A breakthrough in Guided Wave – Excitation of shear-horizontal (SH) wave is not a problem any longer

Excitation of non-dispersive shear-horizontal (SH) waves in plates is always a challenge. Recently, we successfully excited single-mode SH0 wave in plates using a face-shear (d24) mode piezoelectric transducer. Meanwhile, the d24 mode transducer can selectively receive SH0 wave and filter Lamb waves. This work was published as a Letter in the IOP journal Smart Materials and Structures  (http://dx.doi.org/10.1088/0964-1726/25/11/11LT01）.

Ultrasonic guide wave inspection is an advanced technique in the field of nondestructive testing (NDT) and structural health monitoring (SHM). Compared with bulk waves, guided wave has less attenuation and thus can propagate for a long distance, very promising for quick defect detection and monitoring. In plates, there exist both Lamb waves and shear-horizontal waves. Lamb waves are easy to excite and sense, but they are dispersive, which limits their propagation distance. Meanwhile, mode transfer occurs when Lamb wave encounters defects or structure boundaries, which makes the received signal rather complicated. The fundamental shear-horizontal (SH0) wave is the only non-dispersive wave mode in plates, it can hardly transfer to other wave mode during propagation, which makes the signal processing very simple. However, the single-mode SH0 wave is rather difficult to excite.

In late 1970s, single-mode SH0 wave had been excited by Thompson using the electromagnetic acoustic transducers (EMAT). EMAT works in a non-contact mode, which is promising for high-temperature cases. However, the energy conversion efficiency of the EMAT is rather low (excitation power ~kW, receiving signal ~0.1mV), about 5-6 orders lower than the piezoelectric transducers. Thus, EMAT must be operated using high-power amplifiers, which makes the detection system bulky and heavy, not suitable for SHM and long distance NDT. In recent years, some scholars have attempted to excite SH0 wave using piezo-fiber based composites or piezoelectric single crystals, while the single mode SH0 waves still cannot be excited.

Our group firstly realized the d36 face-shear mode in piezoelectric ceramics (APL,107, 122902, 2015; http://doi.org/10.1063/1.4931685), then we excited SH0 waves in plates by using d36 mode transducers. Unfortunately, the single-mode SH0 wave still cannot be excited (JAP, 119, 174101, 2016). In late 2015, we changed the strategy. By using an in-plane poled piezoelectric ceramics and applying electric field along another in-plane direction, we realized pure face-shear (d24) mode in piezoelectric ceramics. Then, by using the d24 mode piezoelectric wafer as actuators, we successfully excited single-mode SH0 wave in an aluminum plate. The d24 mode transducer can also act as a selective SH0 wave sensor by filtering the Lamb waves. More recently, we developed an omni-directional SH0 wave transducer in plate using a planar ring array of d24 piezoelectric wafers (Ultrasonics 74: 167-173, 2017，http://dx.doi.org/10.1016/j.ultras.2016.10.011), and excited single-mode fundamental torsional waves T(0,1) in pipes using ring array of d24 piezoelectric wafers (Smart Mater Struct 26: 025021, 2017, https://doi.org/10.1088/1361-665X/26/2/025021). 

The significance of these works is obvious: it provided an easy and reliable way to excite SH0/T(0,1) wave in plates/pipes, which could greatly promote the application of SH0/T(0,1) waves in NDT/SHM. Meanwhile, as it is cost-effective and does not need external load when mounting, the d24 mode piezoelectric transducer is expected to dominate the SHM system of long-distance pipes and large-area plates in future.