Search

• A tunable bidirectional SH wave transducer was proposed. • The transducer was based on antiparallel d 15 piezoelectric strips (APS) • Simulations and experiments validated the proposed SH wave transducer. • The transducer can selectively suppress SH 0 wave and excited SH 1 wave. • The transducer can also excite circumferential SH wave in pipelines. Guided wave based defects inspection is very promising in the field of structural health monitoring (SHM) and nondestructive testing (NDT) due to its less dissipation and thus long distance coverage. In comparison with the widely used Lamb waves, shear horizontal (SH) waves are relatively simple but less investigated probably due to the traditional notion that SH waves were usually excited by electromagnetic acoustic transducers (EMAT). In this work, we proposed a tunable method to excite single-mode bidirectional SH waves in plates using antiparallel thickness-shear (d 15) piezoelectric strips (APS). The proposed SH wave driving mechanism here is similar to that by using the periodic permanent magnetics (PPM) based EMAT with the period of strips equal to half of the wavelength. Both finite element simulations and experiments were conducted to validate this transducer in excitation of bidirectional SH waves. Results show that the Lamb waves excited by single piezoelectric strip can be suppressed very well. The radiation angle of the excited bidirectional SH wave can be reduced by extending the strip length, increasing the driving frequency or using more strips. Moreover, the APS transducer can selectively excite SH 1 wave and suppress the SH 0 wave at 174 kHz and 273 kHz in a 10 mm-thick aluminum plate. Considering its simple structure, flexible design and low excitation energy, the APS SH wave transducer is expected to be widely used in near future. [ABSTRACT FROM AUTHOR]

• A SH wave phased array system was proposed for monitoring of plate-like structures. • The system can detect both surface defects and inside defects without baseline. • The system can detect a through-thickness hole as small as 2 mm. • The system can detect multi defects simultaneously. Guided wave based structural health monitoring (SHM) has been regarded as an effective tool to detect the early damage in large structures and thus avoid possible catastrophic failure. In recent years, Lamb wave phased array SHM technology had been intensively investigated while the inherent multi-mode and dispersive characteristic of Lamb waves limits its further applications. In comparison, the fundamental shear horizontal (SH 0) wave is non-dispersive with uncoupled displacements and thus more promising for defect detection. In this work, we proposed an SH 0 wave linear phased array SHM system based on our recently proposed omni-directional SH wave piezoelectric transducer (OSH-PT). Firstly, the working principle of the phased array system was presented and the total focusing method (TFM) was employed for imaging. Then the SH 0 wave mode generated by the OSH-PT was confirmed in a defect-free plate. Finally, experiments were carried out to examine the performances of this SHM system. Results showed that the proposed system can detect a through-thickness hole as small as 2 mm in diameter with the location error only about 6.3 mm. Moreover, the proposed phased array system can also detect multi-defects. Due to its low working frequency and thus low attenuation, the proposed phased array system is capable of monitoring large structures. This work will lay the foundations of SH wave based phased array SHM. [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]

Highlights • A novel-joint damage detection algorithm based on Lamb waves and electro-mechanical impedance methods is developed. • A mobile transducer set contains PZT patches is designed. • A baseline-free damage characterizing algorithm is presented. Abstract Lamb waves and electro-mechanical impedance (EMI) based methods are increasingly used in damage detection owing to their high sensitivity to small structural defects. Lamb wave based methods are effective in detecting damages in a large area and electro-impedance based methods are suitable for characterizing the identified damage. Based on these two methods, a novel combined damage detection method is presented in this research. To achieve this, first, a mobile transducer set is developed, which can be used for both the Lamb waves and EMI based methods. Then, a baseline-free damage detection strategy that combines the Lamb waves and EMI methods is presented. Finally, a laboratory-sized test piece is used to validate the effectiveness of the proposed approach. The results achieved with the application of the presented combined method for characterizing an L-shape crack in an aluminum plate show better location accuracy and detection efficiency than those obtained by applying only one method. [ABSTRACT FROM AUTHOR]

