Networks of fiber Bragg grating (FBG) sensors can serve as structural health monitoring (SHM) systems for large-scale structures based on the collection of ultrasonic waves. The demodulation of structural Lamb waves requires a high signal-to-noise ratio because Lamb waves have a low amplitude. This paper investigates the signal transfer between Lamb waves propagating in an aluminum plate collected by an optical fiber containing a FBG. The fiber is bonded to the plate at locations away from the FBG. The Lamb waves are converted into longitudinal and flexural traveling waves propagating along the optical fiber, which are then transmitted to the Bragg grating. The signal wave amplitude is measured for different distances between the bond location and the Bragg grating. Bonding the optical fiber away from the FBG location and closer to the signal source produces a significant increase in signal amplitude, here measured to be 5.1 times that of bonding the Bragg grating itself. The arrival time of the different measured wave coupling paths are also calculated theoretically, verifying the source of the measured signals. The effect of the bond length to Lamb wavelength ratio is investigated, showing a peak response as the bond length is reduced compared to the wavelength. This study demonstrates that coupling Lamb waves into guided traveling waves in an optical fiber away from the FBG increases the signal-to-noise ratio of Lamb wave detection, as compared to direct transfer of the Lamb wave to the optical fiber at the location of the FBG.
Junghyun Wee, Drew Hackney, Kara Peters, Brian Wells, and Philip Bradford, "Sensitivity of contact-free fiber Bragg grating sensor to ultrasonic Lamb wave," Proc. SPIE 9803, Sensors and Smart Structures Technologies for Civil, Mechanical, and Aerospace Systems 2016, 98033Q (Presented at SPIE Smart Structures and Materials + Nondestructive Evaluation and Health Monitoring: March 24, 2016; Published: 20 April 2016); https://doi.org/10.1117/12.2218924.
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