Development and Characterization of Fibre Bragg Grating Sensor Packaging for Aircraft Structural Health Monitoring Applications

Development and Characterization of Fibre Bragg Grating Sensor Packaging for Aircraft Structural Health Monitoring Applications | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Development and Characterization of Fibre Bragg Grating Sensor Packaging for Aircraft Structural Health Monitoring Applications M. J. Augustin, Kundan K. Verma, Saransh Jain, Pooja M. G, Vijay Kumar, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7319127/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 13 You are reading this latest preprint version Abstract The use of fibre optic sensors for structural health monitoring has been persuaded by the aircraft industry and research organizations for more than three decades. The Fiber Bragg Grating (FBG) sensors because of their inherent advantages are considered the best choice for the aircraft industry for structural health monitoring applications. However, these sensors are fragile in nature and are still being used in lab-scale experiments. To make the sensor robust without altering its sensitivity a methodology is developed to package the sensors using a glass-carbon/epoxy composite. Different arrangements are established for the protection of the sensing region and the non-sensing region of the sensor. Sensor packaging is carried out in such a way that the sensor quality and its strain transfer capability are unaltered. Based on spectrum comparison and ease of installation, the sensing region of FBG is packaged between composite layers, and the non-sensing region is protected using Teflon and other flexible tubes. The temperature and strain response of the sensors are studied experimentally and compared with bare FBG sensor and resistance strain gauge (RSG) respectively. These studies showed that packaging has the same response to strain and temperature and is very reliable. Composite Fiber Bragg Grating Optical sensor Strain Temperature Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 1. Introduction and background Various sensing technologies are being offered for Structural Health Monitoring (SHM) for aircraft structures. Amongst them, FBG sensors are considered to be the best due to their inherent advantages like light in weight, small in-size, immunity to electromagnetic interference, ability to multiplex, and low sensor lead out [ 1 ]. Over the last few decades, a lot of research has been carried out using the fibre optic sensors especially the FBG for the SHM of aircraft structures and in other areas. These studies demonstrated the use of the FBG sensors for strain monitoring [ 2 ], damage detection [ 3 – 5 ], load monitoring [ 5 – 7 ], and shape sensing applications [ 8 – 10 ]. In addition to this, there are airworthy interrogators available that can meet the requirements of strain measurements during flight endeavors. Though there are sensors and interrogators available, fiber optic sensor technology has not penetrated much into commercial aircraft primarily because of the fragile nature of the fiber. The fragility of optical sensors makes them difficult to handle during sensor installation, assembly, and end uses. Researchers are looking into this aspect and have recommended different techniques for packaging the FBG sensors. Most of the techniques integrated sensors in composite structures in different ways. The most common ones are the embedment of the sensor within the composite layers [ 11 – 13 ] and the bonding of sensors on the surface [ 14 – 17 ]. The advantage of embedding the sensor within the composite layers is robust and sensors are safe during the handling of composite. However, ingress/egress is a major issue in the embedding technique. Post-manufacturing operation (edge machining, assembly drilling, etc.) is also an issue in case sensor fiber length is left unprotected. Ingress/egress issues have been addressed by a couple of researchers and they have demonstrated their embedment technique [ 18 – 20 ]. However, this technique hinders the flexibility of placing the sensors at the desired location. Further, the anticipated ingress/egress location may not be suitable for the manufacturing and assembly process (21). Surface bonding of sensors are generally carried out after assembly of structures to avoid damage of sensors during different assembly operations. This gives more flexibility for selection of sensor location, cable routing etc. Though, this is more advantageous, this technique has issues like projection of sensors on outer surface, handling of surface during other assembly and maintenance. Further, bonding of bare FBG sensors has issues like breaking of FBG sensors especially at grating locations due to high fragility. The FBG sensors are circular in nature and bonded with low amount of adhesive and sometimes disbond occurs at the bond-line. In case the sensor and its cable are unprotected sometimes it gets damage during handling and final assembly. The bonding schemes should make the strain transfer proper, make the handling easy, providing protection during the installation and service life. Considering the complex shape of the aircraft structures the sensor packaging must be reasonably