Residual strain monitoring during composite manufacturing

This work explores a new possibility in accurate measurement of multi-axial residual strains during the production of composite structures. It investigates the usability of the polarization dependent loss (PDL) of an optical fibre Bragg grating as a sensitive indicator of multi-axial residual strains. The experimental work presented is done on a cross-ply carbon fibre reinforced polymer fabricated using an autoclave cycle

Temperature insensitive cure cycle monitoring of cross-ply composite laminates using the polarization dependent loss property of FBG

very important aspect of the composite manufacturing process is the appearance of residual strains and stresses during the curing cycle. Composites exhibit large residual strains after curing. Therefore, in this paper, we propose to follow the evolution of the polarization depend loss peaks (amplitude and wavelength) of fibre Bragg gratings during the manufacturing of the composite material to highlight the residual strains appearanc

Internal strain monitoring in composite materials with embedded photonic crystal fiber Bragg gratings

The possibility of embedding optical fiber sensors inside carbon fiber reinforced polymer (CFRP) for structural health monitoring purposes has already been demonstrated previously. So far however, these sensors only allowed axial strain measurements because of their low sensitivity for strain in the direction perpendicular to the optical fiber's axis. The design flexibility provided by novel photonic crystal fiber (PCF) technology now allows developing dedicated fibers with substantially enhanced sensitivity to such transverse loads. We exploited that flexibility and we developed a PCF that, when equipped with a fiber Bragg grating (FBG), leads to a sensor that allows measuring transverse strains in reinforced composite materials, with an order of magnitude increase of the sensitivity over the state-of-the-art. In addition it allows shear strain sensing in adhesive bonds, which are used in composite repair patches. This is confirmed both with experiments and finite element simulations on such fibers embedded in CFRP coupons and adhesive bonds. Our sensor brings the achievable transverse strain measurement resolution close to a target value of 1 mu strain and could therefore play an important role for multi-dimensional strain sensing, not only in the domain of structural health monitoring, but also in the field of composite material production monitoring. Our results thereby illustrate the added value that PCFs have to offer for internal strain measurements inside composite materials and structures

Microstructured optical fiber Bragg grating-based shear stress sensing in adhesive bonds

We present shear stress sensing with a Bragg grating sensor fabricated in a highly birefringent microstructured optical fiber. This sensor has a shear strain sensing resolution of 0.04 pm/mu epsilon when embedded in a shear loaded adhesive bond. We achieve discrete shear stress mapping in an adhesive bond by embedding a multitude of these sensors at different locations in the bond line. Experiments and numerical modeling show the limited influence of angular misalignment of the sensor on its shear stress response. Finally, we discuss the cross-sensitivity of this sensor to shear strain and temperature

Fiber Bragg grating-based shear strain sensors for adhesive bond monitoring

The application of shear stress sensors in structural health monitoring remains limited because current sensors are either difficult to implement, they feature a low measurement resolution or the interrogation of the output signal is complex. We propose to use fiber Bragg grating-based sensors fabricated in dedicated highly birefringent microstructured optical fibers. When embedded in a host material, the orientation angle of the fiber should be chosen such that their polarization axes are aligned parallel with the direction of maximum shear stress when the host is mechanically loaded. We present experimental results of sensors embedded in the adhesive layer of single lap and double lap structural joints. These tests demonstrate that when the joints are tension loaded, the embedded sensors have a shear stress sensitivity of around 60 pm/MPa. We study the influence of the adhesive material on the sensor response, as well as the influence of sensor orientation and location in the bond line Finally, we demonstrate the minimal thermal cross-sensitivity of the shear stress sensitivity of this sensor

Microstructured optical fiber Bragg grating-based strain and temperature sensing in the concrete buffer of the Belgian supercontainer concept

We present the use of microstructured optical fiber Bragg grating-based sensors for strain and temperature monitoring inside the concrete buffer of the Belgian supercontainer concept, demonstrated in a half-scale test in 2013. This test incorporated several optical fiber sensors inside the concrete buffer for production and condition monitoring. The optical fiber sensors presented here consist of small carbon-reinforced composite plates in which highly birefringent Butterfly microstructured optical fibers, equipped with fiber Bragg gratings, were embedded. The double reflection spectrum of these MOFGBs allows to simultaneously monitor strain and temperature, as confirmed by comparison with data obtained from thermocouples and vibrating-wire sensors installed near the MOFBGs

Etude des propriétés électro-optiques des cristaux de niobate de lithium en fonction de leur composition et de leur dopage par des ions chrome

Lithium niobate (LiNbO3, LN) is a material used in many acousto and optoelectronic devices thanks to its piezo-electric, electro-optic (EO) and nonlinear optical properties. Recently, new techniques of preparation allowed the achievement of crystals with different compositions. The possibility of preparing stoichiometric samples renewed the interest on studying the basic properties of this material. This study is devoted to the determination and the analyisi of the EO properties of several crystals of LiNbO3 : pure crystals with different compositions (concentration of intrinsic defects) and chromium-doped crystals with different initial compositions. We have determined the compostion of the pure crystals by different analysis techniques calibrated with standard crystals. We have also used the same techniques to determine the composition od doped-crystal by discussing the limit of validity of these methods. We have determined the EO properties of the different samples by using a very sensitive method based on the Sénarmont type compensating arrangement. The behaviour of the EO and dielectric properties are compared as a function of frequency of the applied electric field, composition and doping of the samples. Microscopic analysis of intrinsic defects structure and the dynamic site of substitution for Cr3+ ions in LN matrix, based on the luminescence results, allowed us to interpret the variation of the effective EO rc coefficient in pure and Cr-doped crystalsLe niobate de lithium (LiNbO3, LN) est un matériau utilisé dans un grand nombre de dispositifs acoustiques et optoélectroniques grâce à ses propriétés piézo-électriques, électro-optiques (EO) et optiques non linéaires. Récemment, de nouvelles techniques de préparation permettent d'obtenir des monocristaux de différentes compositions. La possibilité de réaliser des échantillons de composition stœchiométrique a relancé les études fondamentales sur ce matériau. Cette étude concerne la détermination et l'analyse des propriétés EO de plusieurs cristaux de LiNbO3 : cristaux purs dont la composition (concentration en défauts intrinsèques) varie ; cristaux dopés au chrome pour différentes compositions initiales. Nous avons déterminé la composition des échantillons purs par différentes techniques d'analyse à l'aide de la calibration sur des cristaux standards. Nous avons aussi utilisé les mêmes techniques pour déterminer la composition des échantillons dopés en discutant de la limite de validité de ces méthodes. Nous avons déterminé les propriétés électro-optiques des différents échantillons à l'aide d'une méthode très sensible basée sur le montage à compensation à une onde type Sénarmont. Ces propriétés sont comparées au comportement diélectrique du matériau en fonction de la fréquence, de la composition et du dopage. Une analyse microscopique de la structure des défauts intrinsèques et de la dynamique du site de substitution des ions Cr3+ dans la matrice de LN, basée sur les résultats de luminescence, nous a permis d'interpréter les variations du coefficient EO effectif rc obtenues pour les cristaux purs et dopé