128 IEEE SENSORS JOURNAL, VOL. 12, NO. 1, JANUARY 2012

How Reliably do Fiber Bragg Grating PatchesPerform as Strain Sensors?

Vivien Gisela Schukar, Nadine Kusche, and Wolfgang R. Habel

Abstract—In Germany, the first guideline for the use of fiberBragg grating strain sensors, “Optical Strain Sensor based onFiber Bragg Grating” (Berlin, Germany: Beuth-Verlag, 2010), hasbeen developed by the GESA guideline group of VDI, “The Associ-ation of German Engineers” and published by Beuth-Verlag. Thisguideline provides the basic specifications of this sensor type andthe sensor characteristics, which have to be known for a reliablesensor performance. In conformity to this guideline, experimentalinvestigations on the strain transfer characteristics of fiber Bragggrating patches have been carried out. A comparison betweenpatches and resistance strain gauges during tensile tests andcombined temperature and tensile loading was carried out. Theevaluated strain gauge factor and the temperature sensitivity ofthe strain gauge factor have been compared to the manufacturer’sdata. The overall performance of the patches has been evaluated.The experimental investigations showed that there are consider-able disagreements between the manufacturer’s specifications andthe observed characteristics.

Index Terms—Fiber Bragg grating, patch, strain gauge, straintransfer, validation.

I. INTRODUCTION

F IBER BRAGG GRATINGS are widely used as opticalstrain sensors. While previously the bare fiber was applied

to components to monitor strain distributions, nowadays fiberBragg gratings have been integrated or applied as strain sensi-tive elements to a carrier in order to improve the handling ofthis sensor system. These optical strain gauges based on fiberBragg gratings are also called “Patches.” Compared to resis-tance strain gauges, fiber Bragg grating patches have a varietyof benefits. Besides their immunity to electromagnetic fields,their multiplexing capacity and their low weight and small size,patches can easily be applied to composite materials withoutdeep knowledge about Bragg grating technology.

Fiber Bragg grating patches are commercially available froma lot of companies. All these patches have different characteris-tics due to different carrier designs. The choice of a patch typefor the specified measurement setup based on the different spec-ifications of the patches needs to be carefully carried out. Inthis paper, different patches have been compared in order toverify experimentally the different characteristic behaviors of

Manuscript received October 20, 2010; revised March 29, 2011; acceptedMarch 29, 2011. Date of publication April 07, 2011; date of current versionNovember 29, 2011. The associate editor coordinating the review of this paperand approving it for publication was Prof. Brian Culshaw.

The authors are with the BAM Federal Institute for Materials Research andTesting, Div. VIII.1: Measurement and Testing Technology, Sensors, D-12200Berlin, Germany (e-mail: vivien.schukar@bam.de; nadine.kusche@bam.de;wolfgang.habel@bam.de).

Digital Object Identifier 10.1109/JSEN.2011.2139202

the patches. Therefore, the patches with integrated Bragg grat-ings were considered as a complete sensor system. Issues con-cerning the reliability or stability of the embedded grating thatmay rise from its fabrication or embedment, are not part of theseinvestigations. It is assumed that manufacturer of patches com-pletely control these processes. The experimental investigationsof the patch characteristics led to considerably different resultscompared to the specifications indicated by the manufacturers.This justifies the essential need to validate patches for specificmeasurement tasks with particular regard to their reliability.

II. PRINCIPLE OF FIBER BRAGG GRATINGS

Fiber Bragg gratings are inscribed into the core of opticalfibers to form a periodic modulation of the refractive index.When coupling light into an optical fiber with inscribed fiberBragg grating, a small spectrum will be reflected and the restof the light will be transmitted. The maximum of the reflectedsmall spectrum is known as the Bragg wavelength. The Braggwavelength is described as

(1)

with as effective index of refraction and the gratingspacing.

Stretching or compression of the fiber along its length willresult in a wavelength change of the Bragg wavelength. Thesame result occurs when the fiber is subjected to temperaturechanges. Both phenomena are due to a change in the effectiveindex of refraction and the grating spacing [2]

(2)

A fundamental parameter for the fiber Bragg grating’s sen-sitivity to strain is the gauge factor. For bare and not-appliedfiber Bragg gratings, the gauge factor or strain sensitivity is typ-ically in the range of 0.7–0.8 [3], [4]. The gauge factor is derivedfrom the strain optic coefficients and out of the Pockel’stensor and, therefore, depends on the fiber material characteris-tics [5]

(3)

The photoelastic constant is defined as, and is Poisson’s ratio. In

general, the gauge factor can only be determined when the barefiber Bragg grating or the patch is applied to a substrate.