flexible and repeatable in performance [ 22 ]. The bonding also has to withstand operation and handling loads [ 23 ]. Considering strain transfer efficiency, and protection of the sensing and non-sensing region (sensor cable) of the fiber, different sensor packaging and protection techniques are reported for composite structures. These approaches are the packaging of sensors and protecting the sensors with suitable enclosures. Most of this packaging is done by embedding of sensors in a metal casing and a few suggest to embedding in thin composites. Few commercial sensor packages [ 24 – 25 ] are available for composite structures, however, their size and outer tube diameters are bigger which cannot be used in complex composite structures. This further has the issue of routing of sensors in space-restricted regions. To avoid breaking of the optical fiber during operation and discriminate strain and temperature epsilon optics have encapsulated the sensors and its cable with glass fiber and high-density polyethylene (HDPE). They claim to provide efficient strength and appropriate strain transfer. However, due to the increase in stiffness due to continuous glass fiber HDPE composite, routing especially coiling of sensor cable becomes difficult. A ribbon-based packaging approach is developed [ 26 ] where the FBG sensing region and the sensor cable regions are placed in flexible tubes between prepreg layers. However, uses of this technique are limited to a planar structure like skin stiffener panels. Aircraft structures like wings, horizontal tails etc., which are box structures can have greater difficulty in above-said sensor packages. The above-mentioned issues can be resolved if proper installation methodology is worked out with proper bonding of sensors and its cables, routing scheme etc. This minimizes the risk of damaging of sensors during the final assembly of structures. This method also eases the certification process and acceptance. To develop the surface bonded packaged sensors, a novel technique is developed for sensor packaging which can be used for SHM of aircraft applications. In this packaging the FBG sensors are encapsulated with unidirectional (UD) carbon fiber followed by glass fabric using 5052 epoxy resin system. The sensor cable region is protected by Teflon and other tubing. This makes sensor cable very flexible and routing within structures becomes easy. The developed sensor packages can be bonded using suitable adhesives depending on the environmental condition. In this research, Hysol EA 934NA adhesive from M/s Loctite is used for bonding the sensor package. In order to demonstrate the strain transfer capability, reliability of the packaging technique and bonding methodology the packaged sensor is bonded on composite specimen and subjected to static and fatigue loading. An RSG is bonded to compare the strains during testing and comparison of test results. The temperature response of packaged sensor is compared with bare FBG sensors subjected to different temperature range. 2. Development of sensor package The major objectives for the design and development of the sensor package that is kept in consideration are (a) protection of sensing region and cable region of optical sensors from breaking during installation and further use, (b) retaining of optical characteristics of FBG sensor, (c) enough flexibility of sensor cable for installation on complex composite structures and (d) reusability of packaged sensors. Different schemes are explored to package the sensors based on their application. However, the common scheme in all the different methods, packaging of cable region of sensors remains the same. The cable region is covered with a Teflon tube and heat shrink tube. A thin layer of room-temperature curable silicon paste is applied on the tip of the cable region of sensor. The silicon paste further helps in preventing of epoxy resin filling in the tube and breaking of sensor cable. A Teflon tube is then inserted and placed carefully without braking of sensor. Another issue of these kind of sensor is breaking of cable near connector. This area is reinforced with heat shrink tubes. A heat shrink tube of nearly 0.5mm is placed from connector and terminated 10mm before sensor region. Further, another heat shrink tube of 1.5mm is placed covering partially on the connector boot and previously heat shrink tube. The schematic of placement of tubes is shown in Fig. 1 . Packaging of sensors are carried out by encapsulating the sensor between composite layers. A schematic of encapsulation scheme is depicted in Fig. 2 . Initially the complete sensors (sensing region and cable region) is encapsulated between two glass fabric layers of 0.18mm thickness. This is carried out for better strength with enough flexibility which intern helps in installation on composite structure. However, an unacceptable reduction of power level (Fig. 3.a) is observed after packaging therefore this scheme is forbidden. The reduction in power is attributed mainly due to microbending of sensing region, a consequence effect of weaving architecture of glass fabric. In another trial a thin layer of glass fabric of 0.1mm thickness is used to cover the sensing region followed by 0.18mm thickness of glass