Nicolas Terrien
email: nicolas.terrien@cetim.fr

Fethi Dhamene

Hasnae Zejli

Monssef Drissi

STRUCTURAL HEALTH MONITORING OF A SMART COMPOSITE BRIDGE USING GUIDED WAVES AND ACOUSTIC EMISSION TECHNIQUES

Keywords: Structural Health Monitoring, Acoustic emission, Guided waves, composite footbridge

This paper is dealing with the development of a structural health monitoring (SHM) system implemented on a composite footbridge during the regional project "Pays de la Loire" called DECID2. The SHM system was made out of complementary techniques: strain sensors based on optical fibers (out of concern in this present work) and ultrasonic techniques that are presented in this document. Due to the huge size of the composite bridge (20 m * 3 m), only its most critical areas are monitored as the assembling parts and the most solicited areas. To access the structural integrity of the footbridge, two complementary monitoring strategies were presented in this paper: -a real-time acoustic emission monitoring system to detect fibre breaks,… -and a monitoring system using guided waves to evaluate the resin degradation. Both of these SHM systems use the same miniature ultrasonic patches that are used alternately as acoustic emission sensors and as ultrasonic guided waves actuators. The first step of this work was to develop these patches, and then to set up each monitoring systems and characterize their damage sensitivity. Finally, two composite footbridges were built at the EMC2 Technocampus and IFSTTAR in France to serve as demonstrators.

INTRODUCTION

In recent years, advances in composite development and manufacturing have contributed to emerge fiber reinforced polymers (FRPs) as a practical material for structural engineering applications [START_REF] Gand | Civil and structural engineering applications, recent trends, research and developments on pultruded fiber reinforced polymer closed sections: a review[END_REF]. By offering a number of advantages over steel (non-corrosive, high strength and lightweight), FRP are gradually replacing conventional steel reinforcement and rapidly gaining recognition by taking a prominent place in civil engineering. Moreover, much research on the use of pultruded structural shapes for civil engineering applications have shown the undeniable potential for the use of Glassfiber reinforced polymer (GFRP) [START_REF] Liao | Hunston Effects of environmental aging on the properties of pultruded GFRP[END_REF].

Composite bridge may deteriorate due to overloads or changing load patterns which are far from being predictable easily. The lack of sufficient maintenance and defects evolution between two periodic inspections can bring catastrophic consequences. To overcome these limitations and master the durability and reliability in service, civil structure may to be equipped with Structural Health Monitoring (SHM) systems. Various monitoring strategies were proposed in the literature such as Acoustic Emission (AE) [START_REF] Hamdi | Acoustic emission pattern recognition approach based on Hilbert-Huang transform for structural health monitoring in polymercomposite materials[END_REF][START_REF] Kaphle | Structural health monitoring of bridges using acoustic emission technology and signal processing techniques[END_REF], optical fibre [START_REF] Chapeleau | Embedded optical fiber sensors for in situ and continuous health monitoring of civil engineering structures in composite materials[END_REF][START_REF] Habti | Bending fatigue behaviour of smart glass-fiber reinforced vinylester. composite materials[END_REF] or guided wave (GW) [START_REF] Clarke | Guided wave health monitoring of complex structures[END_REF] which has been proven to be viable SHM techniques. The employment of these three techniques together is very effective as they are complementary.

This paper is dealing with the development of a structural health monitoring (SHM) system implemented on a composite footbridge. Due to the huge size of the composite bridge (20 m * 3 m), only its most critical areas are monitored as the assembling parts and the most solicited areas. To access the structural integrity of the footbridge, two complementary monitoring strategies were discussed in this paper:

-a real-time acoustic emission monitoring system to detect fibre breaks,… -and a monitoring system using guided waves to evaluate the resin degradation. Both of these SHM systems use the same miniature ultrasonic patches that are used alternately as acoustic emission sensors and as ultrasonic guided waves actuators. The first step of this work was to develop these miniaturized ultrasonic patches. Then, the setup of each system was determined and their sensitivity to damage was characterized. Finally, two composite footbridges were built and instrumented with the developed SHM systems.

MATERIALS OF THE COMPOSITE FOOTBRIDGES

The material under study is a pultruded composite material made with continuous glass fibers and a vinylester resin as matrix. The fiber volume fraction is 66% [START_REF] Habti | Bending fatigue behaviour of smart glass-fiber reinforced vinylester. composite materials[END_REF]. Test specimens were manufactured from rectangular shaped profiles (section dimensions: 16mm x 40mm). They were diamond saw-cut and the dimensions of the specimens were 320mmx40mmx16mm. These dimensions follow the recommendations of the EN ISO 178 standards. The distribution of fibers in terms of positions and diameters are inhomogeneous (Figure 1). One may also note that fibers are not perfectly straight. Some fibers intersect the image plane and others make an angle with the main direction (Pultrusion's direction). Finally, some porosity and additives are observed in the microstructure.

MINIATURIZED ULTRASONIC SENSORS

Sensor design

Both of these SHM systems use the same miniature ultrasonic patches that are used alternately as acoustic emission sensors and as ultrasonic guided waves actuators. The first step of this work was to develop these miniaturized ultrasonic patches. The ultrasonic sensor which was chosen for this work is a piezoelectric thin disc with 200 µm thickness and 20 mm diameter. It was coated with two electrodes to get ease of welding to the upper surface. This patch is directly bonded to the pultruded surface with environment-resistant glue. The glue has been tested previously in immersion test for one month at 40°C and showed good aging resistance in terms of mechanical strength and acoustic coupling. The connector includes a coaxial environment resistant "BNC cable" welded directly to the patch upper surface. To protect patch and welds, two different coating are used: an epoxy resin for the mechanical protection and polyurethane clear coats to ensure UV and water resistance. Patches (Figure 2) qualification required several tests to evaluate its resistance to hygro-thermal aging, sensitivity and durability.

Sensitivity and durability of miniaturized sensors

Sensitivity are evaluated with Hsu-Nielsen source (leads break 2H03) at 5 cm distance from patches (after coating). Amplitude measured is about 72 dBae which is less than value obtained for standard AE sensors (90 dBae). Patch sensors background noise are also compared to standard sensors and was evaluated to 10 dBae in the case of both sensors. Through the measurements of these two parameters, it was concluded that theses patches can be used as acoustic emission sensors although they are less sensitive to damage detection than their conventional counterparts. It will remain thereafter to determine the level of sensitivity of theses patches to damage detection in pultruded profiles which will allow the deduction of their spacing in the final structure.

To avoid patch hygrothermal ageing, clear coating polyurethane was used to protect patches and resin layer against moisture. In order to validate this coating, an immersion test was performed at 40°C during one month. This polyurethane coating showed good ability to protection against moisture and hygrothermal aging.

The influence of severe weather like cold and frost on the patches and instrumentation performances was also supported. Safe operability of patches and preamplifiers was verified by inserting them into a freezer at -18°C. Sensitivity measurement tests before and after exposure confirm that preamplifiers and patches remain operational during the cold. Thus, it was concluded that low temperature does not deteriorate the patches protection and their performance.

The mechanical strength of patches depends on the piezoelectric ceramic disc and its welds because they are the limiting factors in terms of shock-resistance, knowing that the bonded interface will face fatigue phenomena. Mechanical durability is therefore evaluated by fatigue bending tests performed on pultruded composite bars (vinylester matrix and glass-fibers) that were instrumented by two ultrasonic patches. The fatigue cycle was conducted at a constant amplitude triangular waveform with a frequency of 2 Hz and 36000 cycles per strain level. Durability was assessed by measuring sensitivity by leads break after each strain level cycles (0.12% to 1.065%). The results show that mechanical test did not alter patches sensitivity and their coating.

ACTIVE MONITORING OF PULTRUDED BEAMS USING GUIDED WAVES

Monitoring of pultruded profiles is based on two complementary methods (active and passive) using the same sensors (patches). In the case of active surveillance, slow mechanical resin degradation is detected when using guided waves.

The first step was to simulate analytical wave propagation in a composite bar and then to select the most sensitive Lamb waves to the resin ageing. Figure 3 shows wave propagation modelling results in the cases of intact resin and 50% degraded resin. The mode A 0 generated at low frequency seems to be a good candidate to evaluate vinylester resin degradation. This is explained by the fact that low frequency A 0 mode has a perpendicular displacement component to the plane of the bar. To meet the health monitoring system requirements, ultrasonic patches are chosen for generating and detecting A 0 mode. Diameter and thickness of patches are selected to generate the desired wave length (λ= 34.59 mm; f=30 kHz) in pultruded bars. The wave propagation velocity plotted as a function of the resin Young's modulus highlight the reduction of A 0 mode velocity with the decrease of Young modulus. In order to quantify mode A 0 sensitivity to the decrease of resin stiffness, a compression test (perpendicular to fibres direction, Figure 4) monitored by guided waves was performed on a composite bar at various temperatures (20°C, 50°C, 75°C and 100°C). Two patches are therefore used, one as an emitter and the other as a receiver. A burst signal with a magnitude of 10 V pp was generated every 10 ms at a frequency of 30 kHz. For each temperature, a load test was performed up to 50kN (within the elasticity area) to calculate Young's modulus. Guided wave measurements were made at each temperature in order to determine the influence of stiffness loss on guided wave propagation. It is interesting to note (Figure 5) that the decrease of resin's stiffness due to increasing temperature, results in the increase of the arrival time of the A 0 Lamb mode. For a loss of 39% of resin stiffness, a delay time of 33 µs was measured. Thus, these experiments have confirmed the decrease of the A 0 wave velocity with the reduction of the resin's stiffness.

