Distributed Optical Fibre Sensors for Structural Health Monitoring: Upcoming Challenges

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Multicore optical fiber shape sensors suitable for use under gamma radiation

© 2019 Optical Society of America. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modifications of the content of this paper are prohibited.[EN] We have designed and implemented a fiber optic shape sensor for high-energy ionizing environments based on multicore optical fibers. We inscribed two fiber Bragg gratings arrays in a seven-core optical fiber. One of the arrays has been inscribed in a hydrogen-loaded fiber and the other one in an unloaded fiber in order to have two samples with very different radiation sensitivity. The two samples were coiled in a metallic circular structure and were exposed to gamma radiation. We have analyzed the permanent radiation effects. The radiation-induced Bragg wavelength shift (RI-BWS) in the hydrogen-loaded fiber is near ten times higher than the one observed for the unloaded fiber, with a maximum wavelength shift of 415 pm. However, the use of the multiple cores permits to make these sensors immune to RI-BWS obtaining a similar curvature error in both samples of approximately 1 cm without modifying the composition of the fiber, pre-irradiation or thermal treatment.Ministerio de Economia y Competitividad (DIMENSION TEC2017 88029- R); Generalitat Valenciana (IDI/FEDER/2018, PROMETEO 2017/103); H2020 Marie Sklodowska-Curie Actions (MSCA-ITN-ETN-722509); Universitat Politecnica de Valencia (PAID-01-18); Ministerio de Ciencia, Innovacion y Universidades (IJCI-2017-32476).Barrera, D.; Madrigal-Madrigal, J.; Delepine-Lesoille, S.; Sales Maicas, S. (2019). Multicore optical fiber shape sensors suitable for use under gamma radiation. Optics Express. 27(20):29026-29033. https://doi.org/10.1364/OE.27.029026S2902629033272

Dielectric relaxation behavior of Callovo-Oxfordian clay rock: A hydraulic-mechanical-electromagnetic coupling approach

Water content is a key parameter to monitor in nuclear waste repositories such as the planed underground repository in Bure, France, in the Callovo-Oxfordian (COx) clay formation. High-frequency electromagnetic (HF-EM) measurement techniques, i.e., time or frequency domain reflectometry, offer useful tools for quantitative estimation of water content in porous media. However, despite the efficiency of HF-EM methods, the relationship between water content and dielectric material properties needs to be characterized. Moreover, the high amount of swelling clay in the COx clay leads to dielectric relaxation effects which induce strong dispersion coupled with high absorption of EM waves. Against this background, the dielectric relaxation behavior of the clay rock was studied at frequencies from 1 MHz to 10 GHz with network analyzer technique in combination with coaxial transmission line cells. For this purpose, undisturbed and disturbed clay rock samples were conditioned to achieve a water saturation range from 0.16 to nearly saturation. The relaxation behavior was quantified based on a generalized fractional relaxation model under consideration of an apparent direct current conductivity assuming three relaxation processes: a high-frequency water process and two interface processes which are related to interactions between the aqueous pore solution and mineral particles (adsorbed/hydrated water relaxation, counter ion relaxation and Maxwell-Wagner effects). The frequency-dependent HF-EM properties were further modeled based on a novel hydraulic-mechanical-electromagnetic coupling approach developed for soils. The results show the potential of HF-EM techniques for quantitative monitoring of the hydraulic state in underground repositories in clay formations

Dielectric relaxation behavior of Callovo-Oxfordian clay rock: A hydraulic-mechanical-electromagnetic coupling approach

Water content is a key parameter to monitor in nuclear waste repositories such as the planed underground repository in Bure, France, in the Callovo-Oxfordian (COx) clay formation. High-frequency electromagnetic (HF-EM) measurement techniques, i.e., time or frequency domain reflectometry, offer useful tools for quantitative estimation of water content in porous media. However, despite the efficiency of HF-EM methods, the relationship between water content and dielectric material properties needs to be characterized. Moreover, the high amount of swelling clay in the COx clay leads to dielectric relaxation effects which induce strong dispersion coupled with high absorption of EM waves. Against this background, the dielectric relaxation behavior of the clay rock was studied at frequencies from 1 MHz to 10 GHz with network analyzer technique in combination with coaxial transmission line cells. For this purpose, undisturbed and disturbed clay rock samples were conditioned to achieve a water saturation range from 0.16 to nearly saturation. The relaxation behavior was quantified based on a generalized fractional relaxation model under consideration of an apparent direct current conductivity assuming three relaxation processes: a high-frequency water process and two interface processes which are related to interactions between the aqueous pore solution and mineral particles (adsorbed/hydrated water relaxation, counter ion relaxation and Maxwell-Wagner effects). The frequency-dependent HF-EM properties were further modeled based on a novel hydraulic-mechanical-electromagnetic coupling approach developed for soils. The results show the potential of HF-EM techniques for quantitative monitoring of the hydraulic state in underground repositories in clay formations. Key Points Dielectric relaxation behavior of porous mediaRadio and microwave remote sensing techniquesHydraulic-mechanical-electromagnetic couplin

