Ifsttar PhD subject

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Title : Impact of environmental perturbations on distributed fiber optic sensors

Main host Laboratory - Referent Advisor COSYS - IMSE  -  KHADOUR Aghiad      tél. : +33 181668388

Director of the main host Laboratory LINGUERRI Roberto  -

PhD Speciality Capteurs à fibres optiques repartis; Analyse et traitement de données massives

Axis of the performance contract 2 - COP2017 - More efficient and resilient infrastructure

Main location Marne-la-Vallée

Doctoral affiliation UNIVERSITE GUSTAVE EIFFEL

PhD school SCIENCES, INGENIERIE ET ENVIRONNEMENT (SIE)

Planned PhD supervisor KHADOUR Aghiad  -    -

Planned financing Contrat doctoral  - Université Gustave Eiffel

Abstract

State of the art:
To date, the continuous development in distributed fiber optic sensors (DFOS) technologies for physical parameters measurements (e.g. temperature, mechanical strain, mechanical and acoustic vibrations) has enabled the deployment of these sensors in real applications in numerous domains (e.g. civil engineering, geotechnics, intelligent structures) [1-6]. Therefore, DFOSs can be particularly suitable for instrumentation in areas that are difficult to access or in restrictive environments where conventional sensors are not functional. In the case of complex and large-scale civil engineering structures, the DFOS instrumentation network and the installation geometry require the compatibility between the following three components:
1.The optoelectronic interrogation unit:
This unit is the "brain" of the network that carries out an interrogation on the optical fiber core and translates the intrinsic measured characteristics into strain or thermal values. The interrogation techniques are based on optical time (or frequency) domain reflectometry; making it possible to obtain the profile of strain (or temperature) along the optical fiber with tens of thousands of measurement points. This unit is characterized mainly by:
-the investigated backscattering phenomena (Rayleigh, Brillouin or Raman)
-the unit optical power and detection sensitivity
-the unit electronic processing capacity
These characteristics would allow, depending on the type of sensing optical fiber, to specify the maximum range, the minimum spatial resolution, the mechanical/thermal sensitivity and finally the acquisition frequency.
2.The fiber optic sensing cable (i.e. neuron) installed in a host medium:
Consists of one optical fiber or more assembled in a single cable. The different layers of sensing cable should protect and fully transfer the structural strain/thermal field into the optical fiber core. The sensing cable design should satisfy all the constraints related to the installation, conditions of use and operation [7-9]. DFOSs that are based on optical reflectometry techniques explore the optical fiber as a uniaxial sensor along its axis of symmetry [10-11]. "Neuron" is characterized mainly by its geometry, materials, mechanical/thermal characteristics, mechanical adhesion/thermal conductivity between the various layers of the cable and their long-term chemical stabilities.
3.The connection cables of fiber optics:
The connection fibers must guarantee an optical signal transfer between the optoelectronic unit (i.e. brain) and the sensor cable (i.e. neuron), without degradation. It may contain active and passive optical components (e.g. optical switches, optical amplifiers) to distribute the optical signal to the different “neurons” depending on the complexity of the instrumented structure. However, the implementation of an efficient and sustainable DFOSs network into infrastructures depends on the following instrumentation objectives:
-Providing structural health alerts
-Detection, quantification, classification of anomalies (e.g. cracks in concrete) and monitor their evolution
-Measurement of strain, mechanical vibrations and temperature with high precision [6]
This implantation works perfectly for all structures in laboratory conditions and in controlled environment, however as soon as the structure is exposed to external or environmental disturbances, the different measurements become unusable and the instrumentation network becomes non-functional. At the moment, the majority of DFOSs applications in large structures instrumentation have been carried out with low external disturbances and limited durations.
In general, the acquired information delivered by the optoelectronic interrogation unit needs post-processing operations with a significant investigation time. This procedure becomes very complex for massive data in the presence of various disturbances; particularly, the instrumentation of large structures and various parameters.
Therefore, the use of DFOS has been limited to specific cases such as linear installation of optical fibers, low environmental disturbances within a short term.
LISIS has developed its expertise to deliver an accurate measurement of the strain and thermal variations profile that allows early identification of anomalies in different types of structures and applications [12-17].