PASSIVE MONITORING OF PULTRUDED BEAMS USING ACOUSTIC EMISSION

Acoustic fingerprints of damages occurring in pultruded bars

Miniaturized patches are also used in the acoustic emission health monitoring system to detect sudden damages such as matrix micro-crackings, fibre breaks… close to the assembling parts or to the most stressed areas of the footbridge. To implement this system, the first step was to characterize the acoustic fingerprints of the damages occurring in pultruded samples during mechanical tests. The different types of damage occurring in the pultruded bars were identified using conventional acoustic emission sensors (Mistras R15) during loading-unloading bending test (Figure 6). From the test start to the end of the linear strain-stress region (0.8% deformation), AE multiparametrics analysis revealed one single population associated with multiple matrix damage and few broken fibers (Figure 6a). Upper than 0.8 % deformation, two new AE populations appear related to fiber breaks and delaminations (Figure 6b). During loading-unloading test, the Felicity ratio determines the beginning of damage which is estimated at a strain value of 0.25%. For this strain, acoustic amplitude activity is still lower than 52dB which is indicating the existence of mostly matrix damage. Fatigue tests were performed to assess the mechanical behavior of pultruded material and its AE response. The correlation between AE (Figure 7) and mechanical behaviour gives additional explanation about fatigue damage mechanisms evolution. During the first level (=0.11%), no AE was recorded. It is worthy noting that only one AE population is found during the second stage (=0.23%) in accordance with quasi-static tests and Felicity ratio. This may mainly correspond to matrix damage. During the third stage (=0.42%), some AE signals appear and begin to form a second population. This population strongly increases during the last two stages. Plotting stress versus strain indicates that strain remained constant at the first four stages and increases during the two last stages (Figure 8). This confirms that the exceedance of the linear stress-strain region (which is related to fiber failures) has occurred at 0.8% strain and 241MPa stress in accordance with quasi-static tests. Therefore the second population could be related to fiber breaks.

Definition and qualification of AE SHM system using miniaturized sensors

In order to determine the type of damage detected by ultrasonic patches, the same qualitative analysis was applied for different tests (quasi-static, load-unload, and fatigue). Thus, the ability of patches to detect different damage mechanisms is supported based on results obtained by R15 sensors. Acoustic emission activity recorded during quasi-static and loading-unloading tests is zero for maximum strains lower than 0.8%.

Bending loading-unloading tests showed the existence of a threshold strain from which there is a loss of stiffness. Knowing that matrix damage does not change significantly the stiffness of the composite, this loss of stiffness is mainly attributed to fibers breaks. Patches began to detect AE from a strain of 1%. This early detection fits well with a loss of stiffness due to fibers breaks which corresponds to the apparition of the second AE population recorded by the R15 sensors. Therefore, it can be concluded that patches are only sensitive to fibers failures and they are unable to detect the onset of matrix damage. AE parameters from patches recording were determined. Maximum values of theses parameters (Amplitude, duration, energy) increased with load and strain but mean values vary very slightly. Average amplitudes detected do not exceed 54 dB.

Fatigue bending test confirms that fatigue damage detected by patches (related to fibers breaks) is the same as the one detected during quasi-static tests for strain higher than 1%. The onset of damage detected using patches is located at the middle of the specimen and is characterized by average amplitude of 41dB. According to attenuation measurement (0.58dB/cm) and detection threshold of 35dB, the maximum distance between two patches, for damage detection in bridge composite structure, is set to 15 cm.

Effects of climatic conditions on the reliability of AE system

The climatic conditions such as frost, wind, rain and hail were studied to determine the abilities or limitations of AE system to discriminate them from damages using ultrasonic patches. AE recording was performed outside during cold weather at -9°C and showed that frost, light rain and wind does not disturb AE measurements. Only a low increase of background noise was observed due to wind (very low compared to the threshold). However, for heavy rain and hail, discrimination between damage and noise is difficult using simple analysis. Frequency and statistical analysis could be applied to improve classification. Location analysis appears to be an effective method for noise discrimination.

Conclusions

So, this part permitted us to define the sensor locations, their spacing within the composite bridge and the damage criterion. The influence of climatic conditions such as frost, wind, rain, and hail on the structural health monitoring system was also studied in order avoid false alarms.

CONCLUSIONS: BUILDING OF TWO SMART COMPOSITE FOOTBRIDGES

Finally, two composite footbridges were built at the EMC2 Technocampus and IFSTTAR in France to serve as demonstrators (Figure 10). They are made from pultruded composite (glass fiber/vinylester) that are provided with a structural health system in the form of embedded optical fibers and miniaturized ultrasonic sensors glued to the bars. Both active (guided waves) and passive (acoustic emission) structural health monitoring systems were installed. Active method can detect the resin stiffness degradation using guided wave and passive method (acoustic emission) can detect and locate fibres breaks. In this work, miniaturized ultrasonic sensors are developed and validated by various environmental and mechanical tests. Their ability to detect fibers breaks was demonstrated despite a low sensitivity compared to standard AE sensors. Model prediction of A 0 mode sensitivity to resin stiffness loss was validated by quantitative mechanical experiments at various temperatures. Sensors are installed at critical areas, namely the highest bend area and near bolts. The acoustic emission monitoring system is remotely controlled via an internet connection and configured to warn the supervisor by email-sending once an active damage is detected. The real time detection and location of the degradation provides safety of installation and users and will avoid catastrophic failure.

Figure 1 :

 1 Figure 1: SEM microstructure observation.

Figure 2 :

 2 Figure 2: Miniature ultrasonic patches

Figure 3 :

 3 Figure 3: Dispersion curves on vinylester bar (intact and degraded): a) Wave length, b) Group velocity.

Figure 4 :

 4 Figure 4: Compression test at temperature up to 100°C.

Figure 5 :

 5 Figure 5: Delay for the arrival time of A0 mode versus loss of resin stiffness.

Figure 6 :

 6 Figure 6: Located events population for the linear stress-strain region (< 0,8%); b) Located events population for ≥0,8%. (R15 sensors).

Figure 7 :

 7 Figure 7: AE activity during fatigue test: stage 1 (=0.12%) to stage 6 (=1%). (R15 sensors).

Figure 8 :

 8 Figure 8: Strain evolution during fatigue test.

Figure 9 :

 9 Figure 9: AE detected during the 6 th phase of fatigue test and detected during loadingunloading bending test Compression test at temperature up to 100°C.

  10: a) Demonstrator composite Footbridge; b) Miniatures sensors for bolt assembly monitoring.

EWSHM 2014 -Nantes, France

Acknowledgements: This research work was conducted within the National Project DECID2 that was financially supported by the French Department of Industry and the Area Pays de La Loire. The authors would like to thank the sponsors for their precious support.


Pierre Ferdinand
email: pierre.ferdinand@cea.fr

THE EVOLUTION OF OPTICAL FIBER SENSORS TECHNOLOGIES DURING THE 35 LAST YEARS AND THEIR APPLICATIONS IN STRUCTURAL HEALTH MONITORING

Keywords: Optical Fiber Sensors (OFS), IWSHM, EWSHM, Bibliometry, Statistics

Since late 70s, (quasi-)distributed Optical Fiber Sensors have been developed, 12 countries having produced 85% of the global effort in this field. Since mid-80s, OFSs have caught attention in many sectors where SHM is a matter of concern (civil engineering, composites, oil & gas, renewable energies, safety...). Today, OFS is became a strategic domain, especially in Asia, and China invests a lot since 7-8 years. The top 12 countries involved in SHM are almost the same than in OFS, having published 80% of the total number of SHM papers. Moreover, statistics proves that the "optical fiber" is now the second sensing technology for SHM, and 2/3 concern the FBG-based sensing.