Highlights • Damage localization method by the time-reversed Lamb waves is proposed for plates. • Mode conversion behavior of Lamb waves is considered in the time reversal process. • Localization of a notch-type defect in plates is numerically simulated. • Focused spots appear at the damage location by the proposed time reversal method. Abstract A damage localization method based on the time reversal focusing of the mode-converted scattered Lamb wave is proposed for plate structures with a non-symmetric defect in the thickness direction. Dual transducers are attached symmetrically on the upper and lower surfaces of the plate to selectively emit and receive the lowest-order symmetric (S0) and antisymmetric (A0) modes. The localization of damage is achieved by the numerical time-reversed (TR) simulation of the mode-converted Lamb wave generated at the defect. To investigate the validity of the proposed method, the signals of the Lamb waves in a plate with a partial-thickness notch are numerically simulated by the three-dimensional elastodynamic finite integration technique (EFIT). When the S0 mode is emitted in the damaged plate, not only the S0 mode is scattered but also the A0 mode is generated due to mode conversion at the notch. Similar mode conversion behavior is confirmed when the A0 mode is emitted. The time reversal of the mode-converted scattered Lamb waves creates focused spots at the damage location without using baseline data for the undamaged plate. The proposed method reduces the magnitude of the artifacts compared to the time reversal of the non-mode-converted Lamb wave, and yields the focused spot whose size is associated with the size of the notch and the half wavelength of the time-reversed wave mode. Furthermore, the proposed method is applied to a plate with a notch-type defect adjacent to an a priori known through-thickness hole, demonstrating the damage localization in a relatively complicated structure. [ABSTRACT FROM AUTHOR]

Highlights • Incremental Lamb wave scattering from a notch emanating from a through-hole is analyzed. • Multiple frequencies and hole diameters are considered as well as a notch with no hole. • Numerical results are validated with experimentally measured scattering patterns. • The presence of even a small hole significantly affects notch incremental scattering. • Incremental scattering from off-angle incident waves is a strong function of frequency. Abstract Lamb wave scattering from a crack originating at a through-hole is of practical importance because of the abundance of fastener holes used in engineering structures. Notches are often used to simulate cracks so that Lamb wave methods can be more conveniently investigated in the laboratory. A linear, three-dimensional finite element model is employed in this paper to study incremental scattering of the fundamental anti-symmetric (A 0 ) Lamb wave mode from notches emanating from through-holes. The term “incremental scattering” refers to the change in scattering caused by introduction of the notch and is motivated by structural health monitoring for which transducers are fixed and signal changes are interpreted to detect damage. Far-field angular scattering patterns are generated for multiple incident angles and frequencies, and such patterns are experimentally validated at one frequency by laser vibrometry measurements. Comparisons are made between a vertical notch alone (no hole) and notches located above and below the through-hole. Additionally, holes of different sizes are considered to investigate the effect of hole diameter on incremental scattering patterns. Results show that the presence, location and size of the through-hole affect both the shape and strength of notch incremental scattering patterns. [ABSTRACT FROM AUTHOR]

• Measurement of the reflective function of highly attenuative material by using a pair of ultrasound transducers. • Transducer-pair method is proposed to acquire V(z) by using conventional ultrasound transducers with a high f-number and small aperture. • By comparing the results obtained from both the single transducer and the transducer-pair approach, the feasibility is demonstrated. Ultrasonic reflectivity using a V(z) technique is a powerful characterization method in acoustic microscopy to measure the elastic properties of materials. Conventional techniques generally use a low f-number with high frequency; however, to measure the reflectance function of the highly attenuative material, a low frequency is essential. In this study, the transducer-pair method based on Lamb waves is used to measure the reflectance function of a highly attenuative material. The results demonstrate the feasibility of the proposed method using a commercial ultrasound transducer with high f-number. [ABSTRACT FROM AUTHOR]