Scientific objectives
The objective of this thesis will be to study and validate the concept of a “smart” DFOS: it is required to develop a method with massive processing of measurements obtained by optical fibers in real structures with a complex geometry, that may undergo various environmental disturbances. The main expected results are as follows; -a accurate profile of mechanical strain and thermal variations -an automatic diagnostic of the opening of cracks in contact with the sensing optical fiber and the identification of microcracks.
Therefore, we are considering the following two approaches in laboratory conditions:
a-An optical computation:
To work on optical signal processing in order to improve signal quality and translate the acquired data. Analyzing the instrumentation network taking into account the parameters of the optoelectronic interrogation unit, connection cables and sensors. We will mainly use the optical frequency domain reflectometry technique based on Rayleigh backscattering. This technique provides access to the intrinsic data of the optical fiber with millimetric spatial resolution; particularly the evolution of the polarization of light in the fiber during its propagation.
b-An experimental study:
Fabrication of different samples of reinforced concrete (i.e. host medium). DFOS sensing cables will be installed inside the samples during manufacturing, supplementary cables would be installed on the surface afterword. In the meanwhile, conventional instrumentation will also be put in place. External mechanical disturbances will be introduced via piezoelectric components, cyclic thermal variations and then a controlled mechanical degradation of the samples.
Subsequently, algorithms needed to be developed to obtain strain profiles as a function of the temperature of the host medium to identify cracks and microcracks. In this phase, the development of a mechanical and thermal transfer function of optical fiber cables should be studied.
Later one, we would mainly work on the block structure at different scales. to validatate the concept of the developed "smart" DFOS.

Therefore, the work in the thesis would be focused on the following
-Detailed analysis and a summary of the state of the art within the field of study. Specifically, the different concepts of smart sensors would be analyzed, highlighting their level of maturity, the specifications of their technological implementation and their expected performance.
-Performing numerical modeling to understand in depth the impact of each component of DFOS instrumentation network, and developing the most relevant design and optimization tools.
-Defining and carry-out an experimental study in order to demonstrate the reliability of the digital tools in question.
-Conducting a fusion study of information measured by fiber optic sensors and traditional sensors. Analyzing the experimental results and eliminate the impact of disturbances on the taken measurements.
-Carrying out a study to transform the disturbed acquired data into useful information.

Keywords : Distributed fiber optic Sensors, smart sensor, data analysis, anomalies, large scale infrastructure monitoring

