INSTITUTE OF PHYSICS PUBLISHING SMART MATERIALS AND STRUCTURES Smart Mater. Struct. 14 (2005) 354–362 doi:10.1088/0964-1726/14/2/009 Strain development in curing epoxy resin and glass fibre/epoxy composites monitored by fibre Bragg grating sensors in birefringent optical fibre E Chehura 1 , A A Skordos 2 , C-C Ye 1 , S W James 1 , I K Partridge 2 and R P Tatam 1 1 Optical Sensors Group, Centre for Photonics and Optical Engineering, School of Engineering, Cranfield University, Cranfield, Bedford MK43 0AL, UK 2 Advanced Materials Department, School of Industrial and Manufacturing Sciences, Cranfield University, UK E-mail: r.p.tatam@cranfield.ac.uk Received 1 December 2003, in final form 28 October 2004 Published 15 February 2005 Online at stacks.iop.org/SMS/14/354 Abstract Fibre Bragg gratings (FBGs) fabricated in linearly birefringent fibres were embedded in glass fibre/epoxy composites and in the corresponding unreinforced resin to monitor the effective transverse strain development during the cure process. The optical fibres containing the FBG sensors were aligned either normal or parallel to the reinforcement fibres in unidirectional glass fibre/epoxy prepregs. The chemical cure kinetics of the epoxy resin system used were studied using differential scanning calorimetry, in order to investigate the correlation between the strain monitoring results and the evolution of the curing reaction. A non-parametric cure kinetics model was developed and validated for this purpose. The effective transverse strain measured by the FBGs demonstrated high sensitivity to the degree of cure as a result of the densification of the resin caused by the curing reaction. The effective compressive transverse strain developed during the reaction, and thus the corresponding sensitivity to chemical changes, was higher in the case of the sensing fibre aligned normal to the reinforcement fibres than in the case of the sensor fibre parallel to the reinforcement fibres. Small but measurable sensitivity to cure induced changes was observed in the case of the unreinforced resin. 1. Introduction Fibre Bragg grating (FBG) sensors are a powerful measurement tool that can perform internal and surface monitoring of strain and temperature, useful in manufacturing processes, impact and damage detection, and structural health monitoring. The advantages of FBG sensors include their chemical inertness, light weight, and small size (typically 80–125 µm in diameter), which minimizes degradation to the structure caused by the inclusion of the sensor. The measurand is wavelength encoded which permits sensor wavelength- multiplexing, necessary if measurements are required in more than one location. An important area of application of FBG sensors is in embedded structures where they can be used for structural health monitoring through the lifetime of the structure: from cure monitoring during the production process of composite materials [1] through to in-service health monitoring [2] of the finished product. Composite materials undergo regimes of expansion and shrinkage during the cure process caused by a combination of temperature and chemical/material state changes. The state of residual strain in the final composite part as well as the development of internal stress during the curing 0964-1726/05/020354+09$30.00 © 2005 IOP Publishing Ltd Printed in the UK 354