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To overcome the shortcomings of low energy conversion efficiency of electromagnetic acoustic transducers (EMATs), point-focusing shear horizontal (PFSH) wave EMAT is used to focus the wave energy into a specific area. Many factors will affect the capability of the focusing transducer, and in addition to considering the signal intensity, the detection accuracy is also required to be investigated. Specifically, to simplify the test process, we use the orthogonal test method to study the effect of different influence parameters on signal intensity and focal area dimensions. Seven factors are selected, and three results are determined in the test. Range analysis shows that for signal amplitude $M$ , the top three impact factors are the coil width $w$ , coil turns $n$ , and focal length lF (equal to bandwidth factor $\alpha $). Moreover, magnet number $m$ and frequency $f_{c}$ dominate the effective focal length $l_{\text {fd}}$ , and aperture angle $\theta $ determines the effective focal width $w_{\text {fd}}$. To enable higher signal intensity and smaller focal area dimensions, it is necessary to consider various factors on the PFSH-EMAT focusing performance. The test’s signal intensity with optimized parameters’ combination at the focal point is nearly 144.42% higher than the average of all the tests, $l_{\text {fd}}$ decreased by 37.84%, and $w_{\text {fd}}$ decreased by 50.59%. The experiment also verified that focusing EMAT with optimized parameters has a better focusing performance. [ABSTRACT FROM AUTHOR]

As a kind of nondestructive testing (NDT) method, shear-horizontal (SH)-guided wave detection technology is widely used on an electromagnetic acoustic transducer (EMAT). Although ultrasonic-guided waves perform well in defect location, it is difficult to obtain detailed information about defects, and the low efficiency of EMAT energy conversion still reduces the EMAT’s performance. Therefore, in this work, the defect detection method of different shapes and sizes by point-focusing shear-horizontal (PFSH)-guided wave EMAT with the use of periodic permanent magnet (PPM) is investigated through simulation and experiment. For the purpose of defect classification and quantification, the extraction principles of defect features are obtained through simulation based on the circumferential scatter diagrams, and the neural network (NN) is used to process the features extracted from the experimental data. The results show that by extracting effective defect features from the scatter diagram, high-accuracy classification and high-precision quantification of defects under the influence of the focusing transducer can be achieved. [ABSTRACT FROM AUTHOR]

The electromagnetic ultrasound detection technology provides an efficient method to inspect the structural health status. For the pipeline structure, the guided ultrasonic wave propagating along the axial direction has been the focus of the current research. However, the helical wave owns the advantage of obtaining the defect information in extra angles and has not been explored sufficiently. In this work, a transducer with a sector configuration is designed for generating helical shear horizontal waves in a pipeline. The magnetostrictive effect is analyzed, and the model is built in the finite element simulation software of COMSOL Multiphysics. The electromagnetic field and displacement distribution are studied. To evaluate the performance of the transducer, the half divergence angle is defined to measure the acoustic beam and evaluate the capacity in generating helical waves. Corresponding experiments are also implemented to analyze the influencing factors in the structural design of the transducer. The results indicate that the pulse cycle affects the half divergence angle slightly. The compromise should be considered when designing the central angle of the transducer because it influences the signal amplitude and further imaging resolution in a contradictory way. [ABSTRACT FROM AUTHOR]

We realized a phase-coherent oblique point-focusing shear-horizontal (SH) guided-wave electro- magnetic acoustic transducer (EMAT) composed of variable-spacing periodic permanent magnets (PPMs) and racetrack coils. For traditional focusing transducers, the necessary focal position adjustment in defect detection requires a redesign of the coil structure. More conveniently, this new transducer structure arrangement can achieve wave focusing and focal position change by rationally adjusting the layout of the PPM without changing the coil structure. Simulation results show that this newly designed transducer has good focusing performance: it can successfully focus the signal to a preset point, and the signal amplitude is nearly double that of the nonfocusing side. In addition, the focusing performance optimization of the transducer has been studied. The experimental results agree well with the actual location and size of a defect, which verifies the effectiveness of the newly designed focusing transducer in defect detection. [ABSTRACT FROM AUTHOR]

