Reliability and Accuracy of Embedded Fiber Bragg Grating Sensors for Strain Monitoring in Advanced Composite Structures

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1 Met. Mater. Int., Vol. 20, No. 3 (2014), pp. 537~543 doi: /s Reliability and Accuracy of Embedded Fiber Bragg Grating Sensors for Strain Monitoring in Advanced Composite Structures Raffaella Di Sante *, Lorenzo Donati, Enrico Troiani, and Paolo Proli University of Bologna, Department of Engineering for Industry, Forlì, 47121, Italy (received date: 12 November 2012 / accepted date: 27 September 2013) This work investigated issues for an efficient and reliable embedding and use of Fiber Bragg Grating (FBG) sensors for strain monitoring of composite structures with particular regard to the manufacturing process of components in the nautical field by means of the vacuum bag technique in autoclave. CFRP material laminates with embedded FBGs were produced and the effect of the curing process parameters on the light transmission characteristics of the optical fibers was initially investigated. Two different types of coating, namely polyimide and acrylate, were tested by measuring the light attenuation by an Optical Time Domain Reflectometer. Tensile specimens were subsequently extracted from the laminas and instrumented also with a surface-mounted conventional electrical strain gage (SG). Comparison between the FBG and SG measurements during static tensile tests allowed the evaluation of the strain monitoring capability of the FBGs, in particular of their sensitivity (i.e., gage factor) when embedded. Key words: composites, thermomechanical processing, strain rate, tensile test, fiber bragg gratings (FBGs) 1. INTRODUCTION In recent years, composite materials have become more and more popular because they allow construction of light, but durable and strong, structural components in many fields of application: aeronautics, automotive, power plants, civil engineering, etc. However, anisotropy and inhomogeneity of the materials make it rather difficult to analytically and numerically model their mechanical behavior. For this reason, the experimental evaluation of in-service stresses and strains, especially inside a composite structure, can allow improvement of current design strategies and criteria. Moreover, in-service strain monitoring of this type of structures may permit estimation of their deformation state and of the eventual occurrence of a critical condition in real time. In recent years, optical fiber sensors [1-5] have shown greater benefits in the field of static and dynamic strain monitoring with respect to more traditional sensors, such as immunity to electromagnetic fields, durability, resistance to corrosion and multiplexing capability. Furthermore, due to their small dimensions they can be embedded in the material. Among the available optical fiber sensors, fiber Bragg gratings (FBGs) (see for example [6,7]) show the greatest potential for monitoring of in-service strain in composite materials and structural health monitoring as well, but their effective use is not immediate *Corresponding author: raffaella.disante@unibo.it KIM and Springer and may generate measurement inaccuracies especially due to the embedding process. Embedding optical fibers into a fibrous material such as a composite might appear as a quite easy task; however, the manufacturing processes for industrial production of composite components introduce a number of issues due to the mechanical and thermal stresses that may damage the sensors or alter their performances [8]. Indeed a number of specific problems need to be dealt with, e.g., the ingress-egress points of the fiber in the material need special protection to prevent breaking, the structure resistance may be altered by the presence of the fiber itself, the Bragg spectrum may be distorted depending on the intensity and spatial distribution of the residual strain field and finally the strain transfer from the host material to the fiber core is different in the case of an embedded sensor rather than an adhesively bonded sensor. In this paper, we investigate the optical losses due to thermomechanical stresses generated during the manufacturing process and evaluate the measurement performance of the embedded sensors with respect to strain detection. Indeed, while calibration of non-embedded FBG sensors has been extensively treated in the literature (see for example [9]), that of the embedded sensors has not. To this aim, tensile specimens made of advanced pre-preg composite material are manufactured in autoclave and instrumented with single FBG sensors placed in the laminate mid-plane. Reference electrical strain gages are used during tensile tests to perform an in-situ calibration of the embedded fiber sensors.

