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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2.2. Composite panel and BVIDs The composite panel used in this work, was an CFRP panel with an industry relevant size of of 5 x 3 m 2 and a radius of curvature of 1.67 m, representative for a portion of a regional aircraft barrel. A photograph of the panel is shown in Fig. 2(b). Both skin and stiffeners were manufactured from Hexcel ’s M21/194/34%/T800S CFRP unidirectional prepreg tapes and had a stacking sequence and thickness of [±45/0 2 /90/0] s and 2.208 mm respectively. The stringers were hot formed and co-cured to the skin, after which the aluminium frames were riveted into the skin. A wooden fixture was created to support the panel on both sides for access to the inside (concave) and the outside (convex) side of the panel.

Unmounted optical fibres

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Encapsulated optical fibres

Thermocouple

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Fig. 2. (a) Photograph of the installation of a portion of the optical fibres on the feet of the stringers: ; (b) photograph of the full curved panel, with the fibres installed on the middle two stringer of sections A and B.

2.3. Damage detection methodology Calibrated BVIDs were created under the stringer’s feet, with an impact energy of 35 J, which is representative for an accidental tool drop. The BVIDs were created by a custom-built drop tower. In order to detect the BVID, we used FBG sensors to measure the change in residual strain at the surface of the composite stringer, compared to the measurement in a healthy state. To do so, the ‘inspection measurement’ is compared to a base line measurement. However, as an FBG sensor is cross-sensitive to temperature changes, a temperature compensation must first be performed. If the temperature difference between the inspection and baseline measurement is sufficiently small, the median Bragg wavelength shift of all FBG sensors can be used to compensate for the effect of a temperature change. For larger temperature changes, prior knowledge of the Bragg wavelength as a function of temperature is required (Goossens, Berghmans, Sharif Khodaei, et al., 2021). We performed a temperature calibration of the sensors on the panel, by exposing the panel to temperatures from 25 to 60 ℃ in steps of 5 ℃ in a custom-built climatic chamber. The Bragg wavelengths of all 120 sensors were recorded for 30 sec at 2 Hz, yielding 60 Bragg wavelength values per temperature step. The temperature was simultaneously acquired by thermocouples mounted next to the optical fibres, as can be seen on the bottom of Fig. 2(a). For every FBG sensor, a third-degree polynomial relation was fit to yield the Bragg wavelength as a function of temperature , such as in (1) with λ B,i the Bragg wavelength in nm, T the temperature in ℃ , and a i , b i , c i , and d i the polynomial constants for the i th sensor. λ B,i =a i ⋅ T 3 +b i ⋅ T 2 +c i ⋅ T+d i (1)