A flexible laser ultrasound transducer for Lamb wave based structural health monitoring

Lamb waves are widely used in the structural health monitoring (SHM) for plate-like structures. In this paper, a new and flexible ultrasonic transducer with a high photo-acoustic conversion efficiency was proposed by using candle soot nanoparticles (CSNPs) and polydimethylsiloxane (PDMS). Experimental results demonstrate that the developed transducer can generate a longitudinal wave with a short duration of 0.28 μs under the illumination of a nanosecond laser pulse. The amplitude of the excited longitudinal wave is 10 times to that of the signal generated by the traditional laser ultrasound technique. Further, wedge shape transducers were developed to excite Lamb waves in a 1.5-mm thick aluminium substrate by the oblique incidence method. The specific A0 and S0 modes of the Lamb wave with the central frequency of 647 kHz were successfully excited in the aluminium plate. Based on the synthetic aperture focusing imaging technique, a delay-and-sum signal processing method was adopted for damage location in the plate by using the A0 mode Lamb. A 3.5-mm defect was well imaged and the results demonstrate that the developed flexible photo-acoustic transducer can be a good alternative method for the SHM.