Electromagnetic ultrasonic testing, based on the propagation characteristics of the ultrasonic wave in the medium, is an effective technique for defect detection. The structural of the electromagnetic acoustic transducer (EMAT) determines the detection effect, which should be reasonably designed. We propose an improved unilateral point-focusing EMAT structure with curved coils in this manuscript. The 3D model of the designed EMAT is numerically simulated utilizing the finite element method, and the bilateral point-focusing EMAT is also studied for comparison. The simulation results of the magnetic field intensity and the distributions of the displacement are shown in the manuscript, which prove the effectiveness of the improved unilateral point-focusing EMAT. Five impact parameters such as aperture angle, focal position, excitation current, lift-off distance, and coil winding number are investigated in detail. We verify the calculation results of the aperture angle and excitation current by the experiments. The conclusion shows that the focusing intensity of the improved unilateral point-focusing EMAT is relatively higher, and other factors also have a significant effect on the focal intensity. [ABSTRACT FROM AUTHOR]

As the focal position of a line-focusing electromagnetic acoustic transducer (LF-EMAT) affects the intensity of the focal signal, we measure the displacement signal and the selection method of the focal line position is then studied in this work to improve the signal intensity and detection precision for a shear-vertical (SV) wave LF-EMAT. We calculate the magnetostatics field and bulk wave propagation utilizing the finite element method (FEM). We simulate the characteristics of the SV wave, and we find that the radiant distance and propagation direction has a great influence on the amplitude of the displacement of the specimen. The superposition area of SV waves from different sources possesses better focusing ability, and the triangular area (normalized amplitude of the SV waves lower than 10%) near the surface of a specimen should be avoided to be chosen when designing the meander line coils. As for the line-focusing EMAT described in this manuscript, the selection of focal positions for different thickness specimens or detection depth is completely different. Moreover, we perform an experiment to validate the simulation results and lead to a conclusion that the position of the focal point is required to be appropriately close to the excitation coils to enhance the intensity of the signal, while the best focal position should be calculated. [ABSTRACT FROM AUTHOR]

• Coupled models of EMATs are built via a mid-level method. • Approaches to solve the amplitudes of the S0 and A0 modes are compared. • The single frequency approach is used in characterizing excitability of Lamb waves. Electromagnetic acoustic transducers (EMATs) are versatile non-contact ultrasonic transducers with relatively low transduction efficiency. A fundamental problem not answered completely yet is how the structure of an EMAT influences the generation of various Lamb wave modes. We tackle this problem with fully coupled frequency domain EMAT models where the underlying integro-differential equations are solved directly and the magnitudes of the phasors of the displacement components at a point in the middle plane of a plate are used to represent the strength of the S0 and A0 mode waves. Idealized single-wire, two-wire and wire bundle EMATs with uniform bias magnetic field and practical meander line and tightly wound EMATs with distributed bias field are studied. Polar plots of the idealized EMATs with swept angle of the bias field show that the vertical component of the bias field decides the strength of the S0 mode waves. Sweeping of the width of the magnet for the practical EMATs generates S0 mode curves that could be explained from the distribution of the vertical component of the bias field. This work represents the first attempt to solve the proposed problem of characterizing excitability of Lamb waves with EMATs quantitatively, via the model-based approach. [ABSTRACT FROM AUTHOR]

In this work a multiphysics model of a meander line electromagnetic acoustic transducer (EMAT) to generate S0 mode Lamb waves in an aluminum plate is built and the structural parameters of the transducer are optimized based on the model. Three approaches to solve the amplitude of the in-plane displacement component at an observation point are explored and the single frequency approach is selected because it is the fastest. In the optimization, the objective function to minimize is the negative amplitude of the S0 mode waves, and the design variables are the width and height of the magnet and liftoff values of the magnet and the coil from the top surface of the plate. The liftoff values form a linear constraint of the optimization problem. A genetic algorithm (GA) program capable of handling the linear constraint efficiently is developed. The internal status of the GA program is tracked carefully to avoid unnecessary evaluations of the objective function. With 30 runs of the program, the optimal set of variables leads to the ratio of the width of the magnet to that of the coil of 115.85%. [ABSTRACT FROM AUTHOR]

