Structural health monitoring (SHM) may offer an alternative to time based maintenance of safety critical components. Ultrasonic guided waves have recently emerged as a prominent option because their propagation carries information regarding the location, severity and types of damage. The fundamental shear horizontal ultrasonic guided wave mode has recently attracted interest in SHM because of its unique properties. This mode is not dispersive and has no attenuation due to fluid loading. In order to cover large areas using an SHM system, omnidirectional transduction is desired. Omnidirectional transduction of SH0 is challenging because of the required torsional surface stress. This paper presents a concept based on the discretisation of a torsional surface stress source using shear piezoelectric trapezoidal elements. Finite element simulation and experimental results are used to demonstrate the performance of this concept. The experimental modal selectivity is 17 dB and the transducer has a true omnidirectional behaviour.

We have obtained spectroscopy of a large sample of Wolf–Rayet stars in M33 with the Canada–France–Hawaii Telescope Multi‐Object Spectrograph (CFHT‐MOS), including 26 WC stars, 15 WN stars and a WN/C star. In general, spectral types are merely refined, although the spectral type of X9 from Massey & Johnson is revised from WNL?+abs to WC4+abs, whilst their G1 and C21 candidates are not confirmed as Wolf–Rayet stars. We also re‐examine the metallicity gradient of M33 from H ii regions and identify the present sample, lying in the inner disc, with 8.6 ≤ log(O/H) ≤ 8.9. Spectral types are in accord with similar regions in the Milky Way. Our large sample has allowed us to examine the claimed anticorrelation between WC linewidths and galactocentric distance by Schild et al. We find a much larger scatter, though there remains an absence of broad‐line WC stars in the inner disc and narrow‐line WC stars in the outer galaxy.

Structural health monitoring is a prominent alternative to the scheduled maintenance of safety-critical components. The nondispersive nature as well as the through-thickness mode shape of the fundamental shear horizontal guided wave mode (SH0) make it a particularly attractive candidate for ultrasonic guided wave structural health monitoring. However, plane wave excitation of SH0 at a high level of purity remains challenging because of the existence of the fundamental Lamb modes (A0 and S0) below the cutoff frequency thickness product of high-order modes. This paper presents a piezoelectric transducer concept optimized for plane SH0 wave transduction based on the transducer geometry. The transducer parameter exploration was initially performed using a simple analytical model. A 3D multiphysics finite element model was then used to refine the transducer design. Finally, an experimental validation was conducted with a 3D laser Doppler vibrometer system. The analytical model, the finite element model, and the experimental measurement showed excellent agreement. The modal selectivity of SH0 within a 20∘ beam opening angle at the design frequency of 425 kHz in a 1.59 mm aluminum plate was 23 dB, and the angle of the 6 dB wavefront was 86∘.

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Paratellurite, also known as α-tellurium dioxide, is a ceramic that is primarily employed for its interesting optical properties. However, this material’s crystal structure belongs to the 422 symmetry class that allows a unique piezoelectric behavior to manifest itself: deformation in pure face-shear. This means that crystal symmetry necessitates the piezoelectric tensor to have only a single non-zero coefficient, d123 = d14: such unique behavior has the potential to enable novel gyroscopic sensors and high-precision torsional microelectromechanical systems (MEMS) actuators, as pure face-shear can be used to induce pure torsion. Although α-TeO2 is one of the few known materials belonging to this symmetry class, considerable uncertainty in its single piezoelectric coefficient exists, with the few reported literature values ranging from 6.13 to 14.58 pC/N; this large uncertainty results from the difficulty in using conventional piezoelectric characterization techniques on paratellurite, limiting measurements to indirect methods. The novel applications that would be enabled by the adoption of this extraordinary material are frustrated by this lack of confidence in the literature. We therefore leverage, for the first time, a first-principles analytical physical model with electrochemical impedance spectroscopy (EIS) to determine, directly, the lone piezoelectric coefficient d123 = d14 = 7.92 pC/N.