Three-dimensional strain and temperature monitoring of composite laminates

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 main technical objective of this ESA co-funded project is the development of a monitoring system which measures dynamically the multi-axial stress and strain in thermo hardened composite elements (made of space qualified material). The sensor consists of a 'High Birefringence '-fibre (HiBi-fibre) in which two gratings are 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 possible to determine the total strain field, plus the temperature inside a composite material. Theoretical simulations have indicated a good strain resolution of the proposed design. Experimental calibration of the sensor gave similar results.