fabric. In this case the loss is diminish to an acceptable limit (Fig. 3.b). However, high flexibility of design leads to breakage of sensor package frequently. In order to overcome the fragility and add some stiffness near the sensor, it is opined that a carbon unidirectional (UD) layer to be used. This would also help in complete elimination of sensor microbending and thereby eliminating the optical power loss. In the third trial sensing region is encapsulated between carbon UD fabric of 0.17mm thick followed by 0.18mm thick glass fabric. The carbon layer is used just to cover the sensing region along with tubing region of around 10mm. This would prevent slippage of tube within composite layers. In this the power loss was very minimum and power level remain almost equal in comparison to before packaging and after packaging (Fig. 3.c). In order to check the repeatability and power loss few more packaging is carried out in same fashion and it is found that the response of sensors packages is same with almost no power loss. slight shift in wavelength is observed in all the scheme of packaging, this is attributed to the vacuum pressure applied during solidification of epoxy resin system. This vacuum pressure has elongated FBG sensor that result in very minor shift in wavelength. 3. Experimental method 3.1. Characterization of packaged sensors A single FBG sensor from M/s Avensys Inc ITF Labs, Canada with wavelength of 1535nm is used for the experiment. Figure 4 shows the packaged sensors developed with fiber connector angle polished contact (FCAPC) type end connector. A glass/epoxy composite specimen of 220mm length, 25mm wide and 12.5mm thick specimen is used for mechanical characterization of packaged sensor. The packaged sensor is bonded to a composite specimen. Alongside of FBG sensor an RSG is also bonded for comparison of the test results. This is mainly to ascertain the functionality of packaged sensor. Figure 5 depicts the specimen with bonded packaged sensor and RSG. During service life of aircraft, it experiences subzero temperatures. In order to determine the compensation factor for FBG sensor, which is an important factor to convert the wavelength shift into a meaningful strain, FBG sensor is subjected to subzero temperature. Packaged sensor is first characterized in order to estimate the compensation factor and to check the functionality of packaging. Specimen bonded with packaged sensor is kept in refrigerator (Fig. 6 ) where it is subjected to different temperature range to ascertain the compensation factor and functionality of package. A bare FBG sensor is also kept along with specimen in order to measure the wavelength shift due to temperature change in non-packaged form. A callibrated thermocouple is attached with specimen in order to analyze the response of sensors with temperature. Subsequent to subzero temperature calibration, specimen is also calibrated up to a temperature of 30°C. Further, same specimen is characterized for its mechanical performance. The mechanical characterization is mainly carried out to study parameters like strain response to static loading. Further, specimen is subjected to fatigue loading, wherein the longevity of bonding capability is ascertained. Figure 7 shows the specimen mounted in a universal testing machine for static and fatigue loading test. 4. Results and discussion 4.1. Temperature effect on packaged sensor and determination of compensation factor Temperature response of packaged sensor is studied from + 25°C to -18°C temperature range with understanding that the packaged would function properly in a normal environmental temperature. Packaged sensor bonded on the composite specimen is subjected to different temperature. FBG sensors are connected to FBG interrogator and thermocouple is connected the CA150 calibrator cum date indicator. For FBG, a baseline wavelength is recorded before switching on the refrigerator. Wavelength along with temperature against time is recorded simultaneously at definite temperature after stabilizing the temperature. Wavelength change for bare FBG and packaged FBG sensor is plotted against the temperature and depicted in Fig. 8 . There is change in behavior of wavelength shift for bare FBG and packaged FBG sensor. However, the change is very marginal and within acceptable limit. This change is attributed to response of FBG sensor encapsulated and bonded on composite specimen. This experiment also helps in determining the compensation factor using bare FBG sensors in case where range of packaged FBG sensors are used for aircraft structure. 