Embeddable multi-channel all-optical-fiber acousto-ultrasonic system for damage evaluation of composites

Abstract

Although conventional guided-wave-based ultrasonic structural health monitoring (SHM) is paramount for ensuring the safe operation of structural components, the piezoelectric materials used to generate and detect ultrasonic guided waves have the intrinsic issues of electromagnetic interference susceptibility, poor embeddability, and limited multiplexing. To address these issues, an embeddable multi-channel all-optical-fiber acousto-ultrasonic (AOF-AU) system is proposed. A pulsed laser was coupled into specialty optical fibers, and ultrasonic-guided waves were generated from photo-thermal-acoustic conversion. After propagating through the target structure, the ultrasonic wave was detected using a fiber Bragg grating (FBG), which was demodulated using the edge-filtering principle. The control section paired the channel in the optical switch and FBG array and synchronized the ultrasonic excitation and detection to achieve multi-channel SHM. A high-quality carbon fiber reinforced polymer (CFRP) embedded with optical fibers was manufactured; however, a flame-shaped ablation area was observed at the end of the specialty optical fiber. The cross-section of the CFRP did not exhibit a resin-rich area or fiber waviness. The correlation between the characteristics of the optical pulse signal and the ultrasonic wave was clarified under different parameters of the pulsed laser. The ultrasonic signal showed barely changed during continuous monitoring, demonstrating that damage does not accumulate further. In the multi-channel AOF-AU system, the multi-channel capability and strong directionality were demonstrated. In addition, the different attenuations of the ultrasonic waves evaluated in the AOF-AU system enabled easy localization of a hole within the CFRP laminate, demonstrating the capability of the newly proposed system for ultrasonic SHM.