applications of fiber bragg grating sensors in the

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Applications of Fiber Bragg Grating sensors in thecomposite industry

Pierre Ferdinand, Sylvain Magne, Véronique Dewynter-Marty, StéphaneRougeault, Laurent Maurin

To cite this version:Pierre Ferdinand, Sylvain Magne, Véronique Dewynter-Marty, Stéphane Rougeault, Laurent Maurin.Applications of Fiber Bragg Grating sensors in the composite industry. MRS Bulletin, CambridgeUniversity Press (CUP), 2002, 27 (5), pp.400-407. �10.1557/mrs2002.126�. �cea-01841910�

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400 MRS BULLETIN/MAY 2002

IntroductionComposite materials have been used for

many years in the aerospace industry,owing to their high specific stiffness andstrength. They are also used more and morein the transportation industry, and in me-chanical engineering and civil engineeringfor the rehabilitation and reinforcement ofstructures and cables. Composites displaygood short-term and long-term (fatigue)mechanical behavior, and good environ-mental stability (withstanding fire, corro-sion, lightning strikes, etc.).

Optical-fiber sensors have been embed-ded into composite materials for morethan 12 years.1 For aerospace applications,we may distinguish three main applica-tions for optical-fiber sensors, one relatedto manufacturing control, a second linked

to “health” diagnostics (in-flight or formaintenance), and a third related to smartstructures (control damping and shapecontrol).2

Embedded optical-fiber sensors may beused to provide in-flight, real-time infor-mation to an on-board controller able todrive actuators with the purpose of damp-ing vibrations or noise, or controlling theshape of a structure: this is the well-knownconcept of smart structures, that is, struc-tures able to sense their environment and tocorrect autonomously for any perturbation.

Until now, much of the research in thisarea has been directed toward the use ofoptical-fiber sensors in the continuous im-provement of manufacturing processes.During the manufacture of composite ma-

terials, it may be necessary to monitortemperature, pressure, void content, resinfront progression, shrinkage, or residualstress, as well as inline cure monitoring(degree of cure). For the last five years,progress has been made in inline manu-facturing monitoring by using fiber remoteabsorption spectrometry3 or refractometricmethods (Fresnel reflection, slanted fiberBragg gratings),4,5 to such an extent thatmanufacturers are willing to consider thesenondestructive methods in order to shortenthe qualification time for new structures,improve the quality of manufacturing,and reduce the number of rejected parts.

Optical-fiber sensors also have beenwidely investigated for strain monitoringin civil engineering applications (mining,6bridges,7–12 highways,13 and nuclear-powerplants14–16) and for operational monitoringand maintenance of aircraft;17,18 once partsare made, the failure modes of the com-posite structures (e.g., delamination) can-not be predicted analytically and are notwell understood. Application of a newcomposite material is slowed down byhigh manufacturing costs and risk arisingfrom the inability to predict damage andunderstand the material’s mechanisms,especially in the case of heavily loadedstructures. It is also inherently difficult toincorporate a nonintrusive optical connec-tion to a composite part — a problem thathas not yet been industrially solved.

This article will focus on recent applica-tions of optical-fiber sensors in compositematerials. The industrial aspects are sum-marized in Table I.

Smart ManufacturingIn many structures, the key parameter

of interest is strain. A fiber Bragg grating(FBG) is formed by exposing the core of a Ge-doped fiber to alternating regions of intense short-wavelength laser light(around 244 nm). This is typically donewith a high-power UV laser and an inter-ferometer or a phase mask forming aninterference pattern imaged on the fibercore. Due to constructive internal interfer-ence, an FBG acts as a sharp reflecting filterfor a characteristic wavelength (the Braggwavelength). When subjected to strain,pressure, or a temperature change, theBragg wavelength of such a filter is shiftedproportionally. Accurate measurements ofthese shifts can be used to extract the straininformation, free of temperature influ-ence, if a differential approach is used (e.g.,one FBG sensing strain and temperature,and a second FBG only sensing tempera-ture). Moreover, in telecommunication sys-tems, for example, spectral demultiplexingallows the measurement of several Braggwavelengths reflected by a set of gratings

Applications of FiberBragg GratingSensors in theComposite IndustryPierre Ferdinand, Sylvain Magne,

Véronique Dewynter-Marty,Stéphane Rougeault, and Laurent Maurin

AbstractOptical-fiber sensors based on fiber Bragg gratings (FBGs) provide accurate,

nonintrusive, and reliable remote measurements of temperature, strain, and pressure,and they are immune to electromagnetic interference. FBGs are extensively used intelecommunications, and their manufacture is now cost-effective. As sensors, FBGs findmany industrial applications in composite structures used in the civil engineering,aeronautics, train transportation, space, and naval sectors. Tiny FBG sensorsembedded in a composite material can provide in situ information about polymer curing(strain, temperature, refractive index) in an elegant and nonintrusive way. Greatimprovements in composite manufacturing processes such as resin transfer molding(RTM) and resin film infusion (RFI) have been obtained through the use of thesesensors. They can also be used in monitoring the “health” of a composite structure andin impact detection to evaluate, for example, the airworthiness of aircraft. Finally, FBGsmay be used in instrumentation as composite extensometers or strain rosettes, primarilyin civil engineering applications.