COUNTRIES INVOLVED IN OFS TECHNOLOGIES

The database containing all the proceedings of the Optical Fiber Sensors conference was used to achieve this bibliometric analysis [START_REF]Proceedings of the International Conferences on Optical Fiber Sensors -Collected Papers of the International Conferences on OFS[END_REF]. From 1983 to 2012, this represents 3827 papers (the last OFS-23 is not considered). One should remind that this lecture series brings together the international OFS community every 18 months and cyclically in Europe, America and Asia. The very first event (OFS-1) was held in London in April 1983 and the last one considered (OFS-22) in Oct. 2012, Beijing, China. We may observe that OFS has occurred 9 times in Europe, 7 times in America and in Asia. The next event is planned to be hosted by Brazil, in the region of Iguazù falls, i.e. close to the triple border area (Brazil, Argentina and Paraguay), during fall 2015. This will be the first time OFS will take place in South America and only the third time in the southern hemisphere. During these 35 last years, 42 countries were involved, with a very variable production of published papers (Fig. 1). In fact, 12 countries produced 85% of the total number of papers. The main contributors are: Japan (16.0%), followed by UK (15.6%), the USA (15.2%) and China (10.6%). The most impressive result of this analysis is certainly the fact that China in now the 4 th contributor. The 'production' of this country was quite small from 1983 to roughly 2004. But, during the past 7-8 years China made a so large effort to invest in fiber sensing that it has been able to get the 4 th ranking (Fig. 2). So, on this base, we may anticipate that China will become first OFS contributor before the end of this decade.

THE LONG EVOLUTION OF THE OPTICAL FIBER SENSOR TECHNOLOGIES

The field of Optical Fiber Sensors (OFS) concerns a very dynamic community for over 35 years. The late 70s and the following years allowed pioneers to explore the potential of an innovative component: the optical fiber, whose intrinsic features have led some researchers to develop the very first OFS prototypes in the dark rooms of their labs, and others to initiate projects in telecommunications. The 80s gave the opportunity to explore possibilities offered by some OFS mock-ups, thanks to the very first single-mode fibers, some optical components (e.g. the famous "3-dB coupler") and to some equipments then available (fusion splicers, multimode optical connectors...). Many principles were explored at that time, particularly those implementing the interferometry (Sagnac, Mach-Zehnder...) respectively for the development of Fiber Optic Gyroscopes (the "FOG"; Sagnac effect), current sensors (Faraday effect), and also hydrophones for the navy. In 1986, the concept of OFS Network (OFSN), distributed and quasi-distributed, was born, i.e. the possibility to multiplex sensors on a given fiber, to reduce cabling, sensor data management, and finally the cost-per-measurement-point. Then, by the end of the 80s, new products based on optical reflectometry, i.e. the OTDR (Optical Time Domain Reflectometer; somehow a guided optics Radar) appeared, as the Raman DTS (Distributed Temperature Sensor) that provides a selective measurement of the fiber temperature profile.

At the early beginning of the 90s (more accurately in Sept. 1989 at OFS-9, Paris, France [START_REF] Morey | Bragg grating Temperature & Strain Sensors OFS[END_REF]), a very important technology for SHM applications has been introduced: the Fiber Bragg Grating (FBG). As a reminder, this transducer (a narrow spectral filter photo-written by laser in the fiber core) reflects a characteristic wavelength used to simultaneously provide its address in the sensor network, and the measurement (temperature, strain...). During this decade, leaving the labs, only robust techniques survived: those based on FBG filters, and to a lesser extent on micro Fabry-Perot (both quasi-distributed sensors), and the reflectometry (distributed sensing) which implement backscattering phenomena, namely Rayleigh effects (OTDR), Raman (DTS) and Brillouin sensing, usually designated by the letters BOTDR (Brillouin Optical Time Domain Reflectometry) and BOTDA (the letter A being for Analysis). Both kind of Brillouin sensing techniques became commercially available towards the end of the 90s. In the same time, the first OFS applications devoted to materials and structures monitoring started to appear [START_REF] Sansonetti | Intelligent Composites containing measuring fiber optic network for continuous self diagnosis[END_REF]. As an example, fibers were soon identified as quasi non-intrusive embeddable sensors, able to monitor composite curing (smart manufacturing), strain testing for qualification, and of course for SHM purposes.

The end of the 90s and the first years of the millennium were that of the optical Telecommunications 'boom', followed in 2001 by the Internet bubble 'crack'. During several years, many SMEs disappeared but others emerged. Some of them were involved in sensors, optoelectronic systems, or applications increasingly SHM-oriented. The 2000s saw the advent of a new optical guides: the "Photonic Crystal Fibers". Indeed, after 30 years of traditional optical fibers manufacturing refinement (OVD, MCVD...), studies concerning fiber design and development have had to adapt to requests from new scientific/industrial sectors (wideband optical sources, sensors, biophotonic...). We may anticipate that these new fibers are likely to offer new opportunities for the SHM in a near future.

THE MAIN OPTICAL FIBER SENSOR TECHNOLOGIES USED IN SHM

As explained in the previous paragraph, a lot of OFS technologies have been developed during the last decades. Nevertheless, the screening imposed both by the end user requests and by field experiments may be compared to the Darwin theory of evolution: only best techniques, of well adapted solutions fitting with specifications may stay alive. Others may also survive, but only on small or very specific niches. That's for instance the case for the Fabry-Perot interferometers (non-optically multiplexable), or the interferometry (very good resolution, but un-multiplexable, difficult to use on real applications, sensitivity non constant (sinus), etc.). On the contrary more versatile and flexible solutions, i.e. the Bragg gratings (FBG), the king of the quasi-distributed sensors, and several technologies based on Reflectometry (OTDR, OFDR, DTS, BOTDR-A, BOCDA...), using the scattering phenomena in silica, are now widely used. All of them are sensor networks leading to a wide multiplexing of measurement points on a stand-alone remote instrumentation. So, due to this wide multiplexing, additionally to good sensing performances, they allow to reduce the cost-per-measurement-point.

Fiber Bragg Gratings

Tiny diffraction gratings photo-written inside a single mode optical fiber core, called Fiber Bragg Gratings (FBGs), are now used for many years in sensing applications for numerous industrial fields. Reflecting a narrow spectral line called the Bragg wavelength, these transducers encode the parameter to measure (temperature, strain, pressure, refractive index...) in the form of a wavelengthshift of the Bragg wavelength [START_REF] Morey | Bragg grating Temperature & Strain Sensors OFS[END_REF]. As a wavelength does not depend on signal power, therefore FBG-based sensing is insensitive to amplitude fluctuations of the optical signal, as could occur in harsh environments, especially in the case of deployment for nuclear applications with radiation induced attenuation (RIA) in the optical fibers. It's now well known that FBGs feature key advantages for sensing in hostile environments: electromagnetic immunity (EMI), low intrusivity, low power consumption, excellent metrological properties, remote interrogation capabilities and wavelength-encoded measurement information. Although, conventional silica FBGs are usually limited for long-term use in high temperature environments. Through thermal engineering of such gratings, FBGs transducers are now made highly resistant even for temperature, and also the FBG photo-written by fs laser, reaching 900°C during several thousands of hours. The so-called regenerated FBG transducers exhibit considerable improvements of their thermal stability with respect to their conventional counterpart [START_REF] Laffont | Regenerated Fiber Bragg Grating sensors for high temperature monitoring in Sodium-cooled Fast Reactor[END_REF][START_REF] Laffont | Multiplexed regenerated Fiber Bragg Gratins for High-temperature measurement[END_REF]. Moreover, FBGs sensors benefit from the wavelength multiplexing technology (called DWDM), used to multiplex the channels in optical telecommunications (Fig. 3). In that sense, several tens of FBG sensors can be distributed over a single optical fiber (or using even more complex routing topology) and each sensor is identified through its own Bragg wavelength, defined during the photo-writing process. For strain sensing the sensitivity b ~ 1.2 pm / (µm/m) at λ = 1.55 µm; for any other wavelength, proportionality may be applied to get the sensitivities)

Optical Time Domain Reflectometry (OTDR)

For three decades, the OTDR is widely and commonly used in telecommunications as it help ensure the availability of the distribution network (detection of any changes in the fiber attenuation profile, location of specific problems like bending, breakage or bad connection...). Many OTDRs are commercially available and used by operators in this industrial sector. Several types of instruments exist, mainly for long distances (x km) and spatial resolution ~ 1 m, while some provide a fine spatial resolution (a few cm with the "ν-OTDR", based on photon counting). In an OTDR, powerful short optical pulses (10 ns to provide 1 m spatial resolution) are repetitively injected into the fiber core and the Rayleigh backscattering detected from the same fiber end. The backscattering intensity, at the wavelength of laser, is due to the fluctuations of the fiber core refractive index (Rayleigh scattering) and to Fresnel reflections created by discontinuities (connectors, splices, fiber ends). The temporal signal analysis provides a spatial image of losses, defects and discontinuities. So, any temporal evolution of this backscattered light refers to a local modification of the fiber attenuation profile.

