Bi-annular shear-horizontal wave MPT tailored to generate the SH1 mode in a plate

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.