Keywords: composite materials, fiber Bragg gratings, laminates, mechanical properties,optical-fiber sensors, optoelectronics, optical properties.

Applications of Fiber Bragg Grating Sensors in the Composite Industry

MRS BULLETIN/MAY 2002 401

photo-written on the same fiber (i.e., toform a distributed measurement system).

Today, one of the most important appli-cations for FBG sensing technology is insmart manufacturing — in particular, thecontrol and monitoring of the cure of acomposite material. Optical fibers are smallenough to be nonintrusive and passivewhen embedded in a composite. They allowdata to be multiplexed from many sensorsalong a single fiber, and due to the factthat FBGs are able to provide in situ infor-mation in real time, they provide a verystable and reproducible measurement(since they are based on a spectral signa-ture). Optical fibers can be embedded incomposite materials during the manufac-turing process and then used to remotelyto measure temperature, strain, pressure,degree of cure, the presence of resin, andother parameters, depending on the proc-ess. Among the well-known methods forforming composite structures are by meansof autoclave, filament winding, resin trans-fer molding (RTM), and resin film infusion(RFI). We will discuss some of these next.

Cure Monitoring in an AutoclaveAs an example of an FBG-based smart

manufacturing process, we describe herethe real-time recording of strain in a com-posite material, as well as temperaturenear the composite plate, inside an auto-clave used for polymer curing.

For these experiments, a coupon of atypical three-layer composite structureused in aeronautics was prepared in thefollowing way. First, two square layers(150 mm � 150 mm) of a glass/epoxy skin1 mm thick and a foam core 3 mm thickwere glued together. Next, a polyimide-coated FBG (�B � 1310 nm), used as astrain sensor but sensitive to temperatureas well, was placed in a U-shaped groovemachined in the upper surface of the foamlayer. Finally, the second glass/epoxy skinwas placed onto the foam and glued. Asecond FBG (�B � 1305 nm), isolated fromstrain and spliced in series, was used as a

temperature sensor. This FBG was placedoutside of but close to the compositesample within the autoclave.19,20 The dif-ference of the spectral shift of these twogratings gives the absolute strain insidethe composite. Both Bragg wavelengthsare monitored with a homemade scanningFabry–Pérot-based wavelength-divisionmultiplexing (WDM) system, as describedin References 2 and 15.

The full curing cycle takes 12 h, includ-ing the controlled cooling steps. Duringthe cycle, continuous measurements oftemperature and strain are performed.The temperatures measured by the FBGtemperature sensor (Figure 1, solid curve)are compared with those given by the au-toclave controller (Figure 1, open circles)used for regulating the cure. At the sametime, the strain inside the material isrecorded (Figure 2). Any induced strain isdue to two contributions, thermal expan-sion and polymer shrinkage, which bothresult from the curing process.

An easy interpretation of the FBG meas-urements allows one to identify and con-trol every step of the process. The wholeprocess can be divided into the followingsteps (see Figure 2):1. Cycle is initiated at time t � 5 min byincreasing the hydrostatic pressure up to afew bar. This action induces a wavelengthshift of –20 pm for the embedded FBGstrain sensor (i.e., a compressive strain),and �50 pm for the FBG temperature sen-sor in the autoclave. One criticism of theexperimental setup is that the optical fiberlinked to the temperature sensor is notabsolutely free in the pressure chamber,but is firmly attached near the compositesample, which could modify the pressureeffect.2. Temperature is increased to T1. Thestrain sensor records a positive change(maximum 700 ��), linked to the thermalexpansion of the epoxy resin. A somewhatchaotic behavior is observed, perhaps dueto a rapid change in the adhesive condi-tions between the epoxy and the FBG.

3. Temperature is kept constant at T1 for afew tens of minutes. From the curves de-picted in Figure 1, the temperature inertiaof the autoclave can be estimated by com-paring the data given by the temperaturesensor and the regulation probe. This iner-tia could be explained by the fact that thetemperature probe in the autoclave is farfrom the FBG location near the compositeplate. On the strain curve, a sudden de-crease (Point 3 in Figure 2), down to–400 ��, can be observed. The epoxypolymerization begins and, simultaneously,shrinkage occurs.4. Temperature is increased from T1 to T2.The strain sensor shows successive trac-tions and compressions of about 100 �� inamplitude centered around –400 ��. At thisstage in the cure, two phenomena are com-peting: the material’s thermal expansion,due to the temperature increase inducinga positive strain; and the resin shrinkagelinked to the ongoing polymerization.5. Temperature is kept stable at T2 for 3 h.The strain remains constant in the composite.

Table I: Overview of the Use of Optical-Fiber Sensors in Monitoring Composite Structures.

