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• 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]

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]

• 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]

• 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]

• 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]