Relating the state of cure to the real-time internal strain development in a curing composite using in-fibre Bragg gratings and dielectric sensors

Abstract

The paper reports results of preliminary experiments on a continuous curing carbon fibre reinforced epoxy composite, designed to achieve simultaneous characterizations of the state of cure of the resin and the development of strain in the reinforcing fibres. The measurements were carried out using embedded dielectric microsensors and in-fibre Bragg gratings (IFBGs) to follow changes in the conductivity of the matrix resin and in the internal strain in the embedded optical fibre respectively. This was performed using an automated data acquisition system, part of which uses real-time dielectric measurements to follow the industrially relevant parameters of curing. The system can multiplex several sensors and could be used in a quasi-distributed system. The IFBG sensors were demodulated using a scanning fibre Fabry-Pérot interferometer. The results obtained with this combined cure monitoring system show that it is possible to monitor the strain levels in the optical fibre resulting from the onsets of liquification, gelation and vitrification within the surrounding resin matrix. The combined measurements constitute a first step towards a scientific study of the development of internal residual stresses in a cured composite, which will be crucial to the ultimate elimination of warpage and `spring-back' in large composite structures.