Industry Applications Parameters Specific Problems

Composite Aeronautics Smart processing Strain/stress Obtaining optical information from structuresmaterials: Space Quality control Temperature in industrial usecarbon, Defense “Health” monitoring Pressure Monitoring large structuresglass-epoxy, Nuclear power Off-line and on-line Degree of cure Repairing sensors in case of failure or break inglass-polyester Transportation (trains, control of Delamination optical fibers

automobiles, ships) reinforcement in Internal defects High manufacturing cost of compositeCivil engineering (e.g., old structures Impact detection and materials

rehabilitation of damage assessment Long-term behavior in harsh environmentsstructures) (extreme cold, moisture, humidity)

Figure 1.Temperature measurementsperformed by a fiber Bragg grating(FBG) sensor embedded in aglass/epoxy and foam-core compositeduring cure in an autoclave (solidcurve), compared with the autoclave’scontroller (open circles).19,20



6. Temperature is decreased step-by-step.As the temperature decreases, the embed-ded FBG detects an increasing negativestrain from –400 �� to –2200 ��, due to the

resin’s thermal compression. This residualcompression, which is relatively small,suggests that the properties of the FBG areprobably not affected by the embedmentin the composite material and that theFBG sensor can be used for other kinds ofmeasurements (see the section on “ImpactDetection”).

Similar results, showing a compressionof about –2400 ��, have been previouslyreported by Dunphy et al.21 In the laststep, a dramatic decrease in the signal-to-noise ratio occurring at the end of the ex-periment can be observed. This is due toan optical power decrease, induced by sig-nificant stresses (sharp bends, micro-bends) along the optical fiber supportingthe FBGs, that not only considerably re-duces the signal-to-noise ratio but also thespectral Bragg peak resolution at the sametime. This detrimental effect may occur intextile composites showing periodic micro-bends. It may be avoided by applying athicker polymer coating to the fiber.

Smart Resin Transfer MoldingControl Process

In recent years, research laboratoriesand now the manufacturers of compositestructures have perceived the advantagesoffered by optical-fiber sensors in enhanc-ing the quality of their processes and re-ducing costs associated with these phasesof development. This aspect is particularlyacute with regard to the resin transfermolding (RTM) process. RTM is a low-pressure molding process that involvespacking layers of dry reinforcement fibersor a preform into a tight mold into whicha mixed resin catalyst is injected, followedby thermal polymerization. After curing,the mold can be opened and the compos-ite part removed.

The RTM process allows industrial pro-duction of parts to their final dimensions,but the long adjustment phase required toachieve the optimal physical parameters isa drawback to its use. It is rather difficultto guarantee injection of the resin intoevery part of the mold without leaving dryzones that then become points of struc-tural weakness; furthermore, the highreinforcement-fiber volume implies a verydense composite preform, which makesaccurate resin-flow modeling difficult.Moreover, as injection followed by poly-merization is done within a closed metal-lic mold, the possibilities for in situ controlare limited. So, the process adjustment re-quires many prototypes, since only “post-mortem” analysis (slicing followed by amicroscopic analysis) can provide infor-mation feedback on suitable parameters.Such an “open loop” approach is very ex-pensive and time-consuming. Therefore,

manufacturers are actively looking for atechnique that can provide in situ meas-urements of the process as the compositeis formed. Optical-fiber sensors and FBGsare able to help in this area, as they aresmall enough to be nonintrusive, they pro-vide very accurate measurements, they areelectromagnetically inert, and they allowmultiplexing of many sensing points.With these advantages, they look like adecided asset for this type of problem.22

To illustrate this, we next describe aproject that uses optical-fiber sensors tosupervise the manufacturing airplane pro-peller blades of the type shown in Figures 3and 4. The objective of this project, led byFrench aeronautics equipment manufac-turer Ratier-Figeac in collaboration withthe CEA-LIST (the Atomic Energy Commis-sion, Laboratory for Systems and Technol-ogy Integration), was to use an advancedmeasurement system, based on embeddedFBG sensor technology, to obtain insightsinto the process sequence, map resin flowby detecting air-to-resin transitions duringthe injection, check for dry zones or voidsin the structure, and consequently improvethe quality of manufacturing and reducedevelopment costs.23

In this experiment, 38 FBG sensors wereinstalled in an RTM propeller-blade mold(36 strain sensors and 2 temperature sen-sors), divided into 4 lines on the face ofeach blade (Figure 5). The FBG tempera-ture sensors, located at either end of anoptical fiber, are based on a CEA proprie-tary design and make use of an FBG trans-ducer placed inside a microcapillary(20 mm in length with a 350-�m externaldiameter) externally coated with polyimide(Figure 6). The optical fiber is maintainedat the input of the capillary with a specialhigh-temperature (350�C) sealing cement.At the other closed end, the fiber is free tomove inside the capillary. In this way, theFBG temperature sensor is isolated frommechanical stress induced by the compos-ite structure, enabling differential strainmeasurements free from the influence oftemperature.

