Response of Bragg grating fiber-optic sensors when embedded in composite laminates

Fibre-optic Bragg gratings have been extensively used as longitudinal strain sensors in a large number of applications, fulfilling the same tasks as conventional resistive strain gages. Comparative results obtained in composite laminates with both types of sensors show an excellent correlation in those cases where the strain distributions are smooth. Optical sensors offer additional advantages: unnecessary recalibration, multiplexing capability, small size, embedding ability, etc. An experimentally observed phenomenon is that the single peak of the spectrum of a fibre Bragg grating, written on a standard low-birefringence optical fibre, splits into two peaks when a transverse stress field is applied, owing to the promotion of a strain-induced birefringence. At first sight, this would be an undesirable effect that should be avoided by cladding the sensor. The knowledge of the optical behaviour of fibre Bragg gratings subjected to transverse loads affords additional information about the residual stress field created during the manufacturing process, and on the stress relaxation when machining the part. This paper demonstrates theoretically and experimentally how fibre Bragg gratings can be valuable tools for monitoring composite structures in service, and for obtaining the internal stress and strain states in laminates.