Ultrasonic Surface Wave Transducers Made of Piezoelectric Polylactic Acid for Structural Health Monitoring

Ultrasonic-based structural health monitoring (SHM) allows continuous automated monitoring of structures for improved safety and reduced maintenance time and cost. Guided waves are often used for SHM for the long propagation distance with minimal attenuation. Conventionally, Lamb waves propagate in thin plate-like structures with thickness in order of magnitude of the wavelength. Therefore, they cannot propagate in very thick structures, whereas surface waves can be used for SHM of thick structures. Surface waves are usually generated using bulky ceramic-based transducers on a wedge or using polymeric-based transducers with comb-shaped electrodes. However, multiple wave modes, such as bulk longitudinal wave in thickness direction or high-order guided wave, may co-exist with surface waves due to the coexistence of both axial and shear-mode electromechanical coupling response in conventional piezoelectric materials. This work presents a flexible, lightweight, and low-profile poly-L-lactic acid (PLLA) polymer-based ultrasonic transducer with only in-plane electromechanical coupling for generation and detection of surface waves. The transducer comprises a piezoelectric film made of PLLA with the molecular chain orientation about 45° from the propagation direction of the surface wave. A method of SHM with surface waves in a thick shaft using such transducers is demonstrated, wherein the surface wave generated and detected by the PLLA transducers is used as the condition indicator. The PLLA-based transducers and their enabled ultrasonic SHM method offer the advantages of clearer targeted signal by eliminating the coexistence of other wave modes and long-term service stability by eliminating electrical poling process and any accompanied depoling degradation.