At the beginning of the injection proc-ess, the temperature difference betweenthe resin and the mold is small (a few de-grees Celsius). Consequently, it is notpractical to use only thermally inducedeffects to map resin flow. The useful meas-urement parameter is the Bragg wave-length shift that happens when the resinflow reaches a sensor location. This effect,however, occurring inside the tightlypacked preform in the mold, is small, ineither compression or tension. Experimen-tal sensor response ranges between 20 ��and 400 ��. These small values make real-time data interpretation difficult, particu-

Figure 2. Internal-strain measurementsperformed by an FBG sensor embeddedin a glass/epoxy and foam-corecomposite during cure in an autoclave,after themal compensation by FBGtemperature measurement.19,20 (1) Cycleis initiated at time t � 5 min by increasingthe hydrostatic pressure up to a few bar.(2) Temperature is increased to T1.Thestrain sensor records a positive change(maximum 700 ��), linked to the thermalexpansion of the epoxy resin. A some-what chaotic behavior is observed,perhaps due to a rapid change in theadhesive conditions between the epoxyand the FBG. (3) Temperature is keptconstant at T1 for a few tens of minutes.From the curves depicted in Figure 1,the temperature inertia of the autoclavecan be estimated by comparing thedata given by the temperature sensorand the regulation probe.This inertiacould be explained by the fact that thetemperature probe in the autoclave isfar from the FBG location near thecomposite plate. On the strain curve,a sudden decrease down to �400 ��can be observed.The epoxy poly-merization begins and, simultaneously,shrinkage occurs. (4) Temperature isincreased from T1 to T2.The strainsensor shows successive tractionsand compressions of about 100 �� inamplitude centered around �400 ��.(5) Temperature is kept stable at T2 for3 h.The strain remains constant in thecomposite. (6) Temperature is decreasedstep-by-step. As the temperaturedecreases, the embedded FBG detectsan increasing negative strain from�400 �� to �2200 ��, due to theresin’s thermal compression.Thisresidual compression, which isrelatively small, suggests that theproperties of the FBG are probablynot affected by the embedment inthe composite material and that theFBG sensor can be used for otherkinds of measurements. See textfor additional details.



larly defining the strain threshold in orderto identify the interaction of the resin flowwith the sensors. This is the reason whycareful postprocessing of the measure-ments is needed in order to extract valu-able results regarding resin-flow behavior.

With this restriction in mind, resin-flowanalysis is possible using internal con-straint modifications experienced by theoptical FBG sensor network, leading to asmart approach in RTM manufacturing.Figure 7 shows the progression of theresin flow in the mold, based on an inter-pretation of the sensor response.

As a variation of RTM, the resin film in-fusion (RFI) process is now being used bysome manufacturers as an alternative tothe usual pre-impregnation processes be-cause it yields improvements in qualityand promises a significant cost reduction.In RFI, a dry layer of reinforcement is usedalong with a cast layer of catalyzed resin.Under pressure and heating, the resin en-velops the reinforcement fibers, and thusthe part is made. The great benefit of theRFI process is to reduce the void contentof the laminate because the dry reinforce-ment fibers allow air transport out of thestack during polymerization.24 It is advan-tageous for large structures and for theuse of high-viscosity resins (which are dif-

ficult to work with in traditional RTM).With this new process, optical-fiber sen-sors and FBGs should be useful in control-ling physical parameters such as degree ofcure, cure rate, and temperature.

Impact DetectionFor manufacturers and end users of

composite structures, a second motivationfor using optical-fiber sensors, after manu-facturing and testing, is “health” monitor-ing, that is, evaluating the lifetime of acomponent or a whole structure. This isessential in applications in which safety isa concern. An example is the radome(nose) of an airplane, which is typicallymade of a thick, sandwich-type compositestructure that is transparent to electromag-netic waves. Optical-fiber sensors and,more specifically, FBG sensors, are poten-tial candidates for nondestructive instru-mentation to measure stress and damagein such a structure.

One specific topic concerns the evalua-tion of permanent damage induced bylow-energy impacts (less than 30 J, i.e., toosmall to cause visible defects on thesurface) as well as delamination due tofatigue.19,20

The FBG, which is a quasi-distributedsensor localized along the fiber, exhibits aspectral shift proportional to the local per-manent strain added to the influence oftemperature. The idea is to detect perma-nent damage characterized by a perma-nent strain in its surroundings.

To demonstrate the potential of FBG-based instrumentation in the assessmentof material integrity following impacts,several composite samples were manufac-tured with embedded FBGs. One of themincludes three embedded FBGs placed10 mm, 30 mm, and 50 mm, respectively,away from the point of impact located inthe center of the sample, as described inFigure 8.

Figure 5. FBG instrumentation on oneface of a propeller blade.23

Figure 6. FBG temperature sensor forin situ measurements in a compositepropeller blade. Courtesy of CEA/Marty.

Figure 7. Progression of the resin flow in a propeller blade mold, as inferred from the FBGdata.23

Figure 3. Airplane propeller blade withembedded FBG sensors. Courtesy ofRatier-Figeac, France.

Figure 4. Propeller blade mold, withFBG sensors installed, after curing.Courtesy of Ratier-Figeac, France.



