Composite materials are increasingly used in aerospace structures, because of their superior stiffness-to-mass ratio. Due to their ply-stacking nature, they show excellent mechanical performance in the in-plane directions but are much weaker in the out-of-plane direction. Manufacturers and end-users are therefore keen to acquire strength information along that direction. Here, we report on the measurement of three-dimensional strain using fibre Bragg gratings (FBGs) in highly birefringent (HiBi) microstructured optical fibers (MOFs).
By combining two MOF-based FBG sensors, we demonstrated three-dimensional strain-based monitoring of the curing of a fiber reinforced composite subcomponent manufactured by a resin transfer molding (RTM) technique. We also investigated the influence of the presence of the embedded fibers by embedding the MOF in composite coupons manufactured by liquid resin infusion (LRI) and performing a multitude of mechanical tests with these coupons and comparing the mechanical resistance to pristine coupons. It is also the first time that such a comparison has been carried out for this specific combination of optical fiber and composite material system.
Current composite structures used in aircraft can suffer from barely visible impact damage (BVID) that can compromise the load-bearing function of these structures. Especially damage-prone regions, such as the feet of a skin-stiffener structure, must therefore be frequently inspected for such damage. This increases aircraft downtimes and associated costs. A permanently installed structural health monitoring (SHM) network based on optical fiber sensors is an ideal candidate for performing condition-based maintenance (CBM) on such a structure. Individual FBG sensors have a known potential to detect the presence of BVIDs. In this work we propose a Global Damage Index (GDI) for quantifying the health of a composite component in manner of seconds, based on a network of 60 FBG sensors. We first establish a damage detection threshold and then carry out temperature compensated BVID detection with the GDI.
Composite materials are finding their way into aerospace applications thanks to their high stiffness-to-mass ratio. Nevertheless, composite components require frequent inspections because of their sensitivity to critical damage. Damage as small as barely visible impact damage (BVID) can grow as result of structural loading, with component failure as possible outcome. Optical fibre sensors (OFS) are considered excellent candidates for permanently installed structural health monitoring (SHM) systems, owing to their many advantages over electrical sensors. Current state-of-the-art BVID detection with OFS has so far however been limited to proof-of-concept demonstrations at low technology readiness levels. In this work, we equipped a total of 16 coupons, made of 5 different state-of-the-art composite materials, with aerospace compatible embedded or surface mounted optical fibre Bragg gratings (FBGs). We impacted each coupon at two locations and acquired the FBG reflection spectra before and after each impact. We first demonstrate how changes in the Bragg wavelength and in the Bragg peak shape can be quantified when the FBGs are exposed to the (non-)uniform strain field of BVIDs. Second, we show that this method was able to successfully detect the BVID in all considered scenarios and that in most cases, it was able to also locate the damage within an uncertainty of ±1 FBG location. Finally, we show the reliability of this method in terms of repeatability and considering the effects of temperature changes and on-ground airplane vibration. To the best of our knowledge, we are the first to use in-flight-compatible embedded and surface mounted FBG-sensors for the detection and location of BVIDs on aerospace-grade composite materials. These results motivate the use of FBG sensors as a permanent sensor network for cost-efficient damage detection in composite aerospace components for locally monitoring damage-prone locations.
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