Distributed Temperature Sensor -Raman DTS

The DTS principle, is based on the Raman backscattering in the fiber core analyzed by OTDR, i.e. a process of spatial localization along the fiber, based on the spatio-temporal duality (temporal analysis is equivalent to a spatial scaling, the light velocity c/n being known). Traditional OTDRs use the elastic Rayleigh backscattering in the fiber core. However, the Rayleigh effect is not the only scattering phenomenon in optical fibers, Raman and Brillouin effects are also used. In case of Raman scattering, there is interest in the light having undergone a wavelength shift of a few tens of nm from the excitation wavelength (about 40 nm @ 1 µm in wavelength): The inelastic scattering mechanism leads to generate two symmetrical frequencies (two spectral lines) with respect to the laser excitation (Fig. 4). The respectively called ν 0 Stokes line (ν S = ν 0 -ν B ) and anti-Stokes line (ν AS = ν 0 + ν B ), ν B denotes the characteristic vibration frequency of the fiber core. Since Ι AS the signal corresponding to the anti-Stokes line comes from an energy level higher than the fundamental one, it is strongly temperature-dependent according to the well-known Boltzmann statistics: exp(-hν B /kT). The Stokes signal I S , coming from the fundamental energy level, is free from temperature influence. The ratio R = Ι AS /I S is defined by: R(T) = (λ S /λ AS ) 4 . exp(-hν/kT) where h and k are respectively the Planck and Boltzmann constants. Such a relation is only function of temperature and vibrational characteristics of the fiber. Thus, any influence of variations in the transfer function of the instrument (power source, detector sensitivity, amplifier gain...) is removed by the normalization of these two intensities, and a very selective temperature measurement may be obtained [START_REF] Hartog | Distributed temperature sensing in solid-core fibers[END_REF][START_REF] Dakin | Temperature distribution measurement using Raman ratio thermometry[END_REF]. That means the advantage of the Raman technique lies in the fact that the result does not depend on the local value of the intensity of the initial pulses, and provides high insensitivity to mechanical perturbations applied to the fiber, as well as to local or distributed fiber losses. This technique, in addition to OTDR features used, provides a means to determine the temperature profile all along the fiber under test. Since many years, several DTS are commercially available.

Distributed Brillouin Optical Time Domain Analyser & Reflectometer (BOTDA -BOTDR)

Distributed sensors founded on the Brillouin effect are based on the inelastic interaction of the light wave propagating in the fiber core with acoustic phonons. The metrological interest lies in the dependence of this shift with temperature (~ 1 MHz/°C) and strain (1 MHz / 20 µm/m) [START_REF] Thénenaz | Monitoring of large structure using distributed Brillouin fibre sensing[END_REF]. The R&D led to the development of two Brillouin techniques, BOTDR and BOTDA. The main advantage of the BOTDR is that it does not need to have access to both fiber ends. Its drawback is the lower efficiency leading to long measurement times. The advantage of the BOTDA lies to a better signal-to-noise ratio, but the fiber must be looped on the instrument. Due to Brillouin amplification, the BOTDA provides useful signals whose intensity exceeds by two orders of magnitude Rayleigh backscattering. The spatial resolution obtained today with commercial systems are: 30-50 km range (up to 80-100 km, or even more recently in Labs); 1 m resolution (or even better, but for a smaller range), while the Brillouin frequency resolution remains about 1 MHz, equivalent to 1°C in temperature and 20 µm/m in strain. So far, half a dozen manufacturers offer Brillouin instruments. Functionalities and performances differ on how to separate the strain measurement from temperature influence, in terms of spatial resolutions, measurement time, without forgetting the ergonomic aspect (the possibility to easily drive the instrument, the quality of the Man Machine Interface...).

Distributed Rayleigh Optical Frequency Domain Reflectometer (OFDR)

This relatively new approach also involves the Rayleigh backscattering but needs a frequency analysis to determine temperature / strain profiles (it's basically a non-selective measurement). It is based on an homodyne interferometer scan (the source is modulated by frequency ramps), [START_REF] Gifford | Distributed Fiber-Optic Temperature Sensing using Rayleigh Backscatter[END_REF]. Within the system, the laser beam is split into two sub-beams one of which is sent into the fiber under test and the other used to interfere with the backscatter light from the fiber. It allows monitoring of a structure, if this one is equipped with an embedded or attached fiber on its surface, with a spatial resolution of 1 cm and a sensing resolution of few µstrains (µm/m), or 0.1°C in temperature. The duration of a typical acquisition is about 10 s without the signal processing and about 5 min including the data treatment.

The range (maximum fiber length) is basically limited to 70 m, but a 2 km option (by successive windowing) has recently been proposed. So, in terms of performances, let alone its low measurement speed, this technique is quite equivalent to a chain of hundreds FBGs. Recently a faster instrument (200 Hz) has been proposed but its strain range is quite limited. Of course, the above presentation is just an overview, as distributed and quasi-distributed technologies offer many possibilities. These extra options have been developed, or are currently under development, in order to enhance sensing performances in many ways (better resolution, faster measurement, possibility to separate strain from temperature influence, possibility to sense a new parameter through an innovative packaging…). At any new solution, a new acronym. The aim of this paper is not to review them all; nevertheless Fig. 5 and6 present an overview of these options. So, every September, engineers, researchers and academics with interests in SHM and associated research may attend a workshop, in USA or in a European country [START_REF]Proc. of the European Worskshop on Structural Health Monitoring. -Proc. of the First European Workshop[END_REF]. The purpose of these International and European SHM workshops is to allow key and emerging technical issues that are critical and unique in SHM to be identified and discussed, as well as to allow current state-of-the-art and R&D activities in this field to be presented. Those workshops are also intended to promote exchanges and cross-fertilization among disciplines. Twelve years ago, a series of SHM Workshop have been initiated in Asia-Pacific. The first Australian one was held in 2002. While, the first International Workshop on Advanced Sensors, Structural Health Monitoring, and Smart Structures was held in Japan in 2003. During the second Australian workshop in 2004, people observed a drastic increase in the number of researchers in this field in Asia-Pacific region. This led to the extension of these SHM workshops to the Asia-Pacific region. Since, this APWSHM workshop series took place every odd year in December: Yokohama, Japan (2006); Melbourne, Australia (2008 and 2012); Tokyo, Japan (2010). The 5 th APWSHM, organized by the Harbin Institute of Technology, will be held in Shenzhen, China on Dec. 4-5, 2014.

Unfortunately, the corpus of this Asia-Pacific series (8 workshops since 2002) has not been included in the SHM statistics presented below, due to the lack of data available. So, the database used to achieve this bibliometric analysis only contains the papers of the IWSHM series and those of the EWSHM events. From 1997 to 2014, this represents 3249 papers. The table below (Tab. 3) presents the location of these events.

Countries involved in SHM

The bibliometric analysis shows that 57 countries have contributed to the International or European Workshops, between 1997 and 2014 (Fig. 8). Nevertheless, only a few numbers of countries is very active: the top 10 countries having published 80% of the total (Fig. 7). The 12 countries most involved in OFS are also among the top 12 in SHM, except Greece which publishes very little in the field of OFS but which ranks in 12 th position in SHM, unlike Switzerland which is relatively active in OFS but few in SHM. Statistics shows an increasing number of IWSHM papers year after year: from 65 papers in 1997, to reach about 300 communications in 2013. Since 2002, the EWSHM production was quite stable, ranging from 155 to 210 papers, except this year 2014, when it suddenly jumped to 302 publications. So finally, the IWSHM total contribution reaches 1897 papers, while the European series of SHM workshops reaches a total of 1352 papers.

Two specific points may be noticed: it's somehow obvious that the distance between a given country and the Workshop location influences the volume of its participation: for instance the USA production is triple at Stanford, than for any European event.

Extra point, similar as observed with the OFS bibliometry: China's contribution (ranking 8) is growing "exponentially"; Germany does the same (ranking 2) as well as Poland (ranking 6). Considering the corpus of 3249 papers, we observe the following sharing: USA (36%), Germany (9.9%), Japan and UK (5.8%), France (5.5%). So the top 5 countries produced 63% of the total. The weight of the USA is considerable in this ranking per country, but if we consider the production per world region, the ranking is different: Europe (30 countries involved in SHM) produces 43.1% of the total, with half of them coming from Germany + UK + France, America (38.6%, with 36% for the USA), Asia-Pacific (16.6%) and Africa plus middle-east (1.7%). Ranking becomes also very different if the paper production of these countries is normalized by their National Gross Product! More precisely, we may also estimate the influence of the OFS techniques and sub-techniques, presented above. The result is very clear: FBG sensing is widely considered. Since IWSHM1997, an average of 64.2% of the SMH papers involved in fiber sensing are using the FBG technology, the ratio being 50% for this last Workshop EWSHM2014 (Fig. 10). So, we may remember that about 2/3 of the SHM projects which includes fiber sensors, have chosen quasi-distributed Fiber Bragg Gratings. Other optical fiber techniques used in SHM are as follows: the distributed Brillouin sensing (BOTDR -BOTDA), and more recently the Rayleigh OFDR. Other OFS techniques, as Polarimetry and Interferometry (Fabry-Perot…), are less and less considered for SHM.

