8 mm, and between 8 mm and 10.2 mm, thus allowing to detect thickness losses of up to 50% of the nominal thickness of the waveguide.

Unidirectional Shear Horizontal Wave Generation by Periodic Permanent Magnets Electromagnetic Acoustic Transducer With Dual Linear-Coil Array.

Shear horizontal (SH) waves are commonly generated by periodic permanent magnet (PPM) electromagnetic acoustic transducers (EMATs) in metallic media. Conventional PPM EMATs generate ultrasonic waves, which simultaneously propagate both forward and backward. This can be an undesirable characteristic, since the backward wave can be eventually reflected, reaching the receiver transducer where it can mix with the signal of interest. This limitation can be overcome using two side-shifted PPM arrays and racetracks coils to generate SH waves in a single direction. That design relies on the EMAT wavefront diffraction to produce constructive and destructive interference, but produces unwanted backward traveling sidelobes. Here, we present a different design, which uses a conventional PPM array and a dual linear-coil array. The concept was numerically simulated, the main design parameters were assessed and the unidirectional EMAT was experimentally evaluated on an aluminum plate, generating the SH0-guided wave mode nominally in a single direction. The amplitude ratio of the generated waves at the enhanced to the weakened side is above 20 dB. Since the wavefronts from the two sources are perfectly aligned, no obvious backward sidelobes are present in the acoustic field, which can significantly reduce the probability of false alarm of an EMAT detection system.

Bond stiffness estimation with shear horizontal guided waves generated using PPM-EMATs

This paper aims to develop a technique for estimating an aluminium/epoxy/aluminium lap shear joint stiffness. Shear horizontal guided waves are employed using periodic permanent magnet electromagnetic acoustic transducers (PPM EMATs). Based on the source influence of the EMATs and sensitivity of SH-like mode to adhesion, an appropriate transduction regime is selected. Different modality of inspection, such as reflection and transmission mode, is explored. The arrival time of the received signal in transmission mode depends on the adhesion level. A procedure for calculating the energy velocity in the bond and inverse calculation of bond stiffness is proposed. Experiments were performed on samples of different surface treatments. Based on the proposed method, the stiffness of the samples was estimated. Results show the technique's potential to quantify the adhesion level rather than merely classifying as good, weak, and bad bonds.

Optimization of a Focusable and Rotatable Shear-Wave Periodic Permanent Magnet Electromagnetic Acoustic Transducers for Plates Inspection.

Due to the symmetry of conventional periodic-permanent-magnet electromagnetic acoustic transducers (PPM EMATs), two shear (SH) waves can be generated and propagated simultaneously in opposite directions, which makes the signal recognition and interpretation complicatedly. Thus, this work presents a new SH wave PPM EMAT design, rotating the parallel line sources to realize the wave beam focusing in a single-direction. The theoretical model of distributed line sources was deduced firstly, and the effects of some parameters, such as the inner coil width, adjacent line sources spacing and the angle between parallel line sources, on SH wave focusing and directivity were studied mainly with the help of 3D FEM. Employing the proposed PPM EMATs, some experiments are carried out to verify the reliability of FEM simulation. The results indicate that rotating the parallel line sources can strength the wave on the closing side of line sources, decreasing the inner coil width and the adjacent line sources spacing can improve the amplitude and directivity of signals excited by transducers. Compared with traditional PPM EMATs, both the capacity of unidirectional excitation and directivity of the proposed PPM EMATs are improved significantly.

Frequency dependent directivity of periodic permanent magnet electromagnetic acoustic transducers

Beam steering has been achieved using shear horizontal waves generated using a periodic permanent magnet (PPM) electromagnetic transducer (EMAT). Unlike phased arrays, where steering is achieved by carefully controlling the firing of individual elements, the spatial periodicity of the PPM EMAT is ultilised to steer the beam whilst exciting all elements simultaneously. Due to the periodic nature of the array, the interference of individual waves from each of the elements creates a highly frequency dependent angle of propagation, allowing the directivity to be changed by simply varying the frequency of the input signal. Simultaneous excitation precludes the need for complicated and expensive phased array hardware. A frequency domain model is developed so that the beam characteristics, such as steering angle and beam width, can be calculated, allowing for investigation into the beam steering qualities of the PPM transducer. Broadband pulsed generation is also demonstrated, showing how a wave is generated over a large range of angles, meaning a large area can be covered with a single pulse. Interesting properties of this wave, such as a variation of frequency as a function of angle, and how this can be useful, are also discussed.