Abstract

Abstract

Abstract
In this research programme methodologies to improve the accuracy in the results measured with embedded fibre Bragg gratings (FBG) sensors were studied and implemented in order to produce a composite overwrapped pressure vessel (COPV) prototype that incorporate a non-destructive sensing technologies. Using a carbon/epoxy prepreg system, test specimens were manufactured with longitudinally embedded FBG sensors. The combined behaviour of the sensors and the host material was characterized and a calibration rule (correction factor) was determined for the chosen material. The consistency of the results with both theoretical and empirical assumptions suggests that the proposed method is applicable to a wide range of FBG sensors and host materials. In this paper, the experimental setup and procedure used to assess to the calibration rule is addressed and further detailed.

Abstract
Composite structures integrity is sensible to service life. Their application in the aeronautical and space engineering implies the necessity to insure their integrity through non-destructive evaluations. On-line health monitoring procedure capable to detect, acquire, and identify damage in fibre reinforced plastic composite materials are necessary. Among the different non-destructive techniques, acoustic emission was chosen for its ability to detect evolutive defects during in-service life of structures. Traditionally, the AE waves are detected at the surface of the structure by piezoelectric transducers. Such transducers have some limitations (e.g. they can't be used at low/high temperature, and are sensible to electromagnetic interferences). Optic fibre sensors have revealed to be a good alternative. Due to their low dimensions they can be easily embedded in fibre reinforced composite at manufacturing. In this chapter is discussed the use of an optic fibre system developed for damage monitoring in composite materials from the rapid release of elastic strain energy they generate, detected in the form of elastic waves. Among the different optic fibre sensors, the Fabry-Pérot interferometer is chosen for its high sensitivity to transient phenomena. The propagating acoustic emission waves induce variations of the light in the interferometer. The difficulty when using such sensor remains the phase recovery. In this study an original set-up is proposed for phase recovery based on the generation of two quadrature-shifted phase interferometric signals from two fibre Bragg gratings. The optic fibre sensor is embedded in a cross-ply carbon fibre/epoxy laminate. The optic fibre sensor system successfully detects periodic ultrasonic waves propagating into the material as well as simulated acoustic emission waves. These tests demonstrate that the optic fibre system is suitable for damage detection from acoustic emission waves. Such in-service health monitoring methodology can be used to locate damage and to determine its severity.

Abstract
In this Letter, we present a fiber loop mirror configuration based on a suspended twin-core fiber for sensing applications. Using the suspended twin-core fiber, the fringe pattern is due to the differential optical patch of the light in the two cores associated with a refractive index difference of similar to 10(-3), which indicates an advantage of this approach compared with those based on high-birefringent fibers, namely, the possibility of using a small length of fiber. The sensing configuration was characterized for torsion, temperature, and strain. Using the fast Fourier transform technique, it is possible to obtain measurand-induced amplitude variations of the fringe pattern. The results obtained indicate the viability of a temperature- and strain-independent torsion sensor. (C) 2010 Optical Society of America

Abstract
Fibre optic Bragg gratings (FBGs) written in normal and reduced diameter high birefringence (HiBi) fibres are studied. Chemical etching is used to reduce the diameter of fibres while the optical properties of the FBG spectrum are measured. The results obtained agree qualitatively with the stress enhanced chemical etching. The birefringence of the fibre is determined as a function of the diameter. Optical characterization of the FBG under transverse strain and temperature is also performed. The results obtained show the feasibility of the simultaneous measurement of those parameters with a HiBi FBG sensor.