2 538 Raffaella Di Sante et al. 2. BASICS OF FBG OPERATION A fiber Bragg grating is a periodic modulation of the effective refractive index n eff in the optical fiber core. Light travelling at the Bragg wavelength, λ B, which depends on the grating features, is reflected back by the grating itself and as a result it is missing in the transmission spectrum. The principle of operation of a Fiber Bragg Grating is illustrated in Fig. 1. The reflected spectrum is centered at the Bragg wavelength and depends on the mentioned effective index of refraction (n eff ) and on the Bragg period (Λ) of the grating, according to the following equation: ment issues related to the embedding of a fiber Bragg gratings in advanced composite material used in the marine field, we considered the manufacturing process of a sailboat mast. Successful embedding of the sensors in this case would in fact allow the development of a FBG sensors-based system able to accurately monitor in-service strains of composite components of a sailboat during operation, especially the mast which usually is not monitored and may also break during sailing if subjected to severe deformations. The typical mast manufacturing cycle considered in this work is carried out in autoclave with the vacuum bag technique at a temperature of 90 C or 120 C and at a pressure of 3 bar. The specimen is left in autoclave for 8 h in the first case (90 C) and for 1.5 h in the second. In Fig. 2 the autoclave and some of the vacuum bags used in this work are shown. Small samples were extracted from the cured laminas and shaped according to the ASTM 3039 standard for tensile test with the aim of characterizing the sensors behavior under controlled conditions and in the presence of an applied unidirectional force. In this way, in fact, it becomes possible to evaluate whether there is a difference in the FBG sensor calibration with respect to the non-embedded case and study the strain transfer characteristics without adding further uncerλ B = 2n eff Λ which is the Bragg equation. There are different methods to inscribe the grating on the fiber core based on appropriate illumination of the photosensitive fiber by UV light fringe pattern in the nm wavelength range. A holographic method [10] can be used, for example, or a phase mask method. A combination of both methods is also possible [11]. In the case of long period gratings, a pulsed laser technique is preferred [12]. By inscribing an array of FBG sensors with different grating periods in the same fiber, it is possible to monitor different locations on a structure. When a local deformation is present, the grating s period varies and the reflected wavelength varies accordingly, allowing the detection of the local strain via the Bragg equation (1). The strain and the wavelength variations can be put in a oneto-one relationship only in the case of unidirectional and uniform deformation, provided that the effect of the temperature on the Bragg wavelength is suitably compensated for [13]. (1) In order to investigate the major technical and measure- 3. EXPERIMENTAL PROCEDURES Fig. 2. Autoclave and vacuum bags used in this work. Fig. 1. Fiber Bragg Grating s principle of operation.

3 Reliability and accuracy of Embedded Fiber Bragg Grating Sensors for Strain Monitoring in Advanced Composite Structures 539 Fig. 3. Machine used for the tensile tests (a) and drawing of the composite tensile specimen (b). tainty due for example to the existence of transversal or more complex strain fields. Based on the mentioned standard for tensile specimens, the dimensions of the sample were a 250 mm length, 50 mm width and 2.4 mm thickness. The material used for the samples was a typical pre-preg material (type MTM 57 T700S, with a thickness of 0.12 mm and specific weight of 150 g/mm 2 ), unidirectional, with plies at 0 and 90 laid down according to the following sequence: [0, 90, 90, 0, 90 ]2s. The resin used for all the pre-pregs was a MTM 57 standard type (it has intermediate viscosity and tack suitable for full impregnation of light and medium weight unidirectional reinforcements). Each specimen was instrumented with one FBG sensor with a 10 mm length and 125 μm diameter (corresponding to a standard telecom fiber), placed between the two 0 central plies. Two different types of coatings, acrylate and polyimide, were considered in order to analyze their capability to protect the grating spectrum from distortion with respect to the manufacturing process and in particular the residual strains. Polyamide-coated FBG sensors were embedded into samples manufactured at both the curing temperatures (95 and 120 C), while acrylate-coated FBGs were embedded only at the lower temperature (95 C). In fact, preliminary tests executed at a temperature higher than 100 C using acrylate showed that this type of coating can be seriously damaged by the hightemperature process in autoclave and therefore that it is not able to withstand temperatures higher than approximately 100 C. After autoclave curing, all specimens were instrumented also with a traditional electrical strain gage (SG) with a 10 mm length, the same as the FBG sensor (HBM, width of 5 mm, resistance