In the case of patches, the gauge factor depends also on thedesign of the carrier material and can be determined for someexemplary patches. Using statistical methods, the gauge factorcan then be calculated for the manufactured batches.

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SCHUKAR et al.: HOW RELIABLY DO FIBER BRAGG GRATING PATCHES PERFORM AS STRAIN SENSORS? 129

TABLE ICOMPARISON OF THE TEMPERATURE-CORRELATED STRAINS OF

DIFFERENT PATCH TYPES

III. TEMPERATURE-CORRELATED STRAIN

In conformity to the guideline for strain sensors based on fiberBragg grating [1], a random sample of patches have been ana-lyzed concerning their temperature-correlated strain, the straingauge factor and the temperature dependence of the strain gaugefactor. The experimental results have been compared to the man-ufacturer’s specifications in the data sheet. In order to selectthe proper patch adapted to the measurement conditions and theloading of the component, it is essential to know the character-istics of the patch. The specifications of the patches provided bythe manufacturers are sometimes incomplete or incorrect. Thismakes it necessary to prove the patch’s behavior experimentallyunder measurement conditions and to compare the patch char-acteristics with each other. Now, the manufacturer should indi-cate the sensor characteristics according to the datasheet samplerecommended in the guideline VDI/VDE/GESA 2660 [1]. Ad-ditionally, in the COST guideline 299, the important terms typ-ically used in fiber sensing technology are defined [6].

First, the temperature-correlated strain of the patches was an-alyzed. Therefore, the patches, a temperature sensor PT100 anda resistive strain gauge have been applied according to the man-ufacturer’s instruction to a 10 mm thick steel plate. The plateswere subjected to a temperature cycling in the specified tem-perature range of the datasheet. It was possible to determine thethermal coefficient of expansion of the plate and the apparentstrain with the strain gauge. This information was then used tocalculate the temperature-correlated strain of the patches. Thethermal expansion coefficient of the steel plates was found to be

. According to the guideline [1], the tem-perature-correlated strain of the fiber Bragg grating patchis defined as

(4)

with as the strain gauge factor of the patch as specified by themanufacturer, as temperature, as the Bragg wavelength,and as the reference Bragg wavelength. The temperature-correlated strain depends mainly on the optical fiber and thepatch material. For bare optical fibers, 7.8 is a typicalvalue.

The temperature-correlated strain for the patches was de-termined and compared to the manufacturer’s specificationsin Table I. It is obvious that different patch configurationsshow different temperature-correlated strain factors, dependingon the carrier material and the structure of the patch itself.

Fig. 1. Comparison of the temperature-correlated Bragg wavelength change oftwo different patches (A and B) over a temperature range of��� C to��� C.The relative temperature change is related to ��� C.

The majority of the manufacturer did not specify the tem-perature-correlated strain. This information is though veryimportant for the determination of the apparent strain of a com-ponent with temperature. The apparent strain at a measuringpoint consists of the sum of the component’s thermal expansionand the temperature-correlated strain of the patch itself.

The change in Bragg wavelength due to a change in temper-ature over the specified temperature range is shown in Fig. 1for two different patches. The temperature change refers to anabsolute temperature value of 80 C and the Bragg wavelengthchange refers to the corresponding Bragg wavelength at 80 Cof the applied patch. The temperature changes of the experi-ment were in the range of to 80 . It can be observedthat patch B shows a linear functional behaviour. Patch A re-laxes nearly 100 at 80 (circle). This may indicatethat Patch A is not temperature-stable in the specified tempera-ture range.

IV. STRAIN GAUGE FACTOR

In the second part of the experimental characterization ofthe fiber Bragg grating patches, the strain gauge factorwas determined. The patches have been applied to stainlesssteel tensile test coupons. Additionally, resistance strain gaugeswere also applied to the tensile test coupons to verify thestrain gauge factor. Resistance strain gauges can be used todetermine the strain gauge factor of fiber-optic sensors, whilethe coupons are only loaded statically with relative small loads(e.g., 1000 ). When determining the strain transfer char-acteristics for example during fatigue tests, strain gauges areinadequate because of their limited fatigue life. Furthermore,strain gauges cannot be used for measurements on highly ten-sile materials like carbon-reinforced composites (cfrp) or glassfiber-reinforced composites (gfrp). This was demonstratedwith a tensile test coupon which was loaded until it deformedplastically, as shown in Fig. 2.

