Unidirectional Frequency-Steerable Acoustic Transducer for guided ultrasonic wave damage imaging

Ultrasonic guided waves (GWs) are extensively utilized in nondestructive evaluation and structural health monitoring (SHM) fields. Typically, phased-array GW-based inspections consist of numerous piezoelectric transducers permanently attached to the monitored structure. However, these systems face challenges such as bulky hardware, a large number of transducers and cables for individual element control, complex circuitry and signal processing, high power consumption, and consequently high integration costs. To overcome these limitations, shaped transducers featuring inherent beam steering properties, such as Frequency Steerable Acoustic Transducers (FSATs) can be adopted. FSATs exploit a frequency-dependent spatial filtering effect, which is achieved by properly patterning the electrodes of the piezoelectric transducers. This allows the direction of the generated or sensed wave to be controlled simply by the spectral content of the actuated or received signal, a process so-called "In-sensor" signal processing. Initial generations of FSATs face a 180° ambiguity, where waves are simultaneously generated or sensed in both forward and backward directions. This could lead to uncertainty in defect localization or generate undesirable reflections. In this work, a novel unidirectional FSAT is proposed to eliminate this ambiguity through a new design strategy for unidirectional wave generation and sensing, addressed in the wavenumber domain. Finite element simulations and experimental testing on an aluminum plate validated the proposed frequency-dependent unidirectional beam steering concept. Additionally, the transducer was successfully used in pulse-echo mode for damage imaging, demonstrating 98% localization accuracy. The proposed embedded system can substantially reduce the software and hardware requirements of conventional solutions, paving the way for the development of permanent inspection systems.