Localized structural thinning monitoring with high-order Lamb wave mode enabled by direct-write ultrasonic transducers

This article presents a method for monitoring depth of a localized thinning in a structure by analyzing the arrival time and peak absolute amplitude of selected high-order ultrasonic Lamb waves. The high-order Lamb waves are generated and detected by piezoelectric ultrasonic transducers that are in-situ fabricated on a structure, known as direct-write ultrasonic transducers (DWTs). Unlike conventional discrete ultrasonic transducers, a DWT is lightweight, low-profile, and has high conformality, allowing implementation in confined space due to its reduced design footprint. Additionally, the DWT exhibits enhanced excitability and detectability for the high-order Lamb wave due to its enhanced Lamb wave mode selectivity attributed to its comb-shaped electrode design. This design simplifies signal analysis by directly selecting the high-order Lamb wave mode that possesses the fastest group velocity as shown in the dispersion curves. Our experimental test demonstrates great agreement with the theoretical analyses with the highest percentage error of only 13.46%. Comparing to fundamental Lamb wave mode (S0), selecting high-order Lamb wave mode (S4) for ultrasonic structural health monitoring enables smaller transducer footprint particularly suitable for the applications in small and complex structures.