of 350 Ω), surface-mounted above the FBG sensor position inside the sample. Some of the FBG sensors were also placed on the sample surface before the curing cycle, and remained embedded on the sample surface due to the curing of the resin. Figure 3 shows the machine used for the tensile tests, an Italsigma tensile testing machine with 10 kn as the maximum applicable load, and a drawing of the tensile specimen. In the ingress-egress points, the fiber was protected using PTFE foils or small Sterling sleeves. These points are in fact a major issue in fiber embedding, due to the sharp edge of the structure border and the sudden change of stress acting on the fiber [14], which may cause strong light attenuation or fiber breaking. The use of PTFE foil and a Sterling sleeve way proved able to ensure fiber integrity at the curing conditions considered for both acrylate and polyimide fiber coatings. One of the technical problems to be considered when embedding an FBG sensor in a composite structure is the distortion of the structure itself in the surroundings of the optical fiber. This issue was not further analyzed in this work because it has been proven in the literature [15] that standard 125 μm optical fibers generate very low distortion of the hosting material when embedded parallel to the composite fibers in the laminate. All optical fiber sensors were therefore aligned to the unidirectional plies along the reinforcing fibers direction (0 ), thus ensuring no significant reduction in strength of the composite material. Tensile loading and unloading static cycles were finally performed on the specimens in order to evaluate also a possible hysteresis effect in the optical fiber sensors. 4. RESULTS AND DISCUSSION Before testing the sample using the tensile testing machine, we investigated light attenuation in the fiber, which is an important factor in long composite products like a sailboat mast. Based on the tensile test results and the comparison with traditional strain gages, the embedded sensor s response was analyzed. This issue also includes the sensor s ability to measure the strains that are transferred from the surrounding material to the fiber core material Light attenuation The high-temperature and pressure curing process required for polymerizing the embedding matrix of the composite may be able to soften or even melt the coating of the optical fiber and therefore to introduce sharp-radius permanent deformations that may heavily affect the optical attenuation through microbending losses [16] due to light dispersion. Estimation of the optical loss in the embedded sensor and in the whole light path in the fiber after the embedding process therefore becomes important to determine whether the attenuation still allows accurate and reliable measurements. In fact, even if strain measurement with FBG sensors is based on the wavelength shift determination rather than on the amplitude of the back-reflected light, when long composite structures are to be monitored (boat masts can be as long as 40 m) using multiplexed sensors, optical losses can become an issue. Light attenuation in the samples was evaluated using an Optical Time Domain Reflectometer (OTDR) according to

4 540 Raffaella Di Sante et al. Fig. 4. Optical losses measurement chain. Fig. 5. OTDR spectrum. the experimental arrangement shown in Fig. 4. An OTDR injects a series of optical pulses into the fiber and collects the light that is scattered or reflected back from the fiber itself, like an electronic time-domain reflectometer detects reflection from a change of impedance inside a cable [17]. The amplitude of the return pulses is measured and integrated as a function of time and plotted as a function of the fiber length. This plot in the case of a polyimide-coated FBG embedded at 95 C and 3 bar is shown in Fig. 5. The fibers were embedded in the sample and cured without connectors, which were added afterwards through a fusion-splicing machine. All the losses due to connectors, the splicing section, the ingressegress points and the fiber section outside the sample were accounted for in the final evaluation of the losses inside the sample. The overall light attenuation in the material was quantified and reported in Table 1. Losses are in the order of 2 to 3.41 db and therefore measurement at long distances from the interrogation unit are still allowed. The values for the two types of coatings are quite similar. At the highest temperature the acrylate coating was avoided, as mentioned, for its intrinsic resistance limits, while the polyimide coating resisted quite well also from the point of view of optical losses, which prevented degradation of the fiber performance Embedded FBGs calibration The sensitivity and accuracy of the embedded sensors are influenced by the physical coupling between the sensor and the composite embedding matrix. Testing the structure with embedded sensors in laboratory conditions and evaluating the actual metrological performances are issues of primary importance to validate the development and allow exploitation of the results obtained from a composite structure under operating conditions. There are two