Bandwidth-optimised flexible ultrasonic sensor based on PZT thick-film fabrication for nondestructive testing

Ultrasonic non-destructive testing (NDT) is widely used in industrial applications; however, conventional rigid ultrasonic sensors suffer from poor conformability and degraded signal quality when applied to complex curved surfaces. In this study, a flexible lead zirconate titanate (PZT)-based ultrasonic sensor is developed using a silver foil substrate and electrohydrodynamic jet (E-jet) printing to fabricate thickness-controllable PZT thick films. By controlling the thicknesses of the substrate and the piezoelectric functional layer, a thickness-coordinated design strategy enables optimised operating bandwidth while preserving mechanical flexibility. The fabricated sensor exhibits a high piezoelectric coefficient of 126 pC/N and demonstrates effective sensitivity across a broad frequency range, from hundreds of kilohertz to several megahertz, through thickness-coordinated design. Experimental validations using thickness-tailored sensors show that high-frequency bulk-wave testing can detect internal defects as small as 0.7 mm, while low-frequency guided-wave inspection enables large-area defect localisation within a 500 mm × 500 mm region, highlighting the potential of the proposed sensor for versatile industrial NDT applications.