Preferential directional coupling to ultrasonic sensor using adhesive tape

Abstract

For structural health monitoring applications, recent studies have demonstrated an increased ultrasonic detection sensitivity of fiber Bragg grating (FBG) sensors through remote bonding of the FBG to a structure. In this case, the S0 or A0 Lamb waves in a structure are coupled to guided L01 waves in an optical fiber at the adhesive bond location, resulting in L01 modes of equal amplitudes propagating in both directions along the optical fiber. We demonstrate that when the adhesive bond is replaced with adhesive tape the S0 mode couples to the same L01 modes, however, with a preferential direction. Therefore, our study investigates the mechanism causing the preferential direction. We first identify different ultrasonic coupling pathways from a plate to an FBG through adhesive tape, demonstrating that both direct and indirect pathways are active, but that the indirect pathway produces the directional coupling. Then we test different adhesive tape parameters through experiments and simulations, demonstrating that flexural rigidity and bond length are parameters that can be used to control the directionality. The results of this work could be applied to design multiplexed FBG sensor arrays with specified signal pathways through the optical fiber networks.