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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. [ABSTRACT FROM AUTHOR]

Periodic permanent magnet (PPM) array electromagnetic acoustic transducers (EMATs) can efficiently generate and receive shear horizontal (SH) ultrasonic waves. Conventional PPM EMATs typically generate waves which simultaneously propagate both forward and backward. This can complicate the received signals and make it difficult to locate the position of scatterers. Unidirectional generation of ultrasounds can be achieved if two ultrasonic sources are separated by a predefined distance and are excited with the proper delay. Relying on this principle, EMATs have been previously designed aiming to generate other modes of ultrasonic waves. The main challenge when extending this conception to an SH-wave EMAT is how to restrict each coil to its specific magnet array. We present the concept of a unidirectional SH EMAT consisting of two racetrack coils and two interlaced PPM arrays, that are slightly shifted sideways, in such a way that the generated wavefronts still properly interfere. The design was fabricated and experimentally evaluated in an aluminum plate generating the SH0 guided wave mode. The forward to backward generated wave ratio is above 20 dB and well agrees with finite element simulations. [ABSTRACT FROM AUTHOR]

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. • A single dual-array transducer excites distinct unidirectional waves simultaneously. • Pure wave excitation and direction are addressed in the frequency–wavenumber domain. • Superposition of optimal excitation signals ensures simultaneous wave generation. • A dual PPM-EMAT simultaneously generated SH0 and SH1 modes in distinct directions. • The generation direction of each wave mode can be conveniently controlled. [ABSTRACT FROM AUTHOR]

The main disadvantage of electromagnetic acoustic transducers (EMATs) is their inefficiency in generating ultrasound. A new design of Rayleigh-wave EMAT is proposed with improved ultrasonic generation efficiency on non-ferromagnetic materials. The new EMAT's magnet is narrower than the meander-line coil used in the transducer and the coil itself has an uneven distribution of conductors. The principle of the new design is informed by finite element simulations, which show that the EMAT generates Rayleigh waves more efficiently by taking advantage of both horizontal and vertical magnetic fields of the magnet simultaneously. Experimental measurements verify that the maximum amplitude of the ultrasonic signal generated by the new design of EMAT working under a pulse-echo configuration has been enhanced by 90%, when compared to conventional designs. [ABSTRACT FROM AUTHOR]

Array transducers are commonly used to generate several types of waves, such as ultrasonic guided waves. They consist of a series of periodic wave source elements, and thus define a nominal or dominant wavelength of generated waves. A common approach to generating waves in a single direction is to use two identical wave sources, or two arrays of sources forming a dual-array transducer, that are separated by a quarter wavelength. Then, one source is activated a quarter of a time period after the other. In this way, constructive interference arises in one direction, whilst destructive interference arises in the opposite direction, making the total wave source unidirectional. This principle assumes that the frequency content of the generated wave is monochromatic. When there is some frequency bandwidth to the wave generated, such as with finite-length pulses, one cannot satisfy simultaneously constructive and destructive interference in opposite directions with dual-array transducers. For a non-dispersive wave mode, ideal destructive interference for pulsed waveform is obtained if one of the sources is inverted relative to the other, so that, in theory, complete cancellation can be achieved. Moreover, most guided wave modes are dispersive, meaning that, different frequencies propagate at different phase speeds. This limits the amount of wave cancellation in one direction and constructive interference in the other. Here, we describe the operation of the dual-array transducer in the frequency–wavenumber domain and use it to propose two new excitation methods, based on a frequency-dependent phase-shift strategy, that is designed through the dispersion relationship of the wave mode of interest. These provide ideal constructive or destructive interferences for dispersive waves, and achieve optimal unidirectional generation behaviour for dispersive wave modes. The new methods were experimentally assessed with shear horizontal ultrasonic waves, generated by a dual periodic permanent magnet electromagnet acoustic transducer. The optimal excitation signal yielded up to 30 dB unidirectionality, when generating the dispersive SH1 wave mode in an aluminium plate. • Unidirectional wave generation is addressed in the frequency–wavenumber domain. • Usual time-delay excitation methods were proved not appropriate for dispersive waves. • We propose a method that optimally generates dispersive waves unidirectionally. • A dual PPM-EMAT was used to generate shear horizontal waves in a single direction. • About 30 dB unidirectionality was achieved for the dispersive SH1 mode experimentally. [ABSTRACT FROM AUTHOR]

Array transducers can allow wavelength and frequency selectivity and can be used to generate different types of waves, such as ultrasonic bulk or guided waves. Dual-array transducers consist of two interlaced arrays, where the elements of each array are electrically connected. Therefore, by driving each array with a pair of phased pulses one can achieve a degree of wave generation control that allows unidirectional generation, unlike single array transducers that generate waves in both 0° and 180° directions with respect to the array's longitudinal axis. In this paper, we present a one-dimensional analytical model to determine the ultrasonic waves generated by dual-array transducers based on the excitability of the array in the frequency–wavenumber domain, the so-called operation region, determined by the joint spatial and temporal spectrum of the dual-array. We further exploited it to analyse the effectiveness of unidirectional generation with time-delayed excitation signals that provide ideal constructive or destructive interferences. The model also provides the time-domain received waveforms, which were compared to experimentally generated shear-horizontal ultrasonic guided waves, with a dual periodic permanent magnet array electromagnetic acoustic transducer, showing very good agreement. The adequate selection of the excitation signal allowed one to obtain up to about 40 dB unidirectionality experimentally. • Dual-array transducers are modelled in the frequency–wavenumber domain. • The interference mechanism of two types of time-delayed pulses is analysed. • A Dual-PPM EMAT is used to experimentally generate SH guided waves unidirectionally. • Model-predicted time-domain signals and experimental ones show excellent agreement. • About 40 dB unidirectionality was achieved with the ideal excitation pulses. [ABSTRACT FROM AUTHOR]