Sensors for SHM of Composite materials based on OFS

Carbon and Glass Fiber Reinforced Plastics (CFPR and GFRP) are widely used as structural materials due to their high strength/weight ratio vs. metallic ones. However, some issues concern their durability, the possibility of damage detection and prediction, the characterization of residual stresses, and the identification of their global mechanical properties. In addition to this, the aerospace sector for instance has specified the need for multi-functional sensor systems. With time, this aim became more and more realistic with optical fibers as they can be embedded at the early stage of structure manufacturing and used up to its end of life. Hence, the same distributed sensor, or the same sensor network, may be used to monitor the manufacturing, to qualify the structure, and in-service to monitor its structural integrity, and to control smart structures. These various uses may be resumed as follows:

• 1/ Smart Manufacturing: An SHM based on optical fibers can readily be applied to any fiber reinforced matrix system. In particular, it may be used to control the injection of resin to avoid voids, or to monitor residual stresses at specified locations within the composite material while curing. This remains important for large and thick composite structures, for which it is very difficult to measure the temperature profile during manufacturing. Furthermore, the quantification of the magnitude of the residual stress is also just as important for composites as these stresses will influence the nature, and extent of damage development. A better knowledge of manufacturing process would help to understand the material's structural integrity and the inevitable variability of properties induced by this process. In this way, the monitoring of composite material manufacturing is a key feature for better process understanding and quality improvement. To perform such monitoring, e.g. in RTM, it's possible to use small and weakly intrusive optical Fibre Bragg Gratings (FBG)based sensor for both contact pressure and temperature variation measurements [START_REF] Maurin | Development of an integrated Fibre Bragg Grating contact pressure and temperature sensor for composite smart manufacturing[END_REF], • 2/ In situ Health Monitoring: An SHM based on optical fibers may provide real-time strain data which in turn can be used to assess the composite structure integrity, in-service. Any damage may be detected, as the structure's rigidity will change versus the extent of damages. Fiber-based sensing is fully compatible with any fiber reinforced composite technology (prepreg, filament winding, RTM, infusion…) currently used in the aerospace, marine, automotive, or railway industries [START_REF] Ferdinand | High performances Fiber Bragg Grating systems for Smart Structures: Application to Overhead Contact Line monitoring in the railway industry[END_REF]… The durability of sensors is also a matter of concern for such long term monitoring. On this point too, optical fibers are performing. When embedded into the composite material they have an expected life time equal those of the structure itself, much longer than that of surface mounted sensors [START_REF] Maurin | FBG-based smart composite bogies for railway applications[END_REF],

• 3/ Active Vibration & Shape Control: Strain informations provided by an OFS-based SHM can also be used to control the vibration characteristics of a structure (i.e. for control damping). In addition, strain data may be useful to control the shape of a composite panel for instance.

Composite materials are now used in more and more industrial sectors. In term of volume, aeronautics is certainly the first one, but others like marine sector and civil engineering are also concerned. In North America for instance, the ISIS Canada network association gained a world leadership position through the use of advance composites materials and their applications of SHM to civil structures, such as bridges. Such composites often used for repairs include OFS.

SHM of civil structures based on OFS

The civil engineering covers roads, highways, bridges, tunnels, dams, earthworks, as well as some additional areas related to road safety (weather: fog, sleet, snow... state of the surface road: vehiclepavement interaction ...) and operation (traffic growth, Weigh-In-Motion, driving assistance ...), inducing many needs. Moreover, the monitoring of public works include the need for design assistance for infrastructures and structures still in project, controls during construction phase, and the monitoring to schedule the maintenance for those already in operation.

Many people believe that civil engineering is a traditional sector, but high technologies start to broadcast in this industry. The optical technologies can provide rich information around the needs of the user in real time. End user needs concern reliable systems, turnkey, at optimized cost (purchase + operating + maintenance). This was an impossible mission in the past, but it becomes now possible to fulfill such needs, because costs of optoelectronic components (lasers, detectors, couplers, and fibers) have been drastically reduced by the telecommunications market.

Civil infrastructures deteriorate with time, due to aging of materials, over usage, overloading, climatic influences, corrosion, inadequate maintenance… leading to accelerate their obsolescence. As a consequence, repair, retrofit and rehabilitation become more and more necessary with time to insure safety of use. This is typically the case for bridges. Modern approaches based on optical means include several techniques (imaging, topography, guided laser beam, GPS...). But, they do not provide an analytical sensing and the determination of internal origins of local effects they determine. Considering the age of existing civil works, there is a growing need for performing remote monitoring due to various attacks suffered by structures: pollution, acid, rain ... both for economic reasons induced by increased traffic, as well as traditional/human inspection and maintenance become more and more costly.

An SHM system based on OFS, and especially on FBGs embedded into or attached to a structure may monitor these evolutions throughout his life. Moreover, it may determine the load distribution, strain levels or temperature profiles, internal damages, and trigger alarm if measurements exceed predetermined thresholds [14 to 18]. As for composite materials, such a system can also monitor structural components during construction phases, possibly leading to improved manufacturing and so increase the quality of building. In addition, for example, in the case of a bridge, the optical SHM may be used to generate the traffic information. We may imagine such a structure properly equipped that can monitor not only the number of vehicles per minute, but also their Weight-In-Motion (vehicle identification / classification), the flow direction or speed as well as detection of incidents or accidents. In terms of degradation, it is also possible to be early informed by the SHM system, allowing an early repair, therefore reducing cost.

OFS-based SHM in the industry

The SHM is a particularly promising sector that has attracted significant attention in recent years for its wide field of applications, particularly for civil and aircraft structures. Of course, the application sectors for OFS in SHM are numerous; they relate to almost all types of structures at risk or linked to a strategic use. Beyond the fields of civil engineering and sectors using composite materials, as mentioned above, OSF-based SHM may include many other industries, as for instance the nuclear power industry where safety will have to be strengthened after the nuclear accident at the plant Fukushima-Daiichi, Japan, in March 2011 [START_REF] Ferdinand | Enhancing safety in Nuclear Power Plant with Optical Fiber Sensors[END_REF], the steel industry seeking new ways to improve the quality of its productions and stay ahead of the competition, the oil and gas industry requesting solutions enabling to improve the security of its production and installations, both new and renewable energy (wind, heat pumps [START_REF] Giuseffi | Raman Distributed Temperature Sensing in underground GeoExchange System[END_REF]...) without forgetting the rail sector where electromagnetic insensitivity of optical fibers is a great advantage over traditional electrical measurement techniques [START_REF] Maurin | High speed real-time contact measurements between a smart train pantograph with embedded fibre Bragg grating sensors and its overhead contact line[END_REF][START_REF] Ferdinand | High performances Fiber Bragg Grating systems for Smart Structures: Application to Overhead Contact Line monitoring in the railway industry[END_REF]. Of course, it is irrelevant in this paper to address all these areas in details, but figure 11 illustrates them, while the scientific and technical literature will allow the reader to be more informed if needed.

Figure 11: Industrial SHM applications based on Optical Fiber Sensors. From [START_REF] Magne | Capteurs à Fibres Optiques pour l'industrie, la médecine, et les applications scientifiques et technologiques[END_REF] CONCLUSION During past decades, OFS have been developed in the more industrial countries, and 12 of them have produced 85% of the global effort. Today, complementary optical fiber technologies, named "Distributed" and "quasi-Distributed" offer the same functionalities as traditional approaches (measurement, detection, alarm...) and many additional benefits due to optical fiber properties (small diameter, flexibility, wide bandwidth, low attenuation, immunity to electromagnetic interferences, resistance to ionizing radiations...) without forgetting those of optoelectronic measurement systems (metrological performances, multi-parameter sensing, wide and remote multiplexing…). The bibliometric analysis for SHM shows that many countries are involved, but the top 10 countries have published 80% of the total. The 12 countries most involved in OFS are almost the same among the top 12 in SHM.

For a given SHM application, the choice of the most appropriate OFS technology is obviously a matter of concern for designers, which in practice depends on a few number of interleaved parameters: Size of the structure, number of measurement points, acquisition speed, and resolution. A wide size structure, e.g. 1 km-long, combined to a 1-m spatial resolution specification, i.e. thousand sensing points, will definitely drive the user to select the distributed sensing. On the other hand, a fast acquisition request, and a very accurate measurement will orientate the engineer to select the quasi-distributed FBG sensing. By chance, to meet requirements of industrial applications, especially those from SHM, distributed and quasi-distributed sensors are increasingly performing and commercially available. Thus, many market sectors are now looking for these optical technologies (civil engineering, oil & gas, conventional and renewable energies, safety domain, nuclear... and of course those linked to composites materials and structures.