Implementation of an Embedded Sensor Based on Electrical Resistivity to Monitor Drying in Thick Concrete Structures

Electrical resistivity is a parameter sensitive to several properties of concrete, including water content, which is one of the key parameters governing concrete long-term durability. In this paper, the monitoring of the concrete water content profile throughout its entire thickness is discussed using an electrical approach as a measurement method. This is very relevant to applications requiring a centimeter resolution over a large thickness. The aim of this paper is to implement a multi-electrode embedded sensor in a concrete slab to determine the resistivity profile over concrete depth in order to monitor its drying. The sensor, designed as a printed circuit board (PCB), is integrated in two 30 cm thick concrete slabs. Different measurement configurations are presented. Following qualification in laboratory and controlled conditions, the study focuses on characterizing the sensor‘s response during the drying of the slabs. The results demonstrate the capability of the sensor to monitor concrete drying by measuring the resistivity profiles with a spatial centimetric resolution

Réflectométrie fréquentielle (FDR) appliquée à l'évaluation non destructive des conduits de précontrainte extérieure, perspective pour la mesure de la teneur en eau du béton

National audienceThis paper presents an electromagnetic method of diagnosis based on frequency domain reflectometry (FDR) associated with an inversion algorithm developed by INRIA, ISTLTM (Inverse Scattering for Transmission Lines). ISTLTM allows estimating the spatial profile of the electrical impedance of the line from the FDR measurements. Experimental results on two mockups of external post-tensioned ducts with filling defects show the feasibility of the method. We will try to show the similarities between auscultation external post-tensioned ducts and measurement of water content by TDR probes (Time Domain Reflectometry).Ce papier présente une méthode électromagnétique de diagnostic reposant sur la réflectométrie fréquentielle (FDR) associée à un algorithme d'inversion développé par l'INRIA, ISTLTM (Inverse Scattering for Transmission Lines). ISTLTM permet d'estimer le profil spatial d'impédance électrique de la ligne à partir de la mesure FDR. Des résultats expérimentaux obtenus sur deux maquettes de conduits de précontrainte extérieure ayant des défauts de remplissage montrent la faisabilité de la méthode. Nous tacherons de montrer les similitudes qui existent entre l'auscultation des conduits de précontrainte extérieure et la mesure de teneur en eau par des sondes TDR (Time Domain Reflectometry)

Truly Distributed Optical Fiber Sensors for Structural Health Monitoring: From the Telecommunication Optical Fiber Drawling Tower to Water Leakage Detection in Dikes and Concrete Structure Strain Monitoring

Although optical fiber sensors have been developed for 30 years, there is a gap between lab experiments and field applications. This article focuses on specific methods developed to evaluate the whole sensing chain, with an emphasis on (i) commercially-available optoelectronic instruments and (ii) sensing cable. A number of additional considerations for a successful pairing of these two must be taken into account for successful field applications. These considerations are further developed within this article and illustrated with practical applications of water leakage detection in dikes and concrete structures monitoring, making use of distributed temperature and strain sensing based on Rayleigh, Raman, and Brillouin scattering in optical fibers. They include an adequate choice of working wavelengths, dedicated localization processes, choices of connector type, and further include a useful selection of traditional reference sensors to be installed nearby the optical fiber sensors, as well as temperature compensation in case of strain sensing

Extensomètres à fibre optique pour le génie civil : Etat de l'art, applications industrielles et perspectives

Les capteurs à fibre optique sont des outils exceptionnels pour le contrôle de santé des structures. Le présent article vise à dresser l'état de l'art des extensomètres à fibre optique en génie civil, près de 20 ans après leurs premiers développements. Après des descriptions béotiennes de la fibre optique, les différents types de capteurs à fibre optique sont présentés de manière pragmatique, en listant les performances et les limitations, l'état d'avancement, et pour les plus avancées les fournisseurs et le coût. Trois catégories de capteurs à fibre optique sont ainsi distinguées : les capteurs ponctuels, les extensomètres de longue base de mesure, et les capteurs réalisant des mesures réparties, c'est-à-dire continues tout le long de la fibre optique. Dans une seconde partie, différentes applications industrielles spécifiques au génie civil développées par deux entreprises françaises sont décrites. Enfin, l'exemple du développement d'un extensomètre de longue base de mesure destiné à être noyé dans le béton est présenté, depuis la conception, jusqu'aux tests en laboratoire et sur un pont à béquilles près d'Angoulême