Patch D applied to the tensile test coupon follows very wellits deformation. At almost 8000 , the patch delaminatesfrom the specimen, but actually does not fail in its function. The

130 IEEE SENSORS JOURNAL, VOL. 12, NO. 1, JANUARY 2012

Fig. 2. Plastically deformed tensile test coupon. The resistance strain gaugereaches its saturation limit, while Patch D follows the deformation of the couponuntil delamination from the coupon.

TABLE IICOMPARISON OF STRAIN GAUGE FACTORS OF DIFFERENT PATCH TYPES.

measurement signal returns to the reference Bragg wavelengthafter delamination. If adhesive interface did not fail, the patchwould continue to measure at even higher strain rates. Com-pared to this patch, the resistance strain gauge already reachedits saturation limit at about 4500 . This shows the greatadvantage of optical strain sensors compared to resistance straingauges: they can be loaded up to very high strain values.

In order to determine the strain gauge factor, the tensile testcoupons were loaded up to 1200 in three cycles, eachcycle consisted of ten steps of 1 kN up and down. The temper-ature was kept constant at 23 C.

All experimentally determined strain gauge factors are pre-sented in Table II and compared with the manufacturer’s specifi-cations. The difference between the specified data of the patchesand the real data is quite large.

As shown in Table II, different patches have different straingauge factors. The strain gauge factor 1.2 , as in-dicated by the manufacturer, corresponds to a dimensionlessgauge factor of 0.78. The dimensionless description should beused according to [1]. Otherwise, the strain gauge factor still hasto be related to the reference Bragg wavelength, which has to beknown additionally.

The strain gauge factors depend mainly on the design andinternal structure of the patches. This leads to the advantagethat the strain gauge factor can be adjusted depending on themeasurement conditions. In order to make it straightforward forthe use of this technology, the strain gauge factor can be setto one. This has been done for Patch C, as shown in Fig. 3.

Some of the experimentally investigated patches showed non-linear strain behavior, as shown in Fig. 4. This behavior can onlybe explained by the manufacturing process and/or the design of

Fig. 3. Comparison of the strain gauge factor� for two different patches.

Fig. 4. Nonlinear strain behavior of Patch A.

the patches. In this case, Patch A does not follow the strain ap-plied to the tensile test coupon when the load is very small. Athigh loads, the patch follows the higher strain very well. Fromthis behavior can be concluded that depending on the load of thecomponents, some patches are better suited for high strains thanothers that can only be used for low strain loading.

V. TEMPERATURE DEPENDENCE OF THE

STRAIN GAUGE FACTOR

The temperature dependence of the strain gauge factor wasdone experimentally in the last part of the test series. The tensiletest coupons have been prepared in the same manner as for thetests to determine the strain gauge factor at room temperature.Additionally, the tensile test coupons have been loaded with themaximum and minimum temperature in which they should workaccordingly to the specifications of the manufacturers. Whenthe temperature chamber reached thermal stability at the desiredtemperature, the loading cycles have then been carried out. It isshown in Table III that there is only a very small temperaturedependence on the strain gauge factor.

The temperature dependence on the strain gauge factor hasnot been explicitly mentioned in the datasheets of the manu-facturers. It is assumed that the strain gauge factor does not

SCHUKAR et al.: HOW RELIABLY DO FIBER BRAGG GRATING PATCHES PERFORM AS STRAIN SENSORS? 131

TABLE IIICOMPARISON OF STRAIN GAUGE FACTORS AT DIFFERENT TEMPERATURES FOR

DIFFERENT PATCH TYPES

Fig. 5. Relaxation of the strain determined by Patch A. The patch decays inbetween the temperature range indicated by the manufacturer.

vary with temperature. In order to facilitate the measurementsetup for the user, the strain gauge factor should be indeed con-stant over the indicated temperature range. In fact, slight differ-ences of the strain gauge factors can be experimentally deter-mined with temperature, as shown in Table III. Depending onthe overall measurement uncertainty, the users must be awareof the significance of the strain gauge’s temperature dependencefor their intended use.

From the combined temperature and loading cycles can beseen that Patch E is not temperature stable for the maximumtemperature indicated by the manufacturer. At a first test at80 C, this patch type showed a gauge factor of 0.765, indi-cated by 1) in Table III, and the Bragg wavelength peak wascreeping at maximum load. It is assumed that either the adhe-sive softens at that temperature and a full strain transfer cannotbe carried out, or the patch material relaxes with the same ef-fect that the strain is not fully transferred into the Bragg grating.When repeating the loading cycles at 80 C, the gauge factorrose to 0.816, indicated by 2) in Table III, and the Bragg wave-

length peak was stable. This behavior could be caused by a fur-ther hardening process of the adhesive or the patch material at80 C, while carrying out the tests. Carrying out the loadingtest at 60 C, it led to repeatable strain transfer behavior with agauge factor of 0.843. At this temperature, adhesive and patchmaterial showed good performance.

