Phononic crystal sensing devices for enhanced nonlinear ultrasonic inspection

Nonlinear ultrasonic methods have proven to be far more sensitive than conventional linear ultrasounds to early detection of damage such as micro- or partially closed cracks, by measuring small nonlinear ultrasonic waves generated by the defect. However, the efficiency of nonlinear ultrasonic testing and their large-scale use in industrial applications is still limited by undesired instrumentation effects caused by high-power ultrasonic signal transmission, which may mask nonlinear ultrasonic waves and prevent damage detection. This paper aims to overcome the challenge of accurate detection of nonlinear ultrasonic signals by designing and developing new portable and compact phononic crystal (PC) waveguide transducers, which enhance the performance of nonlinear ultrasonic methods by achieving unprecedented sensitivity and reliability. These innovative PC sensing devices are material-based tunable filters that allow natural propagation of nonlinear ultrasonic waves and suppression of undesired instrumentation effects. PC waveguide transducers can be integrated directly on the monitored component and are easily fabricated using additive manufacturing (3D printing) processes. Their design and optimisation is driven by a pioneering theoretical model based on perturbative couple-mode theory of guided wave propagation in structures with periodic corrugated surface profiles. Experimental nonlinear ultrasonic tests confirmed these theoretical predictions and indicated that PC sensing device are able to enhance the sensitivity of nonlinear ultrasonic inspection for various materials and components.