main aspects related to the ability of the FBG sensor of performing accurate strain measurements when embedded: the possible distortion of the Bragg spectrum and the effective strain transfer from the composite material to the optical fiber sensor. Regarding the first issue, the reflected Bragg spectrum after curing may show distortion due mainly to residual strains caused by the manufacturing process. The latter can originate from the different mechanical properties of the reinforcement fibers and the resin and from anisotropy of the plies. It must be noted though that even if from a measurement point of view such aspects may only look like a problematic issue, from an industrial point of view their determination is of key importance, because residual stresses play a significant role in the mechanical performance of the composite structure produced. Moreover, residual stresses are still difficult to evaluate experimentally once the item has been manufactured. Before they can be evaluated with sufficient accuracy using an embedded FBG sensor though, the measuring performances of the sensor must be carefully checked. In the literature, light to severe deformation of the spectrum has been observed, depending on the type of composite, the relative orientation of the fiber with respect to the reinforcement fibers, the type of coating and the existence of strong transversal strains. In the present case, sensors with protective coating were embedded parallel to the material fibers in a unidirectional laminate, and therefore, possible effects of broadening or even splitting of the spectrum should be contained and are eventually to be attributed to the existence of strong transversal residual stresses. The experimentally measured Bragg spectrum is shown in Fig. 6 for the case of an embedded polyimide-coated fiber and compared to the non-embedded spectrum of the same FBG sensor to check the existence of a possible distortion occurring inside the material. The same comparison for the acrylate coating at the same temperature showed a quite similar result, proving that no significant distortion takes place in this case, given the used coating and the technical solutions adopted. This is an important result as it also means that classical Bragg spectrum analyzing methods, such as the Full Width at Half Maximum (FWHM) method and the center of gravity (COG) algorithm Table 1. Estimated optical losses in the different samples with FBG sensors (SUP: surface-mounted sensor, EMB: embedded sensor) at the curing conditions indicated Acrylate SUP Acrylate EMB Polyimide SUP (120 C, 3 bar) Optical Loss [db]

5 Reliability and accuracy of Embedded Fiber Bragg Grating Sensors for Strain Monitoring in Advanced Composite Structures 541 Fig. 6. Bragg spectrum of the embedded sensor compared to the nonembedded sensor in the case of the polyimide-coated fiber (curing conditions: 95 C, 3 bar). [18], can still be used to evaluate the reflected Bragg wavelength during the measurement. For the polyamide-coating fiber, in the case of curing at higher temperature (120 C), we assumed that no major changes occurred because of the demonstrated protective ability of the coating. Regarding the strain transfer from the material to the fiber core, this again is strongly dependent on the coating, which acts as the interface between the two. Much research in this field focuses on surface bonded FBG sensors (see, for example, [19]) and shows that the strain transfer is dominated by the adhesive thickness and the bond length of the fiber. However, this situation is quite different from the embedded sensor case, where the sensor is completely surrounded by the material and speaking of adhesive thickness and bond length becomes meaningless. In this work, the surface-mounted FBGs are in any case partially embedded, as no additional adhesive is used and the bond length is comparable to that of the sensor embedded between the inner plies. Indeed, the surfacemounted sensors undergo the curing cycle in autoclave and are bonded to the specimen surface through the same resin used inside the composite matrix. Their behavior should therefore be closer to that of the embedded sensors. In order to evaluate the strain monitoring capability and reliability, data obtained from the strain gages (SGs) was used for an in-situ calibration of the FBG sensors. In fact, SGs are not subjected to any relevant interfering input and may therefore constitute a reference for the accurate determination of the tensile strains during the experimental static tests. The calibration factor of both the superficial and embedded FBG sensors was derived from the linear interpolation of the data, i.e., reference strain values obtained from the esg and the corresponding wavelength shifts of the FBG sensors cal- Fig. 7. Calibration curve for acrylate-coated fibers for samples cured at 95 C temperature, 3 bar pressure and (a) with FBG sensor on the surface; (b) with FBG sensor embedded. Fig. 8. Calibration curve for polyimide-coated fibers for samples cured at 95 C temperature, 3 bar pressure and (a) with FBG sensor on the surface; (b) with FBG sensor embedded between the central plies.

