Abstract
Multiplexed optical fibre sensors were embedded into a carbon-fibre-reinforced-preform during the industrial production of a full-sized, one-piece tail cone assembly for a regional jet aircraft. Optical fibre Fresnel sensors monitored both the infusion of the resin, via measurement of the refractive index-dependent attenuation in the reflected light signal, and the degree of cure of the resin, via measurement of the chemical cure reaction-dependent change in refractive index. The resin cure was also monitored by optical fibre Bragg gratings (FBGs) fabricated in high linearly birefringent optical fibre, which measured through-thickness strain development, while FBGs in standard single mode optical fibre measured longitudinal strain development. The magnitudes and profiles of the transverse and longitudinal strains developed during the curing process were consistent across different locations on the tail cone. Typical transverse and longitudinal strains, related to cure reaction-induced shrinkage, were −1500 ± 17 μϵ and −500 ± 5 μϵ, respectively. Post-production, the same embedded FBG sensors were used subsequently to monitor structural strains when the tail cone was subjected to vacuum pressure loading. The longitudinal strains measured using the embedded FBG sensors were generally in good agreement with the longitudinal strains measured by the surface-bonded resistance foil strain gauge (RFSG) sensors, both qualitatively and quantitatively. The in-plane transverse and circumferential strains, oriented collinearly, were measured by the embedded FBGs and appropriately oriented surface-bonded RFSG sensors, respectively, and were, qualitatively, in good agreement.
Highlights
There has been a significant increase in the up take of composite material technology across a number of industries, thanks to the development of composite materials with innovative characteristics and to advancements in the manufacturing processes
The general trend and the sign of the strains measured by the embedded high linearly birefringent (HiBi) fibre Bragg gratings (FBGs) and surfaced-mounted resistance foil strain gauge (RFSG) sensors were in good agreement
The corresponding maximum longitudinal strains measured at sensor location S5 were 274 ± 4 με and 100 με for the single mode (SM) FBG and RFSG sensors, respectively (figure 18(b))
Summary
There has been a significant increase in the up take of composite material technology across a number of industries, thanks to the development of composite materials with innovative characteristics and to advancements in the manufacturing processes. Often based on the piezoresistive effect, are used for monitoring resin flow by transducing resin pressure into electrical resistance [8] The use of these non-optical sensors is restricted to laboratory applications, as the sensors are either not suitable for embedding into carbonfibre reinforced composites, or are too large to be embedded into critical components without compromising the structural integrity. The work investigated the feasibility and challenges of incorporating embedded OFS into the carbon-fibre-reinforced-preform (CFRP) to monitor the manufacturing processes, from lay-up of the preform through resin infusion and cure, to post-fabrication strain measurements. Multiplexed optical fibre Fresnel, SM FBG and HiBi FBG sensors monitored the infusion of resin, resin cure and the development of strain in the longitudinal and transverse directions at various locations within the tail cone. Post-production, the embedded fibre grating sensors, along with surface-mounted resistance foil strain gauge (RFSG) rosettes, were used to monitor structural strains when the tail cone was subjected to vacuum pressure loading
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