Electromagnetic acoustic transducers (EMATs) are non-contact ultrasonic transducers used to generate ultrasonic waves directly in the tested metal samples, suitable for special situations like testing moving or hot metal solids. EMATs rely on electromagnetic coupling, and the transduction efficiency of EMATs is relatively low, so it's crucial to improve their performance through building accurate models and implementing structural optimisations. Modern evolutionary optimization algorithms involve a number of evaluations of the objective function of the optimization problem, so it's necessary to explore surrogate models to reduce the time consumption during evaluation of the computationally expensive objective function, especially when the function corresponds to complex numerical models, like the EMAT model. We focused on the application of Kriging surrogate model (DACE) in EMAT modeling in this work. The formulations of the DACE model were reviewed, then the model of an omnidirectional EMAT was introduced. Three methods to calculate the amplitudes of the displacement components were discussed and compared, including the time-domain method, the multifrequency method (with FFT/IFFT processing), and the single frequency method applying directly the phasors. The DACE model of the EMAT was then built to predict outputs on a discrete grid of the design variables. Finally the obtained DACE model was used as the surrogate model in the optimization of the EMAT driven by the simulated annealing algorithm. With the DACE model, the total time consumption of optimization of the EMAT was reduced greatly. [ABSTRACT FROM AUTHOR]

In this paper an axisymmetric model of an omnidirectional electromagnetic acoustic transducer (EMAT) used to generate Lamb waves in conductive plates is introduced. Based on the EMAT model, the structural parameters of the permanent magnet were used as the design variables while other parameters were fixed. The goal of the optimization was to strengthen the generation of the A0 mode and suppress the generation of the S0 mode. The amplitudes of the displacement components at the observation point of the plate were used for calculation of the objective functions. Three approaches to obtain the amplitudes were discussed. The first approach was solving the peak values of the envelopes of the time waveforms from the time domain simulations. The second approach also involved calculation of the peaks, but the waveforms were from frequency domain model combined with the forward and inverse Fourier transforms. The third approach involved a single frequency in the frequency domain model. Single and multi-objective optimizations were attempted, implemented with the genetic algorithms. In the single objective optimizations, the goal was decreasing the ratio of the amplitudes of the S0 and A0 modes, while in the multi-objective optimizations, an extra goal was strengthening the A0 mode directly. The Pareto front from the multi-objective optimizations was compared with the estimation from the data on the discrete grid of the design variables. From the analysis of the results, it could be concluded that for a linearized steel plate with a thickness of 10 mm and testing frequency of 50 kHz, the point with minimum S0/A0 could be selected, thus the multi-objective optimization effectively degenerated to the single objective optimization. While for an aluminum plate with a thickness of 3 mm and frequency of 150 kHz, without further information it would be difficult to select one particular solution from the Pareto front. [ABSTRACT FROM AUTHOR]

• In this manuscript, a new unidirectional SH wave EMAT with oblique permanent magnet for the inspection of the aluminum plate is presented for the first time.The oblique angle of the permanent magnet enhances the guided signal on one side and weakens the ultrasonic signal on the other side. • Therefore, the angled bias magnetic field can improve the focusing ability of the SH guided wave transducers effectively. • It is concluded that the oblique PPM can improve the focusing ability of the unidirectional point-focusing SH guided wave EMAT and suppress the influence of the reflected signal, and then improve the resolution of the ultrasonic signal at the focusing point. • In the study of the influence of the oblique angle of the permanent magnet on the focusing SH guided wave EMAT, the displacement amplitude at the focal point increases as the angle increases while the growth rate decreases. • For the other side of the focal position, the displacement amplitude at that point decreases linearly as the angle increases. We propose a new unidirectional point focusing shear horizontal (SH) guided wave electromagnetic ultrasonic transducer (EMAT) with an angled periodic permanent magnet (PPM) in this work. The angled PPM developed here provides an angled bias magnetic field to achieve the EMAT's unidirectional focusing capability. The characteristics of the magnetic field distribution are analyzed by numerical and theoretical calculations. The simulation and experimental results are proven to be in good agreement when studying the bidirectional normalized amplitudes of the displacement of the single-coil SH-guided EMAT with oblique permanent magnets. Both the proposed angled transducer structure and the traditional paralleled transducer structure are performed and simulated using the three-dimensional Finite Element Method (FEM) to compare their unidirectional focusing capabilities. The results show that the SH guided wave EMAT of the focusing coils with an oblique permanent magnet enhances the signal on the focusing side and weakens the signal on the other side effectively. This performance can suppress the influence of the reflected signal from the unfocused side and further improve the ultrasonic signal's resolution. Moreover, it is shown in the study that increasing the oblique angle of the PPM makes it difficult to increase the signal strength at the focal point when the angle reaches a certain value, but it is still effective at weakening the signal on the unfocused side. [ABSTRACT FROM AUTHOR]

