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• 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]

• An omnidirectional high-order SH wave piezoelectric transducer (OSH n -PT) was proposed. • The excitation function of the OSH n -PT was derived explicitly. • Simulations and experiments validate the significant mode selectivity of the OSH n -PT. • The SNR of the OSH 1 -PT was over 19 dB in pitch-catch tests (30.7 dB at 490 kHz). • An OSH 2 -PT was theoretically designed and validated by FE simulations. Guided wave tomography, as an advanced structural health monitoring (SHM) method, has offered a feasible solution to wall thickness quantification which is essential in petrochemical industries. However, previously used low-frequency Lamb waves (A 0 and S 0) limit the resolution of tomography. Recently, the first-order shear horizontal guided wave (SH 1) was found very promising in tomography for its capability in resolution improvement. However, the SHM-required omnidirectional piezoelectric transducers for selectively generating and receiving the SH 1 wave, namely OSH 1 -PT, are not available yet. In this work, a general method was developed to design an OSH 1 -PT based on the thickness-poled PZT half-ring configuration. Firstly, the excitation function of the OSH 1 -PT was explicitly derived and validated through finite element simulations. Secondly, a design formula was obtained and used to determine the size of the OSH 1 -PT. Then, the designed OSH 1 -PT was fabricated and tested by using a 2D laser Doppler vibrometer. Significant mode selectivity was observed in all directions (0 ∼ 90°) with the excited SH 1 to SH 0 ratio higher than 15 dB. Pitch-catch tests were conducted from 400 ∼ 520 kHz and the received SH 1 to SH 0 ratio was found higher than 19 dB at all frequencies and reached its maxima of 30.7 dB at 490 kHz, which is very close to the designed working frequency of 500 kHz. Finally, an OSH 2 -PT was theoretically designed and validated by FE simulations. Due to its simplicity and effectiveness in designing the OSH n -PT, the proposed method is expected to pave the road to wide applications of high-order SH wave tomography. [ABSTRACT FROM AUTHOR]