Lamb Wave Excitation Using a Flexible Laser Ultrasound Transducer for Structural Health Monitoring

Using Lamb waves for detection is a desirable method for the structural health monitoring (SHM) and has been widely used in plate-like structure. In this paper, an originally flexible ultrasonic transducer with high photo-acoustic conversion efficiency was proposed by using candle soot nanoparticles (CSNPs) and polydimethylsiloxane (PDMS). Experiments results demonstrate that the developed transducer can generate a longitudinal wave in the aluminum block with the amplitude of 10 times to that of the conventional laser ultrasound technique, demonstrating the high energy conversion efficiency of the proposed transducer. Wedge-shape transducers were developed to excited Lamb wave in the 1.5 mm aluminum substrate by the oblique incident method. The specific A0 and S0 modes of the Lamb wave with the central frequency of 647 kHz were successfully excited in the aluminum plate of 1.5 mm thickness. A0 mode Lamb wave was selected for damage detection application. According to the synthetic aperture focusing imaging technique, a delay-and-sum signal processing method was adopted for damage location in the plate. A 3.5 mm defect was well imaged and the results demonstrate that the developed flexible photo-acoustic transducer can be a promising method for the SHM.