In Lamb wave imaging, MVDR (minimum variance distortionless response) is a promising approach for the detection and monitoring of large areas with sparse transducer network. Previous studies in MVDR use signal amplitude as the input damage feature, and the imaging performance is closely related to the evaluation accuracy of the scattering characteristic. However, scattering characteristic is highly dependent on damage parameters (e.g. type, orientation and size), which are unknown beforehand. The evaluation error can degrade imaging performance severely. In this study, a more reliable damage feature, LSCC (local signal correlation coefficient), is established to replace signal amplitude. In comparison with signal amplitude, one attractive feature of LSCC is its independence of damage parameters. Therefore, LSCC model in the transducer network could be accurately evaluated, the imaging performance is improved subsequently. Both theoretical analysis and experimental investigation are given to validate the effectiveness of the LSCC-based MVDR algorithm in improving imaging performance. [ABSTRACT FROM AUTHOR]

To develop an effective baseline-free damage detection strategy using the time-reversal process (TRP) of Lamb waves in thin walled structures, it is essential to develop a good understanding of the parameters that affect the amplitude dispersion and consequently the time reversibility of the Lamb wave signal. In this paper, the effects of adhesive layer between the transducers and the host plate, the tone burst count of the excitation signal, the plate thickness, and the piezoelectric transducer thickness on the time reversibility of Lamb waves in metallic plates are studied using experiments and finite element simulations. The effect of adhesive layer on the forward propagation response and frequency tuning has been also studied. The results show that contrary to the general expectation, the quality of the reconstruction of the input signal after the TRP may increase with the increase in the adhesive layer thickness at certain frequency ranges. Similarly, an increase in the tone burst count resulting in a narrowband signal does not necessarily enhance the time reversibility at all frequencies, contrary to what has been reported earlier. For a given plate thickness, a thinner transducer yields a better reconstruction, but for a given transducer thickness, the similarity of the reconstructed signal may not be always higher for a thicker plate. It is important to study these effects to achieve the best quality of reconstruction in undamaged plates, for effective damage detection. [ABSTRACT FROM AUTHOR]

This study proposes a modified virtual time-reversal (VTR) algorithm for baseline signal-free damage detection in plate-like structures. The physical actuation and sensing of Lamb waves are performed using a broadband Gaussian excitation instead of the conventional narrowband modulated tone burst excitations. The forward response and the reconstructed signal due to the time-reversal process for a narrowband input signal are then constructed virtually using the broadband transfer function. The method eliminates the possibility of numerical errors encountered in the conventional VTR method based on narrowband excitations. It is also more efficient than the traditional VTR algorithm because it can probe at multiple excitation frequencies using a single measurement for each sensing path. This modified VTR algorithm is employed in the recently developed refined time-reversal method (RTRM), which uses an extended signal length of the reconstructed signal for computing damage index (DI) and probes the structure at the best reconstruction frequency. The new method is termed the virtual refined time-reversal method (VRTRM). The DIs based on the VRTRM are used in the reconstruction algorithm for probabilistic inspection of defects to achieve baseline signal-free localization of damages. The efficacy of the proposed VRTRM for damage localization is experimentally verified with the established technique RTRM. Experiments are performed in an aluminium plate equipped with a network of surface-bonded piezoelectric patch transducers to illustrate the conventional VTR's pitfalls and the modified VTR's accuracy for a single mass damage scenario. The results show that the proposed VRTRM is as accurate as the established technique RTRM in estimating the reconstructed signals and localizing a block mass damage. Finally, the VRTRM is shown to localize in a dual damage scenario with excellent accuracy. In contrast, the conventional main mode-based VTR method fails to localize the damages with or without single-mode tuning. • A modified virtual time-reversal (VTR) method is proposed using broadband measurements. • The method eliminates possible numerical errors in conventional VTR method. • It can compute reconstructed signals for multiple frequencies using a single measurement. • The new VTR is combined with refined time-reversal method for baseline-free damage localization. • The method is shown to very efficient and accurate in single and dual-damage scenarios. [ABSTRACT FROM AUTHOR]

