PSI - Issue 52

Sidney Goossens et al. / Procedia Structural Integrity 52 (2024) 647–654 Sidney Goossens / Structural Integrity Procedia 00 (2023) 000 – 000

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2. Materials and methods 2.1. Optical fibre sensors and installation

This work exploits optical fibre Bragg grating sensors, commonly abbreviated as FBGs. FBGs are a local refractive index modulation in the core of the optical fibre. This index modulation results in the reflection of light around a sensor-specific wavelength, named the Bragg wavelength. Whenever a strain or temperature change is applied to the FBG, its Bragg wavelength will shift proportionally with the magnitude of the measurand. By creating a chain of physically spaced FBGs with different refractive index modulation, tens or even hundreds of sensors can be created in one single optical fibre, such that each reflects a different Bragg wavelength. This gives FBG sensors several advantages compared to electronic sensors: they are thin (outer diameter of 125 μm ) and lightweight, they can be multiplexed, and they are immune to electromagnetic interference and to corrosion (Raman Kashyap, 2010). The optical fibre sensors that were used in this work are Draw Tower Gratings (DTGs®) sourced from FBGS International. The DTG sensors are inscribed into the optical fibre during the drawing process of the optical fiber and before applying protective coatings to the fibre. This results in FBG sensors with excellent mechanical strength (Chojetzki, 2005; Johnson D., 2012; Lindner et al., 2011, 2012, 2014). The fibre used here was additionally packaged with glass fibre reinforced polymer (GFRP) and a layer of HDPE for optimal protection against harsh aerospace conditions. We previously developed an installation method, that consists of encapsulating the entire length of the fibre with a two-component epoxy adhesive (3MTM Scotch-WeldTM DP490) for optimal mounting and protection of the OFS. The optical fibre and its installation method were previously extensively tested for their compatibility with standardized in-flight conditions, such as temperature (-50 to +80 ℃ shock cycles), pressure (sea level tot 47.000 ft), relative humidity (> 90 % for 48 h), fluid susceptibility (48 h in aerosol), vibration (1 h random ASPD), and tensile fatigue (10 6 cycles of 5 kN at 5 Hz) (Goossens et al., 2018, 2019). In this work we used four optical fibres, each equipped with 30 FBG sensors, resulting in a total of 120 sensors. The reflection spectrum of one optical fibre is shown in Fig. 1. The Bragg reflection peaks feature an average maximum reflectivity of -20.06 ± 0.55 dBm and an average spectral spacing of 1.87 ± 0.99 nm. The optical fibres were installed on the feet of the stiffeners of the composite panel, according to the installation method described in (Goossens et al., 2019). Fig. 2(a) shows the four fibres installed onto the stringer’s feet. Four sequential feet were instrumented with a fully encapsulated portion of fibre that holds 15 sensors. A portion of fibre without sensors is used to pass underneath the metallic frame on the right, behind which another portion of fibre with 15 sensor is installed onto the same stringer. The physical spacing between two subsequent FBGs is 27 mm, in line with our previous work and guidelines on practicalities for BVID detection with OFS (Goossens et al., 2020; Goossens, Berghmans, Sharif Khodaei, et al., 2021). The fibre design allows for monitoring BVIDs along the entire length and width of the instrumented stringer foot.

Fig. 1. Reflection spectrum of one optical fibre with 30 Bragg peaks, corresponding to the 30 FBG sensors