Analyse vibratoire à partir d'extensomètres à fibre optique longue base

International audienceLa surveillance d'ouvrages d'art est un domaine actif de la recherche, cependant la mise en place in-situ de l'instrumentation peut s'avérer fastidieuse et coûteuse. En effet, équiper une structure de plusieurs dizaines de mètres nécessite des dizaines de capteurs, auxquels il faut rajouter le coût de maintenance. D'autres solutions utilisant moins de capteurs sont souvent préférées. Par exemple, quelques capteurs servant de référence sont fixes et les autres sont déplacés sur l'ensemble de la structure étudiée. Le nombre de capteurs étant réduit, le choix de leur placement peut devenir délicat. Une alternative à ce problème est l'utilisation d'Extensomètres de Longues Bases de mesure. Réalisés à partir de fibres optiques, ces capteurs permettent de mesurer des variations de la distance entre des points espacés de plusieurs mètres tout en conservant une résolution de l'ordre du μm. Ils sont principalement utilisés pour la surveillance des ouvrages à long terme mais aussi, plus récemment, pour l'étude du comportement dynamique des structures. Depuis quelques années, une évolution a permis de les attacher continûment aux structures soit par collage, soit en les noyant au cœur du matériau. Dans un premier temps, le système de mesure optique ainsi que le capteur sont présentés. Ensuite, l'expression de la mesure obtenue par ce type de capteur est introduite et appliquée aux cas des poutres et des plaques minces. Enfin, les notions de « nœud » et de « ventre » de vibrations sont présentées pour ce type capteur

Distributed Optical Fiber Sensors for Structural Health Monitoring : Upcoming challenges. In : Optical Fibre, New Developments, chapitre 9

The civil engineering structures become, on one hand more and more complicated and/or aged and on the other hand, the society seeks to increase their lifetime which require a special attention to prevent the damages: leaks, fires, landslides, earthquakes, etc... Thus to do so, a large network of sensors is necessary, and its insertion inside the structure has often to be planned before its construction. This led to a new field of research and techniques: Structural Health Monitoring (SHM). Thanks of their intrinsic small size, electromagnetic immunity and robustness, optical fibers sensors offered many advantages compared to existing electronic strain sensors (vibrating strings, linear variable differential transformers), or temperature sensors (thermocouples, resistive probes). A lot of point-like optical fiber sensors based on Bragg gratings or Fabry-Perot cavities have been developed in the nineties, and their multiplexing is still an intensive research topic nowadays, mainly on the aim of cutting down the whole sensing system costs. About ten year ago, distributed optical fiber sensing technologies appeared, constituting a breakthrough in sensor technology. Those methods, based on non-linear optical effects (Raman, Brillouin) or light backscattering, allow to measure temperature and strain (and consequently pressure, displacement ) into a high range zone, wherever the user want, with tunable spatial resolution, bringing consequently huge multiplexing possibilities. Now this technology has become quite mature and suitable for industry, thanks to the creation of many companies providing distributed optical fibers sensing devices. Nevertheless, several topics are still intensively studied. Indeed, distributed optical strain sensors are always sensitive to temperature as well. Unfortunately, in some real structures as bridges, temperature and strain can both vary significantly at the same time. We will demonstrate, with the example of a distributed Brillouin sensor located into a reinforced concrete beam, that it is very difficult to distinguish thermal and mechanical effects on measurements, even if the physical origin of the process is well known. Spatial resolution limit of these methods is tightly linked to the chosen technology. Non-linear distributed optical sensors (Raman and Brillouin) are based on optical pulse-echo technique. Then, the resolution is proportional to optical pulse time-width, and limited by the phonon lifetime (10 ns). In these cases the minimum spatial resolution is restricted to approximately 1m, and the range can reach 20km because it is only limited by the optical losses (about 0.2dB/km). For some applications, requiring high resolution (sink-hole detection), the Optical Backscattering Reflectometry (OBR) technique could be a better solution, thanks to its resolution better than 1cm. However, the range is then limited to about 100m, mainly due to the light coherence length. Thus, the sensor choice is always a trade-off between long-range and high-resolution. However, between those extreme cases, despite a few promising laboratory experiments, there is no commercial solution for both middle-range (1km) and middle-resolution (5 cm) requirements at the present time, to the best of our knowledge