Unidirectional torsional T(0,1) wave meta-transducer for circumferential-axial defect localization in pipes

Unidirectional excitation of axially propagated guided waves (GWs) is critical for pipeline structural integrity diagnostics, as it eliminates directional ambiguity in defect localization. Conventional methods achieve unidirectional GW excitation by arranging a pair of identical bidirectional wave transducers in a phased array configuration, which requires costly multichannel electronic systems. This study proposes a metasubstrate-based piezoelectric transducer (MBPT) for tunable generation and reception of the unidirectional torsional T(0,1) mode in pipes. The MBPT consists of a ring-shaped metasubstrate and thickness-shear PZT arrays. The designed metasubstrate enables the formation of a tailored phase gradient, thereby removing the need for additional electronic components to introduce time delays. The performance of the MBPT is thoroughly assessed through finite element simulations and experimental validation. Results show that by selecting appropriate combinations of MBPT arrays as actuators and sensors, the directionality of emission and detection for the T(0,1) mode can be accurately controlled. The efficacy of the proposed MBPT in defect detection is further confirmed. Using the Common Source Method, two artificial defects symmetrically positioned along the axial direction but located at distinct circumferential positions are successfully identified and localized. Due to its tunable unidirectional capabilities and cost-effectiveness, the MBPT demonstrates significant potential for structural health monitoring (SHM) and non-destructive testing (NDT) of pipelines.