Abstract: Defect imaging using scanning laser source technique has been investigated for a plate with rounded defects and notch-type defects in our previous studies. This paper examines frequency dependence of the defect images with both calculations and experiments in order to acquire clearer images. Both calculation and experimental results for a straight notch revealed that clearer images of notch-type defects can be obtained in the range of low frequency-thickness product below about 200kHzmm. Moreover, images of the defects of various shapes were obtained by synthesizing images from eight receiving transducers, and similarly to the case of the straight notches, they became more clearly in the low frequency range. [Copyright &y& Elsevier]

Abstract: The aim of this work is to study the fundamental Lamb modes interaction with defects in isotropic plates. For these experimental investigations, symmetrical notches with various depths milled in aluminum plates are considered. Moreover, the incident Lamb wave of a specific mode is generated by means of two identical thin piezoceramic transducers placed at the opposite sides of the plate. The waves scattered by the notch are recorded with conventional transducers located on the plate surface in front and behind the defect. The selection of the A 0 or the S 0 modes is obtained by exciting the transducers with anti-phased or in-phased signals, respectively. Furthermore, a calibration process is investigated to correct errors caused by the presence of the receiver between the emitters and the defects. The power reflection and transmission coefficients are then obtained and the power balance is verified. Finally, these measurements are compared successfully with those obtained by a numerical method using the finite-element modeling described in a previous work. [Copyright &y& Elsevier]

Abstract: Active fiber composites (AFC) are thin and conformable transducer elements with orthotropic material properties, since they are made of one layer of piezoelectric ceramic fibers. They are suitable for applications in structural health monitoring systems (SHM) with acoustic non-destructive testing methods (NDT). In the presented work the transfer behavior of an AFC as an emitter of transient elastic waves in plate-like structures is investigated. The wave field emitted by an AFC surface bonded on an isotropic plate was simulated with the finite-difference method. The model includes the piezoelectric element and the plate and allows the simulation of the elastic wave propagation. For comparison with the model experiments using a laser interferometer for non-contact measurements of particle velocities at different points around the AFC on the surface of the plate were performed. Transfer functions defined as the ratio of the electric voltage excitation signal and the resulting surface velocity at a specific point are separately determined for the two fundamental Lamb wave modes. In order to take the orthotropic behavior of the AFC into account the transfer functions are determined for several points around the AFC. Results show that the AFC is capable to excite the fundamental symmetric and antisymmetric Lamb wave mode. The antisymmetric mode is mainly radiated in the direction of the piezoelectric fibers, while the symmetric mode is spread over a larger angle. The amplitudes of the emitted waves depend on the frequency of the excitation as well as on the geometric dimensions of the transducer. [Copyright &y& Elsevier]

• Ultrasonic flow meter with wide dynamic range. • Use of novel transducers that cause minimal disturbance to fluid flow. • Very low pressure drop across the flow meter device. This paper describes the development of an ultrasonic flow meter using a new type of transducer called the mode coupling transducer (MCT). The mode coupling transducer consists of a thin plate of a suitable piezoelectric material on which an interdigital transducer (IDT) is deposited for the generation and detection of plate acoustic waves. The flow meter consists of two such transducers mounted on opposite walls of a pipe through which the fluid is flowing. The transducers used in this work were fabricated on 0.5 mm thick plates of lithium niobate and had a center frequency of 1 MHz. The prototype device developed here can measure flow rates over a range from less than 0.2 lpm (liters per minute) to greater than 100 lpm. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a new experimental measurement method for leaky Lamb waves propagating in a piezoelectric plate immersed in a conductive fluid. The measurement system is a low-frequency version of lens-less acoustic microscopy which has been developed based on a line-focus double-layer PVDF transducer. The transducer and its defocusing measurement system can perform V(z) measurements on a sample plate immersed in a fluid, and therefore can obtain the leaky Lamb wave velocities with high accuracy. An X-cut LiNbO3 plate is investigated with this experimental measurement system to find out its fluid-loading effects, especially the conductive loading effects by water of various conductivities. Angular dependence of this conductive loading effect along different propagating directions on the X-cut LiNbO3 plate is measured. It is found out the conductive loading effects are strongly dependent on the piezoelectric coupling factor. Theoretical calculations based on partial wave theory have also been carried out and compared with experimental data. Good agreements have been observed. [Copyright &y& Elsevier]