The plate was submitted to nine succes-sive impacts with energies ranging from2 J to 20 J. The impact-energy threshold forthe three successive FBGs were, respec-tively, 8 J, 10 J, and 12 J (Figure 9). It is in-teresting to note the strain saturation forthe FBGs, about –500 ��, after the impactof 18 J in such a sandwich structure.

The method we have developed torecord a given permanent defect (invisibleon the surface) in sandwich compositematerial arising from an impact is the firststep toward a mapping method able to de-

termine and localize all of the defects pres-ent in the whole structure. Because themethod for measuring impacts is to detectthe strains induced by them (primarily in-ternal delamination or foam crushing), theembedded FBG sensing grid needs to bedense enough (typically 10 cm � 10 cm) tosense any zone of the structure. The nextstep in this global damage-detection studyis to model the sensor response in order to localize and quantify a given defect bytriangulation.

Smart Bogies for Fast TrainsMobility is an important factor for eco-

nomic growth. Enhancing train trans-portation will become a key issue in futureyears, as competing means of transporta-tion such as road and air will graduallybecome saturated in capacity. In a recentstudy, the German government predicteda saturated situation for these systemswithin the next 10 years if today’s rate ofdevelopment continues. One consequencemay be a load shift of transportation torail. The European Union is promotingthis by developing initiatives and enhanc-ing the competitiveness of the rail grid bymeans of deregulation and interoperabil-ity. This process has been nearly completedin Great Britain, with some consequences.Additionally, the European Union is sup-porting this process with a planned up-grade to the existing rail network, theTrans-European Net. This will require theupgrade of 40,000 km of existing railtracks for use at speeds of up to 200 km/hinstead of the 160 km/h typical today, andthe development of 20,000 km of new railtracks for use at speeds of greater than230 km/h.

Within this framework, some compa-nies in Europe are working to develop

new functionalities for trains. In France,Alstom Transport SA, the train division ofthe Alstom Co., has been working for sev-eral months on a new project involvingcomposite materials for a new railcar bogie(Figure 10). Bogies are the swiveling under-carriages at either end of a railroad car onwhich the wheels are fixed and which serveto damp vibrations transmitted to the car.As we have already discussed, compositematerials present many advantages, includ-ing lower density and a higher strength-to-weight ratio than metals. However,their use in this area of the transportationindustry is new; previously, bogies weremade from metallic materials. Conse-quently, one important part of this devel-opment is validating the mechanicalbehavior of the bogies. A mechanicalsimulation was led by DDL Consultants (asmall company specializing in modeling)with Samcef software; the monitoringtechnology that achieved the experimen-tal validation was implemented by theOptical Measurements Laboratory of theCEA-LIST, by the use of several opticalFBGs during static and dynamic trials.

Several parts of composite bogies wereinstrumented with in situ FBG sensors. Anoptical line is composed of several strainFBG sensors for static and dynamic tests,while an extra FBG is devoted to tempera-ture compensation. Accelerated aging testswere carried out by Alstom and the CEAin a climatic chamber to simulate in-useenvironmental conditions (e.g., cycles ofstress, intense cold, heat, moisture, andstrong lighting). For these accelerated lifetests, an optical line containing three FBGstrain sensors and one extra FBG for tem-perature compensation was embeddedduring manufacturing of the structuralelement.

Figure 8. Schematic illustration of acomposite plate embedded with threeFBG sensors placed 10 mm, 30 mm,and 50 mm, respectively, away froma point of impact located in the centerof the sample.This setup is used todemonstrate the potential ofFBG-based instrumentation in theassessment of material integrityfollowing impacts.19,20

Figure 9. Real-time detection of impacts with energies ranging from 2 J to 20 J by means ofthree FBG sensors (labeled 1, 2, and 3) embedded in a composite material. Each stepcorresponds to an impact of increasing energy.19,20

Figure 10. Design of a new compositebogie for fast trains. Bogies are theswiveling undercarriages at either endof a railroad car on which the wheelsare fixed and which serve to dampvibrations transmitted to the car.Courtesy of Alstom, France, fromReference 25.



The stress levels were induced accord-ing to three cycles of dynamic loadingadded to a static one: 27 kN � (8 kN @7 Hz), 32 kN � (10 kN @ 7 Hz), and 38 kN � (11 kN @ 7 Hz). The correspon-ding strain ranged from about 2500 �� upto 3500 �� in the static mode and from700 �� to 1000 �� in the dynamic mode.The thermal cycles (as specified in the In-ternational Union of Railways standardUIC-515-4) were completed every 16 h.The instrumented elements underwentmore than 10 million cycles during threeweeks of testing, which represents, to ourknowledge, the most severe fatigue testscarried out to date with an FBG-basedmonitoring system.25

The results of these experiments showeda perfect agreement of the measurementswith the modeling, which proved that these new bogies retain a constant stiffnesswith no observable aging or mechanical-behavior evolution (Figure 11).