Abstract: The amplitude of an ultrasonic signal generated by electromagnetic acoustic transducers (EMATs) is typically low when compared to those generated by contacting transducers, which restricts the application of EMATs in the fields of nondestructive testing and nondestructive evaluation. The transmission process of a surface wave EMAT is studied, based on a previously established 3-D model, with the aim of enhancing the amplitude of ultrasonic waves generated by the EMAT. The effect of changing various EMAT parameters on the surface wave is investigated, by utilizing the orthogonal test method. Results indicate that after optimization, the signal amplitude of the EMAT has increased by 25.2%. [Copyright &y& Elsevier]

It is usually considered that the wavelength of guided waves propagating in plates is equal to the double spacing interval of the meander-line coil of electromagnetic acoustic transducers (EMATs). However, the actual value of the wavelength might deviate from the double spacing. This paper takes Shear Horizontal (SH) waves for instance to study the relationship between the wavelength deviation and the parameters of EMATs. It is shown that the wavelength deviation occurs when the operating point of EMATs does not locate on the dispersion curves of a certain SH mode. A wavelength measurement method is proposed to evaluate the deviation by the time-frequency analysis of detected wave signals. Experiments are set up to study the phenomenon of wavelength deviation and verify the validity of the proposed method. [ABSTRACT FROM AUTHOR]

Highlights: [•] We develop a spatial transversal filter model for meander-line coil EMATs. [•] The generated wavelengths have to belong to the wavelength spectrum of the EMAT. [•] The characteristics of multiple wavelengths will cause the multi-modes phenomenon. [•] Developing a new meander-line coil with special geometric pattern. [•] Proposing a method for removing the effect of multiple wavelengths. [Copyright &y& Elsevier]

Currently, the finite element method (FEM) and analytical calculation are widely employed for the modeling of electromagnetic acoustic transducers (EMATs). However, it takes long time for finite element calculation. Previous analytical models for bulk wave EMATs are generally considered separately and incompletely, and expressions of radiated wave fields contain infinite integrations and multiple singular points, which result in complex numerical computation. A complete model containing the Lorentz force and radiated wave field calculation for the EMAT with a spiral coil and a NdFeB permanent magnet is established. By introducing a current loop instead of the permanent magnet and adopting the truncated region eigenfunction expansion (TREE) method, the distributions of static and dynamic magnetic fields and their generated Lorentz forces are calculated. A series expansion method is proposed for the computation of radiated wave fields, which replaces the integration by series operation and avoids the solutions of singular points effectively. The Lorentz forces and radiated wave fields of a typical transducer are computed. The validity of the model is verified by FEM and experiments. Their good agreements verify the accuracy and validity of the model. [ABSTRACT FROM AUTHOR]

A three-dimensional model for Rayleigh wave electromagnetic acoustic transducers operating on the Lorentz force mechanism has been established by combining the finite element method and analytical calculation. Rayleigh waves generated by the Lorentz forces due to the static magnetic field and the dynamic magnetic field are calculated. The ratio of wire width to spacing interval (RWWSI) between neighbouring wires is chosen to study the coil parameter influence of Lorentz forces in generating Rayleigh waves. The vibration amplitude of Rayleigh waves due to the dynamic magnetic field is almost proportional to the reciprocal of the RWWSI, whereas that due to the static magnetic field decreases slowly with the increase of the RWWSI. The divergence angles of Rayleigh waves due to the dynamic magnetic field remain invariable when the RWWSI is less than 0.5, and that due to static magnetic field reaches the minimum values and has better detectability when the RWWSI is 0.5. The critical excitation current, at which Rayleigh waves due to static and dynamic magnetic fields are equal, changes sharply when the RWWSI differs. The accuracy of this method is verified by experiment. [ABSTRACT FROM PUBLISHER]

The elastic waves generated by traditional meander- line coil electromagnetic acoustic transducers (EMATs) propagate in two directions, overlapping the echo signals from defects with the same distances, and the defect echo signal is hard to distinguish from the edge-reflected signal when the EMATs are near the edge of a specimen. In this paper, a unidirectional EMAT with two meander-line coils is proposed. A finite element model is used to simulate the directivity of the Rayleigh and shear vertical waves generated by these EMATs. Six transducers are fabricated using the printed circuit technique. The unidirectional Rayleigh wave and shear vertical wave are tested, and the results agree well with the simulation. [ABSTRACT FROM AUTHOR]

Electromagnetic ultrasonic (EMU) Lamb waves excited by electromagnetic acoustic transducers (EMATs) possess many advantages in NDT. However, their characteristic multi-modes and dispersion are disadvantageous for inspection and restrict further improvements in their real applications. By deducing the excitation equation of EMU Lamb waves, the primary design parameters of EMATs and the characteristic equation of Lamb waves are combined, and excitation curves based on the excitation equation are plotted to aid the design of EMATs. The excitation characteristic of EMU Lamb waves on different thickness of plates is analyzed according to the excitation curves. The influence of multi-modes of EMU Lamb waves is minimized by choosing reasonable operating points and operating zones to excite a single-mode Lamb wave or multi-mode Lamb waves with identical or approximate propagation velocities. The influence of dispersion is minimized by searching corresponding points whose slope of group velocity tends to zero. The validity of the proposed method is verified by experiments. [ABSTRACT FROM AUTHOR]