A strain gauge factor at the defined maximum and minimumtemperature could not be determined for Patch A. This patchcollapsed during its investigation at maximum temperature asindicated by the manufacturer. This behavior is shown in Fig. 5.It is also assumed that the carrier material of the patch tends tosoften at higher temperatures, and the Bragg wavelength couldnot follow then the deformation of the specimen anymore.

VI. CONCLUSION

Different commercially available patches based on fiberBragg gratings were validated with regard to the tempera-ture-correlated strain, the strain gauge factor and the tempera-ture dependence on the strain gauge factor. The experimentaldata were compared to the manufacturer’s specifications. It wasdemonstrated that – for some patch types – there are discrepan-cies between the experimentally determined performance of thepatches and the performance indicated by the manufacturers.When optical strain sensors are to be used with high reliabilityrequirements, the appropriate sensor type has to be chosen fromtests in which the performance has been statistically validated.Further developments driven by manufacturers to improve thepatch performance and to correctly specify the patch charac-teristics are essential for the promotion of this measurementtechnology.

REFERENCES

[1] Experimental Stress Analysis – Optical Strain Sensor Based on FibreBragg Grating; Basics, Characteristics and its Testing. Berlin, Ger-many: Beuth-Verlag, Jul. 2010, VDI/VDE/GESA Guideline 2660 [On-line]. Available: http://www.beuth.de

[2] A. Othonos and K. Kalli, Fiber Bragg Gratings – Fundamentals andApplications in Telecommunications and Sensing. Boston , MA:Artech House, 1999.

[3] E. Riviera and D. J. Thomson, “Accurate strain measurements withfiber Bragg sensors and wavelength references,” J. Smart Materials andStructures, vol. 15, pp. 325–330, 2006.

[4] A. Rocco, G. Coppola, A. Di Maio, P. Ferraro, M. Iodice, and P. De Na-tale, “An interferometric demodulation method for visualizing and de-termining quasi-static strain by FBG sensors,” J. Meas., Sci., Technol.,vol. 17, pp. 1486–1490, 2006.

[5] G. Meltz and W. W. Morey, “Bragg grating formation and ger-manosilicate fiber photosensitivity,” in Proc. SPIE, 1991, vol. 1516,pp. 185–199.

[6] Cost 299, “Guideline for use of fibre optic sensors,” Sep. 2009. [On-line]. Available: http://www.cost299.org

Vivien Gisela Schukar received the Diploma degree in mechanical engineeringfrom the Technical University of Braunschweig, Braunschweig, Germany, in2006, and received the Ph.D. degree from the Technical University of Berlin,Berlin, Germany, in 2010. In her Ph.D. thesis, she developed an experimentalvalidation technique for the optimization and characterization of strain transferof surface-applied fiber Bragg grating sensors.

Since 2006, she has been at the BAM Federal Institute for Materials Researchand Testing in the Fiber-Optic Sensors Group of Dr. Habel. Currently, she isinvolved in different research projects dealing with the characterization of sur-face-applied and materials-integrated fiber-optic sensors.

132 IEEE SENSORS JOURNAL, VOL. 12, NO. 1, JANUARY 2012

Nadine Kusche received the Diploma degree in Mechanical Engineering fromthe Beuth Hochschule für Technik Berlin, Berlin, Germany, in 2009.

Since 2008, she has been at the BAM Federal Institute for Materials Researchand Testing in the Fiber-Optic Sensors Group of Dr. Habel. Currently, she isinvolved in different research projects dealing with the development and thecharacterization of surface-applied fiber-optic sensors.

Wolfgang R. Habel received the Diploma degree in theoretical methods of elec-trical engineering and information technology from the Technical Universityof Ilmenau, Ilmenau, Germany, and the Ph.D. degree in engineering (Dr.-Ing.)from the Technical University of Berlin, Faculty for Civil Engineering and Ap-plied Geosciences, Berlin, Germany.

Since 1997, he has been at the BAM Federal Institute for Materials Researchand Testing, Berlin, and is heading the research group “fiber-optic sensors.” Heis a leading member of several national and international societies related tofiber-optic sensors and structural health monitoring. His main research activi-ties concern fiber-optic sensors for characterization of the materials behavior,stress analysis, and early detection of damage. Another activity is focused onreliability and validation aspects when fiber-optic sensors are practically used,and on the development of guidelines and standards for definition, characteri-zation and providing the evidence of appropriate sensor application.