2022 journal article

Ultrasonic frequency response of fiber Bragg grating under direct and remote adhesive bonding configurations

MEASUREMENT SCIENCE AND TECHNOLOGY, 33(1).

By: A. Navratil n, J. Wee n & K. Peters n

co-author countries: United States of America 🇺🇸
author keywords: structural health monitoring; non-destructive evaluation; ultrasonic inspection; optical fiber sensor; fiber Bragg grating
Source: Web Of Science
Added: November 15, 2021

Abstract Ultrasonic inspection based structural health monitoring is a powerful technique for damage detection in a structure. Ultrasonic waves are often excited at different frequencies to detect damages of different sizes, therefore understanding the frequency response of the sensor can be used to optimize the sensor performance. Fiber Bragg gratings (FBGs) are widely used for this ultrasound collection. The sensitivity of FBGs to a particular ultrasonic frequency is a function of the FBG length and the ultrasonic wavelength. Recently the authors demonstrated that its ultrasound sensitivity is improved for some conditions when the FBG is bonded at a distance away from the adhesive bond, referred to as remote bonding. However, the frequency response of this configuration has not been studied. Therefore, in this paper we measure and compare the ultrasonic frequency responses between a conventional directly bonded FBG and remotely bonded FBG. In theory, the FBG sensitivity varies as a function of ultrasound wavelength-to-grating length ( λ / L ) ratio. Therefore, for this experimental study, we maintain L constant and vary λ by changing the frequency of the input ultrasonic waves. We demonstrate that there is a region, below a cut-off values of λ / L , for which the remotely bonded FBG output has a higher sensitivity to the Lamb wave amplitude than the directly bonded FBG. The exact value of this λ / L cut-off depends on the mechanical properties of the structure, the windowing of the input Lamb wave, and the FBG properties. We also demonstrate that windowing the Lamb wave excitation signal has a similar affect to apodizing the FBG sensor in modifying the sensitivity response curve.