The well known zero in the group velocity of the first-order symmetric (S1) plate wave mode has been exploited in air-coupled ultrasonic imaging to obtain significantly higher sensitivity than can be achieved in conventional air-coupled scanning. At the zero group velocity point at the frequency minimum of the S1 mode, a broad range of wavenumbers couple into the first-order symmetric mode at nearly a constant frequency, greatly enhancing transmission at that frequency. Coupled energy remains localized near the coupling point because the group velocity is zero. We excite the mode with a broadband, focussing, air-coupled transducer at the frequency of the zero group velocity point in the S1 mode. By exploiting the efficient coupling at the zero group velocity frequency, we have easily imaged a single layer of Scotch tape attached to a 6.4-mm thick Plexiglas plate and 3.2-mm Teflon inserts in a composite laminate. [Copyright &y& Elsevier]

For inspection of a storage tank and pipeline in service, the application of an automatic inspection system (nondestructive inspection robot) is desirable, because manual inspection is difficult to perfectly and exactly perform due to the enormous amount of inspection needed. However, an ultrasonic nondestructive inspection robot with a piezoelectric oscillator needs to touch only the material surface to be directly inspected using a coupling medium. That is, the material surface and the sensor must always be held by constant pressure in the vertical direction on the material side. Actually, it is difficult to overcome these problems; thus an ultrasonic inspection robot could not be widely applied. We then tried to develop an ultrasonic inspection robot with an electromagnetic acoustic transducer (EMAT) which did not require a coupling medium to inspect the circumferential pipe parts. We developed a special EMAT that could transmit and receive alternately a Lamb wave with high sensitivity and a SH-plate wave without influence by the welded part. The method by which the inspection robot turned around the direction of the steel pipe surroundings was executed by observing the tape pasted in the direction of the steel pipe surroundings with an installed CCD camera. In this report, the basic mechanism of this inspection robot and an examination of results are described. [Copyright &y& Elsevier]

This work describes an investigation into the development of a new health monitoring system for aeronautical applications. The health monitoring system is based on the emission and reception of Lamb waves by multi-element piezoelectric transducers (i.e., arrays) bonded to the structure. The emitter array consists of three different elementary bar transducers. These transducers have the same thickness and length but different widths. The receiver array has 32 same elements. This system offers the possibility to understand the nature of the generated waves and to determine the sensitivity of each mode to possible damage. It presents two principal advantages: Firstly, by exciting all elements in phase, it is possible to generate several Lamb modes in the same time. Secondly, the two-dimensional fourier transform (2D-FT) of the received signal can be easily computed. Experimental results concerning an aluminum plate with different hole sizes will be shown. The A0-, S0-, A1-, S1- and S2-modes are generated at the same time. This study shows that the A0 mode seems particularly interesting to detect flaws of this geometrical type. [Copyright &y& Elsevier]