Main host Laboratory - Referent Advisor	COSYS - IMSE - KHADOUR Aghiad tél. : +33 181668388
Director of the main host Laboratory	LINGUERRI Roberto -
PhD Speciality	Capteurs à fibres optiques repartis; Analyse et traitement de données massives
Axis of the performance contract	2 - COP2017 - More efficient and resilient infrastructure
Main location	Marne-la-Vallée
Doctoral affiliation	UNIVERSITE GUSTAVE EIFFEL
PhD school	SCIENCES, INGENIERIE ET ENVIRONNEMENT (SIE)
Planned PhD supervisor	KHADOUR Aghiad - -
Planned financing	Contrat doctoral - Université Gustave Eiffel
Abstract State of the art: To date, the continuous development in distributed fiber optic sensors (DFOS) technologies for physical parameters measurements (e.g. temperature, mechanical strain, mechanical and acoustic vibrations) has enabled the deployment of these sensors in real applications in numerous domains (e.g. civil engineering, geotechnics, intelligent structures) [1-6]. Therefore, DFOSs can be particularly suitable for instrumentation in areas that are difficult to access or in restrictive environments where conventional sensors are not functional. In the case of complex and large-scale civil engineering structures, the DFOS instrumentation network and the installation geometry require the compatibility between the following three components: 1.The optoelectronic interrogation unit: This unit is the "brain" of the network that carries out an interrogation on the optical fiber core and translates the intrinsic measured characteristics into strain or thermal values. The interrogation techniques are based on optical time (or frequency) domain reflectometry; making it possible to obtain the profile of strain (or temperature) along the optical fiber with tens of thousands of measurement points. This unit is characterized mainly by: -the investigated backscattering phenomena (Rayleigh, Brillouin or Raman) -the unit optical power and detection sensitivity -the unit electronic processing capacity These characteristics would allow, depending on the type of sensing optical fiber, to specify the maximum range, the minimum spatial resolution, the mechanical/thermal sensitivity and finally the acquisition frequency. 2.The fiber optic sensing cable (i.e. neuron) installed in a host medium: Consists of one optical fiber or more assembled in a single cable. The different layers of sensing cable should protect and fully transfer the structural strain/thermal field into the optical fiber core. The sensing cable design should satisfy all the constraints related to the installation, conditions of use and operation [7-9]. DFOSs that are based on optical reflectometry techniques explore the optical fiber as a uniaxial sensor along its axis of symmetry [10-11]. "Neuron" is characterized mainly by its geometry, materials, mechanical/thermal characteristics, mechanical adhesion/thermal conductivity between the various layers of the cable and their long-term chemical stabilities. 3.The connection cables of fiber optics: The connection fibers must guarantee an optical signal transfer between the optoelectronic unit (i.e. brain) and the sensor cable (i.e. neuron), without degradation. It may contain active and passive optical components (e.g. optical switches, optical amplifiers) to distribute the optical signal to the different “neurons” depending on the complexity of the instrumented structure. However, the implementation of an efficient and sustainable DFOSs network into infrastructures depends on the following instrumentation objectives: -Providing structural health alerts -Detection, quantification, classification of anomalies (e.g. cracks in concrete) and monitor their evolution -Measurement of strain, mechanical vibrations and temperature with high precision [6] This implantation works perfectly for all structures in laboratory conditions and in controlled environment, however as soon as the structure is exposed to external or environmental disturbances, the different measurements become unusable and the instrumentation network becomes non-functional. At the moment, the majority of DFOSs applications in large structures instrumentation have been carried out with low external disturbances and limited durations. In general, the acquired information delivered by the optoelectronic interrogation unit needs post-processing operations with a significant investigation time. This procedure becomes very complex for massive data in the presence of various disturbances; particularly, the instrumentation of large structures and various parameters. Therefore, the use of DFOS has been limited to specific cases such as linear installation of optical fibers, low environmental disturbances within a short term. LISIS has developed its expertise to deliver an accurate measurement of the strain and thermal variations profile that allows early identification of anomalies in different types of structures and applications [12-17]. Scientific objectives The objective of this thesis will be to study and validate the concept of a “smart” DFOS: it is required to develop a method with massive processing of measurements obtained by optical fibers in real structures with a complex geometry, that may undergo various environmental disturbances. The main expected results are as follows; -a accurate profile of mechanical strain and thermal variations -an automatic diagnostic of the opening of cracks in contact with the sensing optical fiber and the identification of microcracks. Therefore, we are considering the following two approaches in laboratory conditions: a-An optical computation: To work on optical signal processing in order to improve signal quality and translate the acquired data. Analyzing the instrumentation network taking into account the parameters of the optoelectronic interrogation unit, connection cables and sensors. We will mainly use the optical frequency domain reflectometry technique based on Rayleigh backscattering. This technique provides access to the intrinsic data of the optical fiber with millimetric spatial resolution; particularly the evolution of the polarization of light in the fiber during its propagation. b-An experimental study: Fabrication of different samples of reinforced concrete (i.e. host medium). DFOS sensing cables will be installed inside the samples during manufacturing, supplementary cables would be installed on the surface afterword. In the meanwhile, conventional instrumentation will also be put in place. External mechanical disturbances will be introduced via piezoelectric components, cyclic thermal variations and then a controlled mechanical degradation of the samples. Subsequently, algorithms needed to be developed to obtain strain profiles as a function of the temperature of the host medium to identify cracks and microcracks. In this phase, the development of a mechanical and thermal transfer function of optical fiber cables should be studied. Later one, we would mainly work on the block structure at different scales. to validatate the concept of the developed "smart" DFOS. Therefore, the work in the thesis would be focused on the following -Detailed analysis and a summary of the state of the art within the field of study. Specifically, the different concepts of smart sensors would be analyzed, highlighting their level of maturity, the specifications of their technological implementation and their expected performance. -Performing numerical modeling to understand in depth the impact of each component of DFOS instrumentation network, and developing the most relevant design and optimization tools. -Defining and carry-out an experimental study in order to demonstrate the reliability of the digital tools in question. -Conducting a fusion study of information measured by fiber optic sensors and traditional sensors. Analyzing the experimental results and eliminate the impact of disturbances on the taken measurements. -Carrying out a study to transform the disturbed acquired data into useful information.
Keywords :	Distributed fiber optic Sensors, smart sensor, data analysis, anomalies, large scale infrastructure monitoring

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