• The wave field equations of continuous pulse and burst pulse signals are studied and derived. • An optimal focal position selection method is demonstrated through simulation and experimental methods. • Optimal focal points are approximately distributed along a line, and the slope of the OSH-FL increases with decreasing excitation frequency. As an important application in nondestructive testing (NDT) technology, ultrasonic guided wave testing is widely used in the detection of defects in metal plates, pipes, and bridges. In the detection of defects in metal plates, electromagnetic acoustic transducers (EMATs) can easily excite shear horizontal (SH) guided waves, and a couplant is not required, in contrast to piezoelectric ultrasonic transducers. However, the low energy conversion efficiency of an EMAT has always been its main drawback. Therefore, in this work, we propose a new oblique point-focusing SH (OPFSH) EMAT configuration and obtain two analytical models for both continuous sine pulses and burst signals. The proposed transducer can achieve high-sensitivity detection of defects in the focal area. Moreover, the influence of the focal position on the signal intensity is also studied, and an optimal focal position selection method is demonstrated through simulation and experimental methods. By performing finite element calculations of the wave fields at different focal positions, we find that the optimal focal points are approximately distributed along a line. Therefore, the optimal focal position should be selected carefully to achieve the best OPFSH-EMAT focusing performance. [ABSTRACT FROM AUTHOR]

• In this work, FEM simulation and orthogonal test methods are used to study the effect of impact factors on the PFSV-EMAT's performance. Five factors are selected in the test to investigate the signal intensity and focal area dimensions of the transducer. Range analysis is applied to compare the influence degree of each factor. In addition, drive frequency f and focal depth l Fz are also studied individually. Considering the influence of different factors on different results, the optimal combination of parameters can be obtained. The effectiveness of the optimization method is verified by both simulation and experiment. Moreover, the experiments show that the focusing intensity of the optimized transducer is 400% higher than the average value of the non-optimized transducers, the effective focal length is reduced by 15%, and the effective focal width is reduced by 57%. Although ultrasonic focusing methods have been widely used in electromagnetic acoustic transducers (EMATs) to solve the problem of their low energy conversion efficiency, the influence of focusing accuracy on defect identification also warrants consideration. The dimension of the focal area that acts as an important factor and affects the detection accuracy has not been fully investigated recently to our knowledge. In this work, we report a parameter optimization method using an orthogonal test when considering the focusing intensity and focal area together. The results of the range analysis show that the factor lift-off distance h l has the greatest impact on signal intensity M. Considering the dimensions of the focal area, bandwidth factor α has the largest effect on the effective focal length L fd. For effective focal width W fd , the concentric line source (CLS) number n has the largest effect. Therefore, a smaller lift-off distance, larger CLS number, and suitable bandwidth factor α are required in a point-focusing SV EMAT (PFSV-EMAT). The optimal combination of parameters can be obtained by considering the influence of different factors on the results. The experiment shows that the signal intensity of the optimized transducer is nearly 400% higher than non-optimized ones and the effective focal length and width are reduced by 15% and 57%, respectively. [ABSTRACT FROM AUTHOR]