Non-destructive evaluation of composite structures using an innovative Bragg sensor

This paper discusses a new sensor design based on optical fibre Bragg gratings which is being developed in the framework of the MASSFOS-project ('Multi-Axial Stress and Strain sensing of thermo hardened composite elements using Fibre Optic Sensors'-project). The objective of this ESA co-funded project is to develop a monitoring system which measures dynamically the multi-axial stress and strain plus temperature in thermo hardened composite elements. The sensor consists of a 'High Birefringence'-fibre in which two gratings have been inscribed; each grating yields two distinct Bragg peaks. The first grating is sensitive to the total stress field in the material, while the second one is isolated from transverse stress components. By measuring the four Bragg peaks of the sensor, it is theoretically possible to determine the total strain field, plus the temperature inside a composite material. A Static tensile test has already been carried out to prove the feasibility of the sensor embedded in a composite laminate.