Anyway, two kinds of application fields dominate: the Civil Engineering and sectors related to composite materials applications. So, the technico-economic impact of the corresponding applications is huge, many market shares being concerned: on ground (bridges [START_REF] Magne | Health Monitoring of the Saint-Jean Bridge of Bordeaux, France using Fiber Bragg Grating extensometers[END_REF], tunnels [START_REF] Dewynter | Underground tunneling monitoring based on sensitive optical fibre cable and Brillouin reflectometry[END_REF][START_REF] Dewynter | Tunnel Structural Health Monitoring with Brillouin Optical Fiber Distributed Sensing[END_REF], Plants [START_REF] Ferdinand | Enhancing safety in Nuclear Power Plant with Optical Fiber Sensors[END_REF], and underground mines [START_REF] Ferdinand | Mine Operating Accurate STABILity Control with Optical Fiber Sensing and Bragg Grating Technology: the BRITE STABILOS Project[END_REF][START_REF] Dewynter | Instrumentation of borehole with Fiber Bragg Grating thermal probes: Study of the geothermic behaviour of rocks, 17 th Inter[END_REF]), wind turbines, at sea (ships [START_REF] Pran | 13 th Intern. Conf. on Optical Fiber Sensors, OFS-13[END_REF], pipelines, platforms), in flight (planes, helicopters), or even in space (satellite, shuttle). As everybody knows, projects are more and more driven by the economy, and this leads to maintain existing structures as longer as possible. Such trend may explain increasing motivations for SHM and health diagnosis of structures, i.e. in practice for the measurement of strains, displacements, temperature, corrosion ... In such context, the economic impact (saving in maintenance costs, gains in performances and related to safety [START_REF] Ferdinand | Brillouin sensing for perimetric detection: the SMARTFENCE project[END_REF]) of the concerned monitoring applications (concrete, basements, grounds, civil structures, composites materials...) justify the efforts which are devoted to them. Thus, during past years the growing overlap of the increasingly performing optical fiber sensing technologies with industries' needs has allowed fruitful synergies between the OFS and SHM communities. That's why since about two decades, OFS has caught increasing attention of SHM people. As a consequence, many SHM oriented field demonstrations have been performed by actors of the OFS community. Finally, products have left R&D labs to reach the market and industrial applications, very often thanks to start-ups.

Literature and quality of projects show that SHM is now became a well-established speciality and, if we refer to the SHM workshops, the bibliometry proves that fiber optics has become the second sensing technology of interest. In the early 2000s, 40% of applications involving the OFS, concerned the FBG technology; it is quite remarkable that in the recent years this figure reached 64%. This is evidence that the FBG technology is now recognised, and considered as a strategic domain all around the world. In particular, China invests a lot in the OFS and FBG domains since 7-8 years, due to their numerous advantages, and due to a large panel of applications in this country.

To complete this analysis, we must always remember that the story is never ended, as scientific or technological breakthroughs may appears at any time, and so opening new opportunities for developers, as well as for users. This is what just happened during the last decade for the Telecommunication community, as well as for the OFS developers, with the invention followed by a strong phase of development of the micro-structured and Photonic Crystal Fibers. More recently other breakthroughs emerged as the possibility to realize FBG transducers able to resist about 1000°C over several years [START_REF] Laffont | Regenerated Fiber Bragg Grating sensors for high temperature monitoring in Sodium-cooled Fast Reactor[END_REF][START_REF] Laffont | Multiplexed regenerated Fiber Bragg Gratins for High-temperature measurement[END_REF], which now opens new market possibilities for the OFS and SHM communities. Some new paradigms, as the concept of smart skins, very small diameter optical fibers, connected objects, big data... will probably have an impact on both the background and practices of the SHM community. Should we anticipate, in less than a decade, Internet-connected structures through sensitive optical fibers, acting as nerves and remote links, everywhere in our life? I suggest planning an appointment to discuss this question at EWSHM2024.

Figure 1 :Figure 2 :

 12 Figure 1: OFS papers per origin (countries, regions) and per event from OFS-1 (1983) to OFS-22 (2012)

Figure 3 :

 3 Figure 3: FBG multiplexing and sensing in case of temperature measurement.

Figure 4 :

 4 Figure 4: Rayleigh, Raman and Brillouin scattering intensity in silica fibers (excitation at 1.55 µm)

Figure 5 :

 5 Figure 5: Mind-mapping diagram of quasi-distributed OFS technologies.

Figure 6 :

 6 Figure 6: Mind-mapping diagram of distributed OFS.

Figure 7 :

 7 Figure 7: SHM papers per country from IWSHM1999 (Stanford) to EWSHM2014 (Nantes).

Figure 8 :

 8 Figure 8: SHM papers per origin (countries, regions) and per event from IWSHM1999 to EWSHM2014 4.3 Sensors for SHM Concerning sensors, several technologies are considered in the (I or E)WSHM papers, as follows: Piezo, Ultrasonic, Guides waves (Lamb, elastic…), Optical Fiber Sensors (OFS), Sensors arrays, wireless sensors, smart sensors, vibration-based sensors, bio-inspired sensors, etc. As shown on Fig. 9, traditional sensing based on wave propagation (Piezo, Ultrasonic, Lamb waves ...) reaches 20%, and OFS 15%. So, the Optical Fiber Sensing is now the second technology considered for SHM. The total number of IWSHM or EWSHM papers devoted to OFS is quite large: 411 papers, or 12.7% of the total.More precisely, we may also estimate the influence of the OFS techniques and sub-techniques, presented above. The result is very clear: FBG sensing is widely considered. Since IWSHM1997, an average of 64.2% of the SMH papers involved in fiber sensing are using the FBG technology, the ratio being 50% for this last Workshop EWSHM2014 (Fig.10). So, we may remember that about 2/3 of the SHM projects which includes fiber sensors, have chosen quasi-distributed Fiber Bragg Gratings. Other optical fiber techniques used in SHM are as follows: the distributed Brillouin sensing (BOTDR -BOTDA), and more recently the Rayleigh OFDR. Other OFS techniques, as Polarimetry and Interferometry (Fabry-Perot…), are less and less considered for SHM.

Figure 9 :Figure 10 :

 910 Figure 9: Sensing techniques used in SHM, from IWSHM1999 to EWSHM2014

Table 1 :

 1 Location of 24 International OFS Conferences from 1983 to 2015.

	OFS1, Londres, April 1983	OFS9, Florence, May 1993	OFS17, Bruges, May 2005
	OFS2, Stuttgart, Sept. 1984	OFS10, Glasgow, Oct 1994	OFS18, Cancun, Nov. 2006
	OFS3, San Diego, Feb. 1985	OFS11, Sapporo, May 1996	OFS19, Perth, April 2008
	OFS4, Tokyo, Oct. 1986	OFS12, Williamsburg, Oct. 1997	OFS20, Edinburgh, Oct. 2009
	OFS5, New Orleans, Jan. 1988	OFS13, Kyongju, Korea, April 1999	OFS21, Ottawa, May, 2011
	OFS6, Paris, Sept. 1989	OFS14 (OFS2000), Venice, Oct., 2000	OFS22, Beijing, Oct. 2012
	OFS7, Sydney, Dec. 1990	OFS15, Portland, May 2002	OFS23, Santander, June 2014
	OFS8, Monterey, Jan. 1992	OFS16, Nara, Oct. 2003	OFS24, Brazil, fall 2015

Table 3 :

 3 Location of the 18 International or European SHM Conferences, from 1997 to 2016.

	IWSHM 1997 (Stanford Univ., USA)	IWSHM 2005 (Stanford)	IWSHM 2011 (Stanford)
	IWSHM 1999 (Stanford)	EWSHM 2006 (Granada, Spain)	EWSHM 2012 (Dresden, Germany)
	IWSHM 2000 (Stanford)	IWSHM 2007 (Stanford)	IWSHM 2013 (Stanford)
	EWSHM 2002 (Cachan, France)	EWSHM 2008 (Krakov, Poland)	EWSHM 2014 (Nantes, France)
	IWSHM 2003 (Stanford)	IWSHM 2009 (Stanford)	IWSHM 2015 (Stanford)
	EWSHM 2004 (Munich, Germany)	EWSHM 2010 (Sorento, Italy)	EWSHM 2016 (UK)

EWSHM 2014 -Nantes, France


Laurent Maurin
email: laurent.maurin@cea.fr

Pierre Ferdinand

Fabien Nony

Stéphane Villalonga

OFDR DISTRIBUTED STRAIN MEASUREMENTS FOR SHM OF HYDROSTATIC STRESSED STRUCTURES: AN APPLICATION TO HIGH PRESSURE H2 STORAGE TYPE IV COMPOSITE VESSELS -H2E PROJECT

Keywords: pressure vessel, optical fibre, Rayleigh OFDR, damage assessment

This paper deals with SHM of Composite Overwrapped Pressure Vessels (COPV) for high pressure hydrogen storage, based on embedded optical fibres and Rayleigh OFDR. Fibre Bragg Gratings in situ strain measurements on flat samples are first presented to assess the mechanical state of such anisotropic structures, and define a preliminary SHM criterion. Comparative fatigue tests were also performed on NOL rings to evaluate optical fibres mechanical impact, but their analysis in terms of energy revealed the difficulty to provide reliable results if the first cycles energy is not accurately controlled. Therefore, with no detrimental effect of the optical fibre embedment within the composite architecture, several instrumented pressure vessels were manufactured and tested. Accordingly, a mechanical criterion based on OFDR differential strain profiles analysis was defined to bring out internal defects, demonstrating the ability to early detect and locate internal flaws without requirements to inflate vessels at high pressures. Additionally, their sensitivity vs. pressure, analysed during a first burst test, provided additional valuable data about structure integrity. Finally, we define four complementary criteria based on in situ strain measurements to control pressure vessels for damage assessment, or any hydrostatic stressed structure since the underlying principles rely on strains proportionality vs. uniformly applied stresses.