A new 4D space-time-wave number-frequency representation Z(x,t,k,f) is introduced. The Z(x,t,k,f) representation is used for processing 2D space-time signal collection issued from wave propagation along a 1D medium. This representation is an extension along the time dimension of the space-wave number-frequency representation. The Z(x,t,k,f) representation is obtained by short time-space 2D Fourier transforming the space-time collection. The Z(x,t,k,f) representation allows the characterization transient aspects of wave generation and propagation in both space and time dimensions.The Z(x,t,k,f) representation is used to experimentally investigate Lamb wave generation and propagation around a cylindrical shell (relative thickness is equal to 0.03) surrounded by water and excited by a pulse (0.1 μs duration with 1–5 MHz transducers). Three kinds of fluids have been used inside the shell: air, water, propanol. In all the cases, the Z(x,t,k,f) analysis clearly identify the reflected field on the insonified side of the shell and it allows the measurement of the local reflection coefficients R(x,t,k,f). The generation and the propagation of Lamb waves are also quantified.For the liquid filled shells, the multiple internal reflections are revealed by Z(x,t,k,f) analysis: the local transmission coefficients T(x,t,k,f) are also measured. When local matching conditions allows Lamb wave generation, the multiple regeneration of Lamb wave is observed. Based on these results, a link is establish toward the theoretical results obtained by steady state approach and Sommerfeld–Watson transform. [Copyright &y& Elsevier]

Lamb waves are normally utilized for inspecting thin metal sheets. Wheel type probes using piezoelectric oscillators have generally been used as the sensors for Lamb waves. Recently, the electromagnetic acoustic transducer (EMAT) has been developed and is beginning to be used as a Lamb wave detector. We have developed a useful type of transducer for Lamb waves. The new EMAT consists of a meander coil with a narrow distance of 2.5 mm and has a symmetrical structure in the vertical direction for both surface sides. The new EMAT can generate Lamb waves with variable wavelengths corresponding to the frequency range from approximately 300 kHz to 2.5 MHz and multiple modes, and can also generate selected symmetrical and anti-symmetrical mode Lamb waves. It is demonstrated that the optimum Lamb wave mode could be produced by the appropriate positioning of the EMATs and controlling the phase (same or inversed) of the electrical signal driving the device. The described EMAT can be used to examine steel (or other material) sheets of different thickness. It is also shown that the S0 (0.3 MHz) mode Lamb wave is the most effective for the deepest (up to 6 mm) penetration. [Copyright &y& Elsevier]

• The state of the art of SH wave piezoelectric transducers and EMATs is reviewed. • The principles, merits and drawbacks of various SH wave transducers are discussed. • Applications for SH wave-based SHM techniques in plates and pipes are presented. • Piezoelectric, magnetostrictive principles and excitation mechanisms are covered. Shear horizontal (SH) waves are of great importance in structural health monitoring (SHM) and nondestructive testing (NDT), since the lowest order SH wave in isotropic plates is non-dispersive. The SH waves in plates, circumferential SH waves and torsional waves in pipes have remarkable resemblances in dispersion characteristics and wave structures, so the latter two can also be called as SH waves in pipes. This paper reviews the state-of-the-art research on SH wave transducers for SHM and NDT. These transducers are grouped into the following categories: Lorentz-force-based electromagnetic acoustic transducers (EMATs), magnetostrictive EMATs, shear wave piezoelectric wedge transducers, thickness-shear piezoelectric transducers and face-shear piezoelectric transducers. The working principles, applications, merits and limitations of different kinds of SH wave transducers are summarized, with a focus on discussing the various configurations for exciting and receiving directional, omnidirectional SH waves in plates and torsional waves in pipes. This paper is expected to greatly promote the applications of SH waves in SHM, NDT and the related areas such as elastic metamaterials. [ABSTRACT FROM AUTHOR]

• Full-field measurements in two dimensions simulated for calculations. • Cramer-Rao bound is applied to thickness estimates from Lamb wave measurements. • Simulation model used to drive theoretical sensitivity computations. • Reflections accounted for in Cramer-Rao bound calculations. • Relation of sensitivity to frequency, source distance, number of sources explored. A persistent question in wavenumber analysis in the estimation of thickness from a steady-state wave field is identifying the theoretical sensitivity of the system. A well-known trade-off between spatial frequency/wavenumber resolution and thickness resolution exists. The current work presents a calculation of the Cramer-Rao Lower bound (CRLB), specifically as applied to thickness estimates, for a 2-dimensional multi-mode waveform in Gaussian noise. Cases of near-field and far-field excitation are considered, and transducer position with respect to the scan area is also varied. Additionally, we consider the CRLB in a plate with multiple sources, simulated as sources placed on the boundary of the plate. We conclude by presenting the CRLB values in terms of frequency for various thicknesses, and by presenting optimal excitation frequencies for a nominal thickness, based on the CRLB. [ABSTRACT FROM AUTHOR]