FBG-Based Strain RosetteIn the area of strain measurement using

FBG sensors, rosettes can be designed toinstrument existing composite structuresfor which embedded sensors are not pos-sible (Figure 12).26,27 Rosettes are made oftwo or three noncollinear strain gaugesmounted on a common substrate at 45� (toform a rectangular rosette) or at 60� (toform a ”delta” rosette).

Such strain rosettes are used extensivelyin experimental stress analysis to measurethe two principal strains (and stresses)and the orientation of the principal axiswhenever it is not known a priori. Besidesthis classical application, we have also de-scribed an innovative way to use thisrosette as a uniaxial strain gauge that isrigorously independent of temperature ef-

fects as well as its orientation on the struc-ture under test. The uniaxial strain, the an-gular orientation, and the temperature areaccurately recovered from data given bythe three gauges.

FBG rosettes are potentially easy tomanufacture, as they are amenable tobatch processing. They may be assembledby hand or automatically using numericalcommand machines driving a positioningultrasonic transducer head, leading tomass production. At least two layout de-signs may be considered. The first designuses one fiber with three Bragg gratings inseries and operates in WDM. A second de-sign uses three Bragg gratings in parallel(one fiber for each grating) and operateswith an optical switching unit or a parallelanalyzer. The bending radius cannot betoo small, for reasons of mechanical relia-bility (the bending radius in Figure 12 is10 mm). The first layout requires the fibersto cross each other, whereas there are nocrossings in the second design. Some deltarosette “patches” containing one fiberhave been realized in the laboratory, accu-rately positioned with the help of a reticle(Figure 13). The sides of the delta rosetteare 30 mm long, but a smaller length canbe designed.28

After positioning, a polyimide cover is applied to the substrate, sealed withacrylic glue, and cured. This second poly-imide sheet may leave the fiber triangle(sensing region) partially uncovered forfurther direct fiber bonding. For industrialapplications, we start from a connectedoptical cable (0.9 mm or 3 mm in diame-ter) from which the fiber is stripped off forabout 30 cm. Then, the fiber may be hydro-genated, and the Bragg gratings are photo-written. The rosette patch is then assembledfollowing the procedure described, except

that the cable is sealed so that the patchcan be handled as easily as any normalelectrical strain-gauge rosette. The poly-imide support of the rosette can then bebonded onto metallic parts, stitched ontotextiles like a patch, or embedded intocomposite materials.

Long-Gauge Glass-EpoxyExtensometers

Fiber-reinforced plastics may also beused as proof-bodies for extensometers.Unlike extensometers with surface-mounted fibers, the sensing fiber is incor-porated into the composite material itselfduring the pultrusion process.29 Pultrudedstructural composites are lighter and morecorrosion-resistant than their metallicequivalents, resulting in easier handlingand lower cost. Moreover, they are electri-cally insulated (immune to electromag-

Figure 11. Comparison of a numerical simulation with optical FBG measurements.25

Figure 12. Delta strain rosette with threeFBGs.26

Figure 13. A rosette sensor. Courtesy ofCEA/Magne.



netic interference) and display goodmechanical performance (linearity and fa-tigue). Their mechanical properties de-pend on the type of reinforcement used(glass, Kevlar), resin type, and profilegeometry. The fiber may be located at thecenter of the composite rod, preservingthe fiber from degradation and renderingthe measurement free from torsion influ-ence. The fiber runs in and out so that eachextensometer may be placed in seriesalong a single cable line and may operatein transmission or in reflection, dependingon the network configuration.

In the pultrusion process, parallel glassfibers are bundled and projected througha resin-impregnation furnace and throughheating zones. The polymerization is thencarefully controlled, as well as the coolingof the composite part. The composite pro-file may then be cut to the desired length.The composite extensometer has a cylin-drical shape, but other profiles may be ob-tained on demand. Once the compositecylinder is made and cut, threaded sheathsare crimped onto each end. Extensometersare screwed onto fasteners fixed onto con-crete or embedded into the concrete whilecasting (Figure 14).

ConclusionsMonitoring of composite structures is a

10-year-old activity, still a new technicalfield in many aspects. Nevertheless, optical-fiber sensors, and particularly fiber Bragggratings, are being considered more andmore often for such purposes, due to theiradvantages in many industrial compositeapplications.2

In the last decade, a number of nationaland international research and develop-ment programs have addressed the use ofoptical fibers as temperature, strain, stress,pressure, vibration, and refractive-indexsensors, and there are now many proto-types of systems and some productsavailable. The main applications are inaeronautics and transportation as well asin civil engineering sectors. Other applica-tions are found in space (antenna stabi-lization, shuttle or satellite monitoring, thespace station, etc.) and the naval industry.30

In a few years, a “health diagnosis” ofspecific or critical parts of any aircraft willbe able to be carried out periodically, or ondemand according to security requests,thanks to airborne or ground-based FBG-based optical-fiber sensor measurementsystems. Such industrial instrumentationwill allow one to guarantee the conformityof these parts with respect to their speci-fications and consequently to certify thatan aircraft is or is not able to fulfill its mis-sion. Certainly, the checklist carried out bya pilot before takeoff will integrate newparameters such as structure monitoring.