6 542 Raffaella Di Sante et al. Table 2. Estimation of the residual strains (SUP: superficially bonded sensor, EMB: embedded sensor) at the curing conditions indicated Acrylate SUP Acrylate EMB Polyimide SUP (120 C, 3 bar) Nominal wavelength [nm] Residual strain Fig. 9. Calibration curve for polyimide-coated fibers for samples cured at 120 C temperature, 3 bar pressure with FBG sensor embedded between the central plies. culated with respect to the initial Bragg wavelength of the sensors embedded. Data obtained for all specimens are illustrated in Figs. 7, 8 and 9 with the relative estimation of the calibration curve and sensitivity. Using the calculated calibration factor, the residual strains were also estimated by comparison with the nominal Bragg wavelength of the FBG sensors, and are reported on Table 2. As no appreciable distortion of the Bragg spectrum takes place, it can be assumed that residual strains are mainly unidirectional along the sensor axis. It can be seen that in most cases the gage factor in the surface mounted FBG and SG sensors are quite close to the theoretical value for silica fibers (0.78) in both cases (acrylate and polyimide), suggesting that the resin, in the FBG sensor, is able to allow a good strain transfer from the material to the fiber. However, values range from 0.67 to around 0.79, which shows that deviations may not be negligible in some cases and that therefore an in-situ calibration becomes absolutely necessary when sensors are embedded. It was observed that no significant hysteresis or sliding effects of the fiber sensors took place, which is useful information for the practical use of the sensors in composite structures that are subjected to varying positive and negative loads during operation. 5. CONCLUSIONS Technical and measurement aspects related to the embedding process of acrylate- and polyamide-coated FBG sensors in composite structures were investigated in this paper. We found that acrylate-coated fibers can be processed in autoclave at the pressure of 3 bar but only to a maximum of 90 temperature, while polyimide-coated fibers are able to withstand temperatures over 120 C. Particular attention must be given to protect the fibers in the ingress-egress points of the fibers, and small Sterling tubes or special Teflon sheets are sufficient to reduce damage to the fiber by the resin flow and to prevent breaking. Optical losses in the cured specimens were measured through an OTDR to evaluate light attenuation in the sensors and in the embedded fiber sections due to the manufacturing process in autoclave. The estimated values were in all cases acceptable with respect to the optimization of the signal-to-noise ratio in the case of one or more (array) FBG sensors and the monitoring of long composite structures. The Bragg spectrum of the embedded sensors was also measured and showed no significant distortion and therefore that the alignment of the optical fibers to the reinforcing fibers in a unidirectional composite and the use of suitable coatings are effective to this aim. Comparison with traditional SGs allowed us to perform an in-situ calibration of the embedded sensors and to verify the strain transfer capability of the resin used in the composite manufacturing process. We found that the actual sensitivity values of the embedded sensors may differ significantly from the theoretical value for silica fibers, and therefore, in-situ calibration is recommended. Based on the estimated calibration factors and on the fact that the spectra of the sensors are not severely distorted, it was also possible to evaluate the residual strains inside the material after the autoclave processing. ACKNOWLEDGMENTS The authors gratefully acknowledge the collaboration of the companies RiBa Composites and SestoSensor of Dr Filippo Bastianini and the precious support of Dr Ivan Meneghin for the manufacturing of the composite specimens. REFERENCES 1. R. de Oliveira, C. A. Ramos, and A. T. Marques, Comput. Struct. 86, 340 (2008). 2. R. Di Sante and L. Scalise, Proc. SPIE - The International Society for Optical Engineering Vol. 4204, p.115, Newport (2001). 3. R. Di Sante and L. Donati, Measurement 46, 2118 (2013). 4. R. Di Sante and L.Scalise, Rev. Sci. Instrum. 75, 1952 (2004).

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