4.2. Mechanical characterization for static loading Based on the operational strain experienced by the spar (wing component) of a trainer aircraft, specimen is subjected to the loading range of -45kN to + 45kN in steps of 5kN. Table 1 shows the steps followed for static loading. The complete load makes it to one cycle of loading. The base line for FBG and RSG is taken at ‘zero’ load condition. Strain are measured in steps of 5kN. Strain response from FBG sensor and RSG is compared to see if there are any anomalies in the test results. Figure 9 shows the load vs strain response from FBG and RSG. The strain response of FBG is in good agreement with RSG strain. RSG data shows good repeatability and strain during loading and unloading is almost same and thereby no deviation. However, in case of FBG strain the path for loading and unloading is not same. Though, the variation is negligible small change is path could be attributed to residual strain in the encapsulation material and bonding adhesives. Table 1 Loading sequence for static test condition Loading steps Loads 1 0 kN to + 45 kN 2 + 45 kN to 0 kN 3 0 kN to -45 kN 4 -45 kN to 0 kN 4.3. Mechanical characterization for fatigue loading Subsequent to completion of static characterization of specimen, same specimen is subjected to fatigue loading with R = -1 spectrum with the same loading range of -45kN to + 45kn. A proper gripping pressure for gripping of specimen is chosen based on the experience to prevent specimen crushing during testing and withstanding the repeated cycle for the required load spectrum. Specimen is loaded with frequency of 0.5Hz. Specimen is subjected to ten thousand cycles. FBG strain is measured continuously and recorded. Strains are also recorded from RSG sensor but at an interval of approximately one thousand cycles for 20 cycles. After ten thousand cycles, no visible damage is seen in the test specimen. The FBG and RSG is found intact with the specimen without any interface disbond. Strain response from FBG sensor and RSG sensor is plotted at different interval of time of about 20 cycles and shown in Fig. 10 and Fig. 11 . A close view of one complete cycle is plotted and shown in Fig. 12 to access the variation in the strain. The maximum variation at highest load is around 10% which is very minimal and could be attributed to experimental error band and residual strain of encapsulating material and bonding adhesive. Further, the strain response at various load levels for each cycle is identical and reliable for both FBG and RSG sensor. 5. Conclusion Different scheme for FBG sensor packaging is worked out and encapsulating the FBG using 2 layers of carbon UD fabric followed by 2 layers of glass fabric is found to be most suitable. The sensor cable is protected using Teflon and heat shrink tube of different diameter. The compensation factor and functionality of the sensor packaging is determined by subjecting the test specimens at different test temperature. There was no evidence of sensor interface disbond for both FBG and RSG. Strain response from static testing showed very minor difference between RSG and FBG. Further, strain response from fatigue loading also showed very minor difference of maximum of 10% (~ 150µε) at maximum load (+ 45kN & -45kN). This difference could be attributed to band of experimental error. The concept of FNG sensor packaging is established and can be surface bonded for any composite structures at the desired location. Declarations ‘Ethics, Consent to Participate, and Consent to Publish declarations: not applicable. Author Contribution Augustin M.J. was responsible for developing the sensor packaging and experimental design. He also wrote the first draft of the manuscript.Kundan K. Verma handled the design and fabrication, package development, and graph plotting. He contributed to the initial proofreading and corrections.Saransh Jain served as the point of contact for the Department of Science and Technology (DST) in this project. He was responsible for structural testing, the test schedule, fabrication, and data interpretation.Pooja M.G. was the Project Graduate Trainee for this project, responsible for data acquisition, analysis, and developing codes for data analysis.Nitesh Gupta managed the overall project. Acknowledgement The authors are indebted to the DST, Govt. of India, for funding the research (Grant Ref.: SERB/F/11922/2018-2019) under Imprint-2 program. The author thanks their colleagues for support from CSIR-NAL, Bengaluru, for this research work. Data Availability The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request. References Ahmed O, Wang X, Tran M-V, Ismadi M-Z. Advancements in fiber-reinforced polymer composite materials damage detection methods: Towards achieving energy-efficient SHM systems. Composites Part B: Engineering 2021;223:109136. Antonucci V, Esposito M, Ricciardi MR, Giordano M, Zarrelli M. Strain monitoring of composite elements by fibre Bragg grating sensors during a quasi-static indentation. Composites Part B: Engineering 2014;56:34-41. Datta A, Augustin MJ, Gupta N, et al. Strain sensor’s network for low-velocity impact location estimation on carbon reinforced fiber plastic structures: Part-II. Indian J Pure Appl Phys 2021; 59(10): 678–686.. A. Datta, M. J. Augustin, N. Gupta, S. R. Viswamurthy, K. M. Gaddikeri and R. 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Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Revision requested 22 Sep, 2025 Reviews received at journal 16 Sep, 2025 Reviews received at journal 08 Sep, 2025 Reviews received at journal 05 Sep, 2025 Reviews received at journal 28 Aug, 2025 Reviewers agreed at journal 28 Aug, 2025 Reviewers agreed at journal 25 Aug, 2025 Reviewers agreed at journal 25 Aug, 2025 Reviewers agreed at journal 25 Aug, 2025 Reviewers invited by journal 25 Aug, 2025 Editor assigned by journal 20 Aug, 2025 Submission checks completed at journal 20 Aug, 2025 First submitted to journal 20 Aug, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-7319127","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":506894699,"identity":"7b76bebc-55ce-4eba-a557-ad3724b52a2b","order_by":0,"name":"M. J. 