Towards self-diagnosis composites: Detection of moisture diffusion through epoxy by embedded evanescent wave optical fibre sensors

Abstract

This paper reports on an epoxy matrix for glass fibre reinforced polymers equipped with low cost optical fibre sensors for the early detection of water diffusion, with devised applications in the oil and gas industry. Novel evanescent wave optical fibre sensors were designed, fabricated and embedded in epoxy resin samples. The tips of the optical fibre sensors were coated with a silver layer to work in reflection, so that they could be used as probes. Accelerated diffusion tests were performed: the samples were exposed to simulated sea water at 80 °C for up to 148 hours. The water diffusion resulted in a remarkable change of the reflected signal from the sensors, a result that was then confirmed through gravimetric measurements and a theoretical prediction, according to Fick's diffusion law. The results corroborate the feasibility of "sensitive" fibre reinforced polymers in harsh environments and that chemicals diffusion in these materials can be remotely and continuously monitored by means of the presented sensing system.

Dual In-Situ Water Diffusion Monitoring of GFRPs based on Optical Fibres and CNTs

“…Strips of aluminium foil, of dimension of 80 mm length, 3 mm width and 0.5 mm thickness, used as electrodes, had been positioned inside of the mould before casting. The epoxy resin was cured following the curing conditions previously defined [21].…”

“…The thickness of the silver deposited was ∼5 µm. Having the silver at the end of the tip, the transmitted light is reflected and the sensors can perfectly work in a single-ended sensor [21,29]. The OFSs were embedded into epoxy and GFRP samples during the casting, as detailed in the following sections.…”

“…The H-GFRP was fabricated using a custom mould with inner dimensions of 50 mm × 80 mm × 110 mm. It was possible to calculate approximately the water diffusion distance, from the epoxy surface to fibres 1, 2, 4 and 5 in Figure 2, with Fick's law, (Equation (4)), [30] and using the water diffusion coefficient in the epoxy measured in [21,30].…”

“…OFS 1, 4 and 5 were located at depth of 1.4 ± 0.2 mm and showed a signal drop after approximately 21, 19 and 18 h, respectively. Sensor 2 was located at a depth of 1.3 ± 0.2 mm and showed a signal drop after nearly 13 h. Table 1 shows the diffusion times calculated taking into account the OFS measured distance from the surface and the water diffusion coefficient [21]: 16 ± 4 and 13 ± 4 h, respectively, for OFSs 1, 4, 5 and OFS 2. Hence, OFSs were able to detect the water diffusion with acceptable time accuracy, in accordance with the calculated diffusion time (Table 1).…”

“…As an experimental proof-of-concept, the sensors are directly embedded within a GFRP composite. The OFSs, based on evanescent wave sensing, are embedded in the GFRP composite and monitored as described in a recent work by the same authors [21]. The CNTs sensors [19], are realized by addition of CNTs, either mixed into the epoxy matrix or spray-coated onto the GFRP plies.…”