Thermal isolation of FBG optical fibre sensors for composite cure monitoring

This paper reports experimental and numerical thermal isolation techniques for Fibre Bragg Grating (FBG) sensors used for cure monitoring of thermoset matrix composites. The FBG sensor readings are mostly affected by both thermal and mechanical effects therefore; these properties should be isolated from one another. An experimental method was applied to evaluate three different capillary tubes that can be used as isolation material for an FBG sensor. A numerical modelling approach has been developed to model heat transfer through these capillary tubes so that the material selection and geometries can be understood accurately. The results from encapsulating FBGs in glass capillary tubes showed almost no sensitivity to changes in heating rates as compared to two types of stainless-steel tubes. The FBG sensors encased in glass capillary tube produced least thermal lag and forces as compared to FBGs encased in stainless-steel capillary tubes. This indicates that glass capillary tube has less thermal effects on the FBG material than stainless-steel tubes. The isolated sensors had reliably detected the residual compressive strains induced during an actual composite cure cycle. The experimental curves show very good agreement with modelling results, with model over predicting experiments by approximately 5%.