In this context, research studies arebeing carried out around the world, relat-ing in particular to enhancing the manu-facturing process, ensuring quality controlof manufactured elements, and monitor-ing their health in use (impact detectionplus assessment of integrity), as well asachieving cuts in operational and fuel costs.

Indeed, the subject on which the aero-nautical sector will focus in the comingyears concerns smart (or “intelligent”)structures, also called adaptronics. Thisterm relates to all of the means (materialsand software) necessary to obtain a betterunderstanding of the real-time parametersinvolved in any phase of flight, namely,takeoff, cruising, and landing, in order toincrease safety, reduce noise, and decreasefuel consumption.

Fiber-optic measurement methods havealready shown excellent promise in labo-ratory tests and in some field trials. Ofcourse, there is a need for scientific re-search and development to produce somespecific sensors, embed fibers routinely,enhance the performance, reliability, andruggedness of the measurement systems,and reduce their costs. Recent trials haveshown that FBG-based optical-fiber sen-sors offer the possibility of enabling indus-trial smart-structure technology, as theyare able to provide critical information for smart manufacturing, nondestructiveevaluation, health monitoring, and damagecontrol. Moreover, they are fully synergis-tic with the expanding telecommunica-tions market, enabling potential low costsin the mid-term.

References 1. P. Sansonetti, P. Ferdinand, D.H. Bowen, M. Crowther, B. Culshaw, M. Martinelli, B. Fornari et al., in Proc. SPIE Conf. on Fiber OpticSmart Structures and Skins II (SPIE—The Inter-national Society for Optical Engineering,Bellingham, WA, 1989).2. P. Ferdinand, S. Magne, V. Dewynter-Marty,C. Martinez, S. Rougeault, and M. Bugaud,“Applications of Bragg Grating Sensors in Eu-rope,” 12th Int. Conf. on Optical Fiber Sensors(OFS-12), Williamsburg, VA, October 28–31,1997, p. 14.

3. P.A. Crosby and G.P. Fernando, in OpticalFiber Sensor Technology, Vol. 3: Applications andSystems, edited by K.T.V. Grattan and B.T. Meggitt (Kluwer Academic Publishers, NewYork, 1999). 4. G. Laffont and P. Ferdinand, Electron. Lett. 37(5) (2001) p. 289.5. G. Laffont and P. Ferdinand, Meas. Sci.Technol. 12 (7) (2001) p. 765.6. P. Ferdinand, O. Ferragu, J.-L. Lechien, B.Lescop, V. Marty, S. Rougeault, G. Pierre, C.Renouf, B. Jarret, G. Kotrotsios, M.R.H. Voet,and D. Toscano, “Mine Operating AccurateSTABILity Control with Optical Fiber Sensingand Bragg Grating Technology: The Brite-EURAM STABILOS Project,” 10th Int. Conf.on Optical Fiber Sensor (OFS-10), Glasgow,October 11–13, 1994, p.162; J. Lightwave Technol.13 (7) (1995) p. 1303.7. F. Ansari, ed., Proc. Int. Workshop on FiberOptic Sensors for Construction Materials andBridges (Technomic, Lancaster, PA, 1998).8. U. Senhauser, R. Brônnimann, P. Mauron,and P.M. Nellen, in Proc. Int. Workshop on FiberOptic Sensors for Construction Materials and Bridges,edited by F. Ansari (Technomic, Lancaster, PA,1998) p. 117.9. S.T. Vohra, M.D. Todd, G.A. Johnson, C.C.Chang, and B.A. Danver, “Fiber Bragg GratingSensor System for Civil Structure Monitoring:Applications and Field Test,” 13th Int. Conf. onOptical Fiber Sensor (OFS-13), Kyongju, Korea,April 12–16, 1999, Paper No. Tu2-1, p. 32.10. J. Seim, E. Udd, W. Schulz, and H.M. Laylor,“Composite Strengthening and Instrumenta-tion of the Horse Tail Bridge with Long GaugeLength Fiber Bragg Grating Strain Sensors,”13th Int. Conf. on Optical Fiber Sensor (OFS-13), Kyongju, Korea, April 12–16, 1999, PaperNo. P1-3, p. 196.11. J. Seim, E. Udd, W. Schulz, and H.M. Laylor,in Proc. SPIE, Vol. 3671 (SPIE—The Interna-tional Society for Optical Engineering, Belling-ham, WA, 1999) p. 128.12. R.M. Measures, T. Alavie, R. Maaskant, S.Huang, and M. LeBlanc, “Bragg Grating FiberOptic Sensing for Bridges and Other Struc-tures,” 2nd European Conf. on Smart Structuresand Materials, Glasgow, 1994, p. 162.13. R.A. Livingstone, in Proc. Int. Workshop onFiber Optic Sensors for Construction Materials andBridges, edited by F. Ansari (Technomic, Lan-caster, PA, 1998) p. 3.14. P. Ferdinand, S. Magne, V. Marty, S.Rougeault, P. Bernage, M. Douay, E. Fertein,F. Lahoreau, P. Niay, J.F. Bayon, T. Georges, andM. Monerie, “Optical Fiber Bragg Grating Sen-sors for Structure Monitoring within the Nu-clear Power Plants,” presented at the OpticalFiber Sensing and Systems in Nuclear Environ-ments Symposium, SCK-CEN, Mol, Belgium,October 17–18, 1994.15. V. Dewynter-Marty, S. Rougeault, P. Ferdi-nand, D. Chauvel, E. Toppani, M. Leygonie, B.Jarret, and P. Fenaux, “Concrete Strain Mea-surements and Crack Detection with Surfaceand Embedded Bragg Grating Extensometers,”12th Int. Conf. on Optical Fiber Sensor (OFS-12), Williamsburg, VA, October 28–31, 1997,p. 600.16. P. Ferdinand, S. Magne, V. Dewynter-Marty,L. Pichon, S. Rougeault, and M. Bugaud, “Opti-