INTRODUCTION

Traditionally, metallic vessels have to comply with specific regulation rules (Pressure Equipment Directive from the European Parliament [1]) involving periodic requalifications. However, for high pressure composite vessels, standards, and particularly non-destructive techniques, have to be developed, tested and validated.

In this new frame, the French H2E research project aims at funding developments to set-up new control / monitoring techniques applicable to composite vessels, for both future regulatory and inspection needs. To reach the market, these techniques have also to be cost-effective in comparison with traditional ones.

To monitor such vessels and highlight the presence of structural defects, the CEA (LIST Institute and DAM) has conducted R&D based on high resolution distributed strain profiles analysis along singlemode optical fibres embedded into the composite structure.

To elaborate this method, preliminary tests were first carried out on samples to evaluate any significant intrusive effect of the optical fibres on their ultimate tensile strength and fatigue performance.

Then, several composite vessels were fully instrumented with optical fibres and tested up to their working pressure to validate the relevance of in situ distributed strain measurements to detect internal flaws thanks to a new and distributed differential strain criterion. This criterion was validated during a first burst test, and then generalised to potentially lead, in the future, to "real-time" inspection of pressure storage vessels during isothermal filling operations.

This paper presents the results of this work carried out during the H2E project timeframe. It opens the way to future developments to extend this methodology to high pressure storage vessels controls during non-isothermal operations such as regular/fast gas-filling.

FIRST MECHANICAL TESTS ON FLAT SAMPLES -TOWARDS A FIRST CRITERION BASED ON

IN SITU STRAIN MEASUREMENTS

First uniaxial tensile tests and optical fibre influence

Preliminary uniaxial tensile tests were first carried out on composite flat samples made of ten unidirectional plies to check for any significant mechanical influence of embedded optical fibres [START_REF] Sansonetti | Intelligent composites containing measuring fiber-optic networks for continuous selfdiagnosis[END_REF], considering that their quantity is several orders of magnitude greater than planned in pressure vessels.

Three different types of optical fibres were tested, differing by their coating in terms of outer diameter and composition. Results show that the average ultimate tensile strength of the control population still remains the greatest, but standard deviations for each batch are also too important to conclude to a significant optical fibres influence on the samples mechanical properties (Figure 1).

In situ FBG strain measurements during tensile tests -A first damage criterion

Therefore, several composite flat specimens made of different ply orientations, more representative of a pressure vessel structure, were instrumented with Fibre Bragg Grating (FBG) transducers to monitor their internal strain during uniaxial tensile tests according to ∆ ⁄ = 0.78 × Δ [START_REF] Ferdinand | Capteurs à fibres optiques à réseaux de Bragg[END_REF]. The FBG relative orientation vs. tensile effort direction depends on the instrumented plies, and takes values amongst 0°, 10°, 30°, 45°, 70°, 85° and 90°. Then, FBG-based in situ strain measurements are compared with strains given by an electrical strain gauge mounted on surface. Tensile force is also recorded, which gives a good estimation of the sample average strain since carbon reinforcement fibres are purely elastic.

The experimental results clearly show that matrix debonding appears once in situ FBG strain measurements differ significantly from surface and average strains measurements, and this phenomenon appears even earlier if the difference between the tensile effort direction and the instrumented ply orientation is increased (Figure 2).

These observations suggest the elaboration of a first criterion to detect composite structure damages based on the difference between the average strain (standing for the whole structure) and the in situ local strain where damages occur.

Thus, the underlying idea for pressure vessel inspection will be to extend this restricted area criterion, given by a single FBG transducer, to distributed measurements all over the pressure vessel thanks to the Rayleigh OFDR technique ( § 3).

LOW-CYCLE FATIGUE TESTS, AND ENERGY CONSIDERATIONS

Meanwhile, comparative fatigue tests on NOL rings [START_REF] Astm Standard | -08 Standard Test Method for Apparent Hoop Tensile Strength of Plastic or Reinforced Plastic Pipe by Split Disk Method[END_REF], more representative of pressure vessels, were performed to evaluate the optical fibres mechanical impact on the cylindrical shell. As usual, several ply orientations representing four different composite structures were considered (Figure 3). The low-cycle frequency was equal to 0.4 Hz and the maximum displacement ∆ℎ was tuned to reach failure after 15,000 cycles for each batch. Force and position ℎ were continuously recorded to compute an energy parameter according to Equation (1). This parameter also reflects the dissipated strain energy during each cycle.

= + !"

with:

# → # %∆ = & '() +( # %∆ # (1)
This parameter is more relevant in comparison with a traditional force vs. time analysis, since samples strength interpreted in energy always decreases, expressing a progressive damage, especially during the few first cycles, whereas interpreted in force amplitude, it is not systematically true. This difference also probably reflects reorientations of reinforcement carbon fibres under stress (Figure 4). Experimental data gathered during fatigue tests lead to a correlation between the first thousands cycles strain energy average sensitivity 〈-〉 and the first cycles average strain energy 〈 /=0 〉. Therefore, these first cycles are crucial for the subsequent damage rates (Figure 5). The relative contribution on the number of cycles to failure 4/ of this starting energy can therefore be estimated according to Equation ( 2), taking into account:

• the strain energy average sensitivity 〈5 6 〉 during the first thousands cycles,

• the average starting energy 〈6 728 〉 for each batch,

• the displacement ∆9 for each cycle,

• the position reproducibility :9 on the testing machine,

• the strain energy dispersion ∆6 728 at start for a same batch.

4/ ; ≈ = 〈> ? 〉 〈0 @A# 〉 ∆ 4ℎ and 4/ ∆0 @A# ≈ ∆0 @A# 〈> ? 〉 (2)
This analysis highlights the major influence of the strain energy dispersion ∆ 123 during the first fatigue cycles, considering a typical error for 4ℎ better than 2 CD on the test machine. Monitoring these tests in terms of energy would have been a better solution to reduce their dispersion. Concluding about any optical fibre influence on cycles to failure would then have been irrelevant (Table 1). However, post mortem analyses show, as expected [START_REF] Sansonetti | Intelligent composites containing measuring fiber-optic networks for continuous selfdiagnosis[END_REF], the perfect integration of optical fibres in the composite matrix since no void nor delamination appear in their vicinity (Figure 6).

STRUCTURAL DEFECT DETECTION AND DIFFERENTIAL DISTRIBUTED STRAIN ANALYSIS

On this basis, several composite pressure vessels were fully instrumented with optical fibres at different depths inside the matrix, from sub-surface to liner vicinity. Some of these structures were deliberately damaged, then inflated with water to avoid crosstalk temperature effects.

The distributed State Of Polarisation (SOP) for each optical fibre is then recorded thanks to Rayleigh-based Optical Frequency Domain Reflectometry (OFDR) with a Luna™ OBR-4600 instrument, and post-processed to compute strain profiles ∆ 'E) along the curvilinear position E in the composite structure [START_REF] Froggatt | High-Spatial-Resolution Distributed Strain Measurement in Optical Fiber with Rayleigh Scatter[END_REF].

The criterion first defined on flat samples ( § 1.2) is then extended according to the following assumptions, assuming that defects may be scattered anywhere into the structure [START_REF] Maurin | Contrôle santé par fibres optiques de réservoirs composites pour le stockage d'hydrogène sous haute pression -Projet H2E (Horizon Hydrogène Énergie)[END_REF]:

• an hypothesis is to consider internal defects as structural weaknesses resulting, in their vicinity, in local strains different from the rest of the structure supposed undamaged, • by a set of differences between strain measurements, it must be possible to bring them out.