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encapsulation of sensing region with composite layers\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7319127/v1/1ffa62af0baa34dfd86e102a.png"},{"id":90400822,"identity":"f2740947-fc6f-41a8-b5ba-db920df81d0e","added_by":"auto","created_at":"2025-09-02 10:17:15","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":50246,"visible":true,"origin":"","legend":"\u003cp\u003ePower loss measurement in different schemes of packaging, a) sensor encapsulated between 0.18mm glass fabric, b) sensor encapsulated between 0.1mm glass fabric and c) sensor encapsulated between 0.17mm carbon UD layer\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7319127/v1/5a5c88d8e2a67917687eb395.png"},{"id":90400826,"identity":"1c4965b1-5f25-4256-a01d-6030ebf0497b","added_by":"auto","created_at":"2025-09-02 10:17:15","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":254339,"visible":true,"origin":"","legend":"\u003cp\u003ePackaged sensor with single FBG sensor\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-7319127/v1/95e56939a6e76213e40c5f70.png"},{"id":90402158,"identity":"a0e040f7-93cd-42f1-aef5-1d28771d02e3","added_by":"auto","created_at":"2025-09-02 10:25:15","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":321841,"visible":true,"origin":"","legend":"\u003cp\u003eComposite specimens bonded with packaged sensor and RSG temperature and mechanical characterization\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-7319127/v1/eb0b654b4e3dfd649039181d.png"},{"id":90400829,"identity":"3234d331-5b84-41ed-8b0a-a0a738524cfa","added_by":"auto","created_at":"2025-09-02 10:17:15","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":398077,"visible":true,"origin":"","legend":"\u003cp\u003eExperiemnt setup in refrigerator for thermal study\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-7319127/v1/5eac967df9179416ab228ad4.png"},{"id":90400838,"identity":"d5d9e24b-1806-42ca-9f7d-7cffba5b8c19","added_by":"auto","created_at":"2025-09-02 10:17:15","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":916031,"visible":true,"origin":"","legend":"\u003cp\u003eSpecimen mounted on UTM, a) testing apparatus and b) close view of specimen\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-7319127/v1/a974359ba86169fb292ac7cc.png"},{"id":90400824,"identity":"579947c8-75a7-418d-aefc-b9d54d5d10db","added_by":"auto","created_at":"2025-09-02 10:17:15","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":42718,"visible":true,"origin":"","legend":"\u003cp\u003eWavelength change of bare FBG and packaged FBG sensor against temperature\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-7319127/v1/04b6e58c48a98f5443ad99f5.png"},{"id":90400849,"identity":"6be3de91-8c72-4bc6-b652-2e6c553c0adc","added_by":"auto","created_at":"2025-09-02 10:17:16","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":38100,"visible":true,"origin":"","legend":"\u003cp\u003eStrain response from both the RSG and FBG sensors for static test condition\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-7319127/v1/3576835e22bc879ac4ddf64f.png"},{"id":90402160,"identity":"e082c699-6515-4eaa-81cc-2df4a0233138","added_by":"auto","created_at":"2025-09-02 10:25:15","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":289037,"visible":true,"origin":"","legend":"\u003cp\u003eStrain response of RSG at different interval of about 20 cycles\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-7319127/v1/df6218370b55fb790455b384.png"},{"id":90400854,"identity":"b730886c-fb37-4017-a290-e1e6a6a4705c","added_by":"auto","created_at":"2025-09-02 10:17:16","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":143130,"visible":true,"origin":"","legend":"\u003cp\u003eStrain response of FBG at different interval of about 20 cycles\u003c/p\u003e","description":"","filename":"11.png","url":"https://assets-eu.researchsquare.com/files/rs-7319127/v1/c95998b065562a99206d18f9.png"},{"id":90402164,"identity":"a0aa9f5e-9897-45e6-a9ae-218017989894","added_by":"auto","created_at":"2025-09-02 10:25:15","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":37090,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of strain response of RSG and FBG for one cycle\u003c/p\u003e","description":"","filename":"12.png","url":"https://assets-eu.researchsquare.com/files/rs-7319127/v1/801b5f8b1afa47d9c2d5f752.png"},{"id":90403739,"identity":"b16f10bb-9380-4dd8-93e6-4752a6c6e5b6","added_by":"auto","created_at":"2025-09-02 10:49:17","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3620753,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7319127/v1/90ef297d-03b1-4e7a-b037-99845bde840c.