Figure 14. Composite-based FBGextensometers for civil engineeringapplications. Courtesy of CEA/Magne.



cal Fiber Sensors Provide New Means for Mea-surement and Monitoring within the NuclearIndustry,” presented at the Int. Nuclear Con-gress ENC-98, Nice, France, October 25–28,1998. 17. A. Ball, The Newsletter of the MonitorConsortium (1) (Autumn/Winter 1996) and(2) (Autumn/Winter 1997).18. A. Ball, in Proc. 4th ESSM and 2nd MIMRConf., edited by G.B. Tomlinson and W.A. Bul-lough (Institute of Physics Publishing, Bristol,1998) p. 435.19. E. Bocherens, S. Bourasseau, V. Dewynter-Marty, S. Py, M. Dupont, P. Ferdinand, and H.Berenger, in Proc. 4th ESSM and 2nd MIMRConf., edited by G.B. Tomlinson and W.A. Bul-lough (Institute of Physics Publishing, Bristol,1998) p. 381. 20. D. Balageas, S. Bourasseau, M. Dupont, E.Bocherens, V. Dewynter-Marty, and P. Ferdinand,J. Intell. Mater. Sys. Struct. 11 (2000) (Part 6)p. 426.21. J.R. Dunphy, G. Meltz, F.P. Lamm, andW.W. Morey, in Proc. SPIE, Vol. 1370 (SPIE—The International Society for Optical Engineer-

ing, Bellingham, WA, 1990) p. 116.22. V. Dewynter-Marty, S. Rougeault, P. Ferdi-nand, M. Bugaud, P. Brion, G. Marc, and P.Plouvier, “4 technologies de CFO pour le suivide fabrication de matériaux composites,” pre-sented at the 19th Journées Nationales d’Op-tique Guidée (JNOG 1999), December 6–8,1999, Limoges, France (in French).23. V. Dewynter-Marty, S. Rougeault, P. Ferdi-nand, M. Bugaud, P. Brion, G. Marc, and P.Plouvier, “Contrôle de la fabrication de piècesen matériaux composites réalisées par procédéRTM à l’aide de capteurs à réseaux de Bragg,”presented at the 20th Journées Nationalesd’Optique Guidée (JNOG 2000), November20–22, 2000, Toulouse, France (in French).24. D. Ness, in Proc. 22nd Int. SAMPE EuropeConf. Soc. for the Advancement of Materials andProcess Engineering (La Défense, Paris, 2001)p. 89.25. L. Maurin, J. Boussoir, S. Rougeault, M.Bugaud, P. Ferdinand, A. Landrot, T. Chauvin,and Y.-H. Grunevald, “Instrumentation deBogie composite par capteurs à fibers optiquesà réseaux de Bragg,” presented at the 20th

Journées Nationales d’Optique Guidée (JNOG2000), November 20–22, 2000, Toulouse, France(in French).26. S. Magne, S. Rougeault, M. Vilela, and P.Ferdinand, Appl. Opt. 36 (36) (1997) p. 9437.27. S. Magne, S. Rougeault, M. Vilela, and P.Ferdinand, “Rosettes à réseaux de Bragg et ap-plications,” presented at the Journées Na-tionales d’Optique Guidée (JNOG 1996),October 28–30, 1996, Nice, France (in French). 28. P. Ferdinand, S. Magne, V. Dewynter-Marty,S. Rougeault, and M. Bugaud, “Optical FiberBragg Grating Sensors Make Composite Struc-tures Smart,” presented at the SAMPE EuropeConf. ’99, Paris, April 1999.29. M. Bugaud, V. Dewynter-Marty, S. Magne,and P. Ferdinand, French Patent No. 2,791,768(June 10, 2000).30. K. Pran, G.B. Havgard, R. Palmstrom, G.Wang, G.A. Johnson, B.A. Danver, and S.T.Vohra, “Sea-Test of 27 Channel Fiber BraggGrating Sensor System on Air Cushion Cata-maran,” 13th Int. Conf. on Optical Fiber Sensor(OFS-13), Kyongju, Korea, April 12–16, 1999,Paper No. W1-3, p. 145. ■■

applications of fiber bragg grating sensors in the

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