This mechanical criterion : F ∆G can be expressed according to Equation (3) taking into account [START_REF] Maurin | Librairie de calcul de profils différentiels d'écarts de déformations longitudinales le long d'une abscisse curviligne (recherche de défauts de structure)[END_REF]:

• a first strain variation ∆G H 8 →H IJK between inflation pressures L 3 and L ,

• a second strain variation ∆G H 8 →H between inflation pressures L 3 and L,

• a proportionality coefficient F MH 8 ,H IJK ,HO calculated by least-squares minimisation of ‖4 Q ∆ ‖.

4 Q ∆ MR # ,R STU ,RO 'E) = ∆ R # →R 'E) -W MR # ,R STU ,RO × ∆ R # →R STU 'E) (3)
The 4 Q ∆ criterion is in fact an error function whose significant deviations from zero point out local abnormal mechanical behaviours, in other words: structural defects.

Pressure L 3 can be any value; however, for the sake of simplicity, it usually corresponds to an empty pressure vessel. In this frame, several structures were tested, up to 700 XYZ working pressure. Control pressures L and L, for these experiments, were much smaller than working pressure, typically L = 100 XYZ and L = 150 XYZ (Figure 7). [START_REF] Maurin | Contrôle santé par fibres optiques de réservoirs composites pour le stockage d'hydrogène sous haute pression -Projet H2E (Horizon Hydrogène Énergie)[END_REF] Furthermore, OFDR signal-to-noise ratio and centimetre spatial resolution are good enough to enable inspections at even lower pressures, e.g.: L = 100 XYZ and L = 40 XYZ. It is then possible to start to control pressure vessels at minimum pressure as low as 6% of service pressure without any risk for additional damages induced by the inspection protocol (Figure 8, left).

But in some cases, due to strain redistributions under stress, some defects may change to behave like the rest of the structure, disappearing from subsequent analyses. Nevertheless, it is sometimes possible to select more appropriate control pressures to bring out their residual strains once the pressure vessel is deflated (Figure 8, right).

FIRST BURST TEST AND COMPLEMENTARY ANALYSES

Since the 4 Q ∆ parameter has proven to be efficient even below service pressure, it has also been tested at higher pressures, at least one magnitude order greater than previous control pressures, during a first burst test (Figure 9). The reservoir is filled with room-temperature water to avoid crosstalk effects on measurements, pressure steps are typically equal to some tens of bar. In this configuration, it is possible to both [START_REF] Maurin | Librairie de calcul de profils différentiels d'écarts de déformations longitudinales le long d'une abscisse curviligne (recherche de défauts de structure)[END_REF]:

• ensure strain measurements ∆G H `ab →H ` between pressure steps without post-processing error,

• compute, without additional artefact, strain variations ∆G H 8 →H c for any pressure step L d according to Equation (4) (Figure 10). It is also possible to benefit from the pressure steps data to differentiate the differential strain profile 4 Q ∆ 'E) vs. pressure, to evaluate a sensitivity i MR # ,R STU ,RO 'E) according to Equation [START_REF] Froggatt | High-Spatial-Resolution Distributed Strain Measurement in Optical Fiber with Rayleigh Scatter[END_REF]. This parameter gives additional information about structural defects evolution during reservoir filling, and should also be helpful to early detect and locate, e.g.: a crack propagation (Figure 11). Finally, since carbon reinforcement fibres are elastic, there should be a relationship s between the proportionality coefficient W and the inflation pressure L. Then, assuming that without any defect, W = W 3 , such a relationship could be expressed according to Equation [START_REF] Maurin | Contrôle santé par fibres optiques de réservoirs composites pour le stockage d'hydrogène sous haute pression -Projet H2E (Horizon Hydrogène Énergie)[END_REF].

∆ R # →R " 'E) = ∑ ∆ R fag →R f 'E) h2d h2= (4)
s MR # ,R STU O 'L) = RtQ # mn # ,n STU ,no u RtQ mn # ,n STU ,no u R v Swx (6)
This damage assessment parameter s clearly revealed, during the burst test, a critical and global damage roughly starting at 70% burst pressure, and demonstrated its robustness to give relevant results even with sparse data, for instance, in case of optical fibre break (Figure 12, left).

It took a maximum value equal to 2.49% at 98.9% burst pressure during this first experiment (Figure 12, right). Also, this may be the signature of a crack propagation.

CONCLUSION

Based on the fact that composite pressure vessels are subjected to an homogeneous hydrostatic stress (the filling pressure), we demonstrated, according to OFDR measurements on composite vessels interpreted in terms of differential strains, the ability to detect some structural defects without the need to inflate them at high pressures (from few MPa up to working pressure), thus reducing global cost of operation and risks of creation, or evolution, of new or existing defects.

Four complementary criteria based on distributed strain and pressure vessel measurements during the water-filling operation are proposed for pressure vessels monitoring and control. Provided that real-time OFDR measurements will be achievable in a near future, we could therefore rely on an efficient and easily understandable SHM procedure for such structures, but also for any structure submitted to hydrostatic stress.

Besides the fact that these optical measurements provide information of a mechanical nature (i.e.: the distributed strain profile all along the optical fibre), they are performed by the same distributed sensor (the optical fibre itself) embedded into the composite structure throughout its lifetime, eliminating by design any measurement dispersion due to replacement, e.g. for maintenance purposes.

Thus, the optical fibre used in combination with Rayleigh OFDR appears to be a relevant highend and in situ non-destructive measurement technique for long-term SHM of high pressure vessels.

Figure 1 :

 1 Figure1: Uniaxial tensile tests conducted on unidirectional UD 0° flat samples[START_REF] Maurin | Contrôle santé par fibres optiques de réservoirs composites pour le stockage d'hydrogène sous haute pression -Projet H2E (Horizon Hydrogène Énergie)[END_REF]

Figure 2 :

 2 Figure2: In situ (FBG) compared with surface (strain gauge) measurements during uniaxial tensile tests[START_REF] Maurin | Contrôle santé par fibres optiques de réservoirs composites pour le stockage d'hydrogène sous haute pression -Projet H2E (Horizon Hydrogène Énergie)[END_REF]

Figure 3 :

 3 Figure 3: Experimental set-up (left) and NOL ring after failure (right)

Figure 4 :

 4 Figure 4: Typical low-cycle fatigue test expressed in force amplitude (left) and in energy (right)[START_REF] Maurin | Contrôle santé par fibres optiques de réservoirs composites pour le stockage d'hydrogène sous haute pression -Projet H2E (Horizon Hydrogène Énergie)[END_REF]

Figure 5 :

 5 Figure 5: Strain energy average sensitivity 〈-0 〉 vs. first cycles average strain energy 〈 123 〉 [6]

Figure 6 :

 6 Figure6: NOL ring post mortem analysis under microscope after low-cycle fatigue test[START_REF] Maurin | Contrôle santé par fibres optiques de réservoirs composites pour le stockage d'hydrogène sous haute pression -Projet H2E (Horizon Hydrogène Énergie)[END_REF]

Figure 7 :

 7 Figure 7: Differential strain profile 4 Q ∆ 'E) -L = 100 XYZ and L = 150 XYZ (L 3 = L ]^)[START_REF] Maurin | Contrôle santé par fibres optiques de réservoirs composites pour le stockage d'hydrogène sous haute pression -Projet H2E (Horizon Hydrogène Énergie)[END_REF]

Figure 8 :

 8 Figure 8: Differential strains with L = 40 XYZ (left) and residual differential strains with L = L ]^ (right)

Figure 9 :

 9 Figure 9: Composites Aquitaine™ pressure vessel burst test facility

Figure 10 :

 10 Figure 10: Strain profiles ∆ R # →R " 'E) during water-filling operation: baseline for structural monitoring regarding theoretical mechanical properties of the composite structure

Figure 11 :

 11 Figure 11: Differential strain sensitivity vs. pressure profile i MR # ,R STU ,RO 'E): "instantaneous" strain deviations due to structural defects highlighted during the filling operation

Figure 12 :

 12 Figure 12: Pressure vessel damage assessment: evidences of critical damage starting at 70% burst pressure

Table 1 :

 1 Position reproducibility 4/ ; and starting energy dispersion 4/ ∆0 @A# impacts on cycles to failure

	batch	〈5 6 〉 [J/Mcycle]	〈6 728 〉 [J]	∆6 728 [J]	∆9 [mm]	:7 :9 [cycle/µm]	:7 ∆6 7A8 [cycle/J]
	#1 w/o optical fibre	484.64	45	20.9	1.92	48.36	2063
	#1 w/ optical fibre	378.58	40	9.5	1.92	55.03	2641
	#2 w/o optical fibre	77.71	34	9.9	1.65	265.16	12868
	#2 w/ optical fibre	121.2	34	6.6	1.65	170.02	8251

7th European Workshop on Structural Health Monitoring July 8-11, 2014. La Cité, Nantes, France Copyright © Inria (2014)

EWSHM 2014 -Nantes, France

ACKNOWLEDGEMENTS

Authors thank their H2E partners, Composites Aquitaine™ and Air Liquide™, for their technical support and availability, and OSEO for co-funding this project.