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Development and Characterization of Fibre Bragg Grating Sensor Packaging for Aircraft Structural Health Monitoring Applications","fulltext":[{"header":"1. Introduction and background","content":"\u003cp\u003eVarious sensing technologies are being offered for Structural Health Monitoring (SHM) for aircraft structures. Amongst them, FBG sensors are considered to be the best due to their inherent advantages like light in weight, small in-size, immunity to electromagnetic interference, ability to multiplex, and low sensor lead out [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. Over the last few decades, a lot of research has been carried out using the fibre optic sensors especially the FBG for the SHM of aircraft structures and in other areas. These studies demonstrated the use of the FBG sensors for strain monitoring [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e], damage detection [\u003cspan additionalcitationids=\"CR4\" citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e], load monitoring [\u003cspan additionalcitationids=\"CR6\" citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e], and shape sensing applications [\u003cspan additionalcitationids=\"CR9\" citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. In addition to this, there are airworthy interrogators available that can meet the requirements of strain measurements during flight endeavors. Though there are sensors and interrogators available, fiber optic sensor technology has not penetrated much into commercial aircraft primarily because of the fragile nature of the fiber. The fragility of optical sensors makes them difficult to handle during sensor installation, assembly, and end uses. Researchers are looking into this aspect and have recommended different techniques for packaging the FBG sensors. Most of the techniques integrated sensors in composite structures in different ways. The most common ones are the embedment of the sensor within the composite layers [\u003cspan additionalcitationids=\"CR12\" citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e] and the bonding of sensors on the surface [\u003cspan additionalcitationids=\"CR15 CR16\" citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. The advantage of embedding the sensor within the composite layers is robust and sensors are safe during the handling of composite. However, ingress/egress is a major issue in the embedding technique. Post-manufacturing operation (edge machining, assembly drilling, etc.) is also an issue in case sensor fiber length is left unprotected. Ingress/egress issues have been addressed by a couple of researchers and they have demonstrated their embedment technique [\u003cspan additionalcitationids=\"CR19\" citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. However, this technique hinders the flexibility of placing the sensors at the desired location. Further, the anticipated ingress/egress location may not be suitable for the manufacturing and assembly process (21).\u003c/p\u003e\u003cp\u003eSurface bonding of sensors are generally carried out after assembly of structures to avoid damage of sensors during different assembly operations. This gives more flexibility for selection of sensor location, cable routing etc. Though, this is more advantageous, this technique has issues like projection of sensors on outer surface, handling of surface during other assembly and maintenance.\u003c/p\u003e\u003cp\u003eFurther, bonding of bare FBG sensors has issues like breaking of FBG sensors especially at grating locations due to high fragility. The FBG sensors are circular in nature and bonded with low amount of adhesive and sometimes disbond occurs at the bond-line. In case the sensor and its cable are unprotected sometimes it gets damage during handling and final assembly. The bonding schemes should make the strain transfer proper, make the handling easy, providing protection during the installation and service life. Considering the complex shape of the aircraft structures the sensor packaging must be reasonably flexible and repeatable in performance [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. The bonding also has to withstand operation and handling loads [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eConsidering strain transfer efficiency, and protection of the sensing and non-sensing region (sensor cable) of the fiber, different sensor packaging and protection techniques are reported for composite structures. These approaches are the packaging of sensors and protecting the sensors with suitable enclosures. Most of this packaging is done by embedding of sensors in a metal casing and a few suggest to embedding in thin composites. Few commercial sensor packages [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e] are available for composite structures, however, their size and outer tube diameters are bigger which cannot be used in complex composite structures. This further has the issue of routing of sensors in space-restricted regions. To avoid breaking of the optical fiber during operation and discriminate strain and temperature epsilon optics have encapsulated the sensors and its cable with glass fiber and high-density polyethylene (HDPE). They claim to provide efficient strength and appropriate strain transfer. However, due to the increase in stiffness due to continuous glass fiber HDPE composite, routing especially coiling of sensor cable becomes difficult. A ribbon-based packaging approach is developed [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e] where the FBG sensing region and the sensