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.
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