Ultrasonic Lamb wave measurement sensitivity of aligned carbon nanotube coated fiber Bragg grating

Abstract

Fiber Bragg grating (FBG) sensors are typically bonded on the surface of a structure using an adhesive to collect ultrasonic waves for damage detection in structural health monitoring applications. However, the ultrasonic wave transfer from structure to optical fiber suffers signal attenuation due to the adhesive bond layer, which has a significantly different acoustic impedance than the optical fiber. Therefore, this paper develops a systematic procedure to fabricate an aligned carbon nanotube (CNT)-wrapped FBGs for acoustic impedance matching. Specifically, we first develop an automated CNT winding system to fabricate CNT-wrapped FBGs with varying CNT layer thickness, which are bonded to an aluminum plate for ultrasonic sensitivity testing. We demonstrate that CNT wrapped FBGs do not necessarily produce an increased sensitivity as compared to a reference polyimide-coated FBG, however some outliers are observed with a significant improvement. Using a scanning electron microscopy we examine the cross-section of CNT/adhesive layers, identifying a unique CNT/ adhesive bonding morphology with a stiff exterior shell and a relatively compliant inner layer. Finite element simulation validates that this two-layered bonding geometry is most likely the source of the increased FBG ultrasonic sensitivity for the outliers.

Spectral Verification of the Mechanisms behind FBG-Based Ultrasonic Guided Wave Detection

“…In order to consider the strain sufficiently uniform, Takeda et al [ 4 ] and Culshaw et al [ 13 ] numerically determined that the respective conditions of λ UGW /L > 7 and λ UGW /L > 6 needed to be satisfied. These thresholds have been accepted as an important requirement for accurate UGW detection [ 2 , 5 , 14 , 15 , 16 ]. Since the wavelengths of UGWs around the resonance frequency of a plate-like structure are typically of the order of a few cm, this implies that the FBG sensor should be no longer than a few mm, depending on the exact application.…”

“…Shorter FBG sensors, thus, inherently imply lower sensitivities for edge detection, which in its turn implies a lower signal-to-noise ratio, therefore requiring improved filtering and averaging to retrieve the UGW signal. Research has continued to investigate ways to increase the sensitivity of FBG sensors for UGW detection [ 16 , 18 , 19 , 20 , 21 , 22 , 23 ].…”