Process monitoring of fibre reinforced composites using a multi-measurand fibre-optic sensor

Abstract

a b s t r a c tThis paper reports on the design, fabrication, characterisation and deployment of a multi-measurand optical fibre sensor (MMS) that is capable of simultaneously monitoring strain, temperature, refractive index and cross-linking chemistry. The sensor design is based on the extrinsic fibre Fabry-Perot interferometer. A feature of this sensor system is that a conventional multi-channel fibre-coupled near-infrared spectrometer is used to monitor the four independent parameters. The issues relating to the measurement resolution of the individual sensors and the associated interrogation equipment are discussed. The MMS was embedded in between the fourth and fifth plies of an eight-ply E-glass plain-weave fabric. A commercially available thermosetting epoxy/amine resin system was used to impregnate the fabric layers manually. The laminated preform was vacuum-bagged and cured in an autoclave. The following parameters were monitored: the depletion rates of the epoxy and amine functional groups in the resin system; the temperature in close proximity to the "chemical sensor"; the evolution of strain; and the refractive index of the resin system. The effect of post-processing on the output from the embedded optical fibre sensors is also considered.

In-situ monitoring of cross-linking reactions using E-glass fibres and evanescent wave spectroscopy

“…As the isothermal cross-linking temperature was increased from 35 to 45, 55 and 65 • C (Figs. [8][9][10][11], the rate of epoxy conversions were similar for the two cross-linking monitoring methods up to approximately 55%. After this point, the epoxy conversion rate conventional transmission FTIR spectral datasets for the conventional transmission FTIR spectral dataset increased at a faster rate until the depletion of the epoxy functional group.…”

“…An industrial epoxy/amine resin system (LY3505/XB 3403, Huntsman Advanced Materials, UK) was also used to demonstrate that the cross-linking reactions could be monitored using E-glass fibre bundles and evanescent wave spectroscopy. It is known that the refractive indices of thermosetting resins generally increase as a function of the cross-link density [2,8,9]. The refractive indices of the LY3505/XB3403 resin (measured at 25 • C) at the start and after isothermal cross-linking at 70 • C were 1.525 and 1.562 respectively.…”

Monitoring Pre-Stressed Composites Using Optical Fibre Sensors

“…The EFPI sensor is relatively insensitive to temperature because the coefficients of thermal expansion for the optical fibre and the capillary are similar [ 45 , 46 ]. However, the action of the vacuum bag will impress the EFPI sensor on to the surface of the pre-cured composite as the vacuum and pressure is applied.…”

“…The EFPI sensors with cavity lengths in the range 50 to 100 µm were fabricated in-house using SM 800 (5.6/125 µm) optical fibres and 128 µm precision bore fused silica capillaries. The details of the fabrication process and the mode of operation of the sensors have been published previously [ 44 , 45 , 46 ]. The FBGs were inscribed on SM 1550 (9/125 µm) optical fibres using the phase mask technique in conjunction with an excimer laser operating at 248 nm (Bragg Star).…”

Numerical Analysis of the Effect of Microscale Components Interaction on Measurements of Fiber Optic Strain Sensors Used in Composite Structures

“…FOS have a high degree of resolution in terms of the measured parameters, small dimensions, long service life, resistance to corrosion, and electromagnetic interference. Optical fibers are also quite flexible, durable, and heat-resistant and can be easily embedded in laminate composites [3,[9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25]. Existing technologies allow to embed fiber optic sensors directly into the composite material during the technological cycle of manufacturing of composite structures without significant influence on its mechanical properties [26], since the size of optical fibers is relatively small compared to reinforcing fibers [15,27].…”