• An optimization model is proposed to design an array of actuating transducers. • Uniform energy distribution in an arbitrary shaped domain is gained using the model. • Time delay values of the arrays are optimized by a secondary optimization model. • The model is examined in two case studies and a successful experimental validation. The use of an array of transducers to excite guided Lamb waves, within a plate or any complex structure, usually leads to a variation in the energy on the propagation direction. In this study, an optimization model is proposed to design an array of transducers to provide uniform energy distribution in a domain of an arbitrary shape. The model is based on finding the optimal placements of the transducers and the optimal time delay for excitation by using a genetic algorithm. The efficiency of the model was tested on an elliptically shaped domain, then on an arbitrarily shaped domain. Both cases showed promising results using various configurations/patterns of transducers. The method was experimentally validated on an aluminium alloy plate for two patterns of transducers including six and eight piezoelectric elements. [ABSTRACT FROM AUTHOR]

• Lens focused piezoelectric transducers for shock wave lithotripsy. • 3 and 4 confocal transducers setup. • Acoustic characterization of both confocal setup. • In-vitro fragmentation evaluation of both confocal setup. This work focuses on the evaluation of a type of piezoelectric lithotripter with similar dimensions of a commercial lithotripter and composed of either 3 or 4 large lens focused piezoelectric transducers set either in a confocal coplanar C-shape or a confocal spherical shape. Each transducer is made with a 92 mm diameter 220 kHz flat piezoelectric ceramic disc and a 3D printed acoustic lens. Both confocal setups pressure field were measured with a fiber optic hydrophone, and in vitro fragmentations of 13 mm diameter and 14 mm length cylindrical model stones were done in a 2 mm mesh basket. The acoustic characterization of the three transducers confocal setup revealed a disc shaped focal volume, with a 2.2 mm width on one axis and a 9.6 mm width on the other, and a peak positive pressure of 40.9 MPa and a peak negative pressure of −16.9 MPa, while the focus of the four transducers confocal setup was similar to a traditional narrow focus high pressure lithotripter with a focus width of 2.1 mm, and a peak positive pressure of 71.9 MPa and peak negative pressure of −24.3 MPa. Both confocal setups showed in vitro fragmentation efficiency close to a commercial electroconductive lithotripter. [ABSTRACT FROM AUTHOR]

• A nearly pure SH1 wave in a plate is generated by an MPT. • Two optimally configured annular patches are designed for the MPT. • A high ratio of the SH1 mode to the SH0 mode was achieved. The use of a specific wave mode is critical in ultrasonic non-destructive evaluations but it is difficult to generate a specific mode, especially a higher mode at a frequency where there exist multiple wave modes. Here, we propose a compact omnidirectional shear-horizontal wave MPT (magnetostrictive patch transducer) having two annular magnetostrictive patches for the generation of a nearly pure SH1 (second shear-horizontal) mode in a plate for frequencies above the first cutoff frequency. While a common wavelength-matching approach would typically require the use of several patches and does not appear completely to eliminate unwanted omnidirectional wave modes, the proposed MPT, with only two annular patches, generates the desired SH1 mode predominantly with the unwanted SH0 (first shear-horizontal) mode nearly eliminated. For the design, the geometries of the annular patches are optimally configured to maximize the ratio of the SH1 mode to the SH0 mode. Numerical simulations and experiments confirm the effectiveness of the proposed bi-annular shear-horizontal wave MPT. Because the SH1 mode near the first cutoff frequency is highly dispersive, the developed transducer is expected to be critically useful in various applications, such as ultrasonic inspections of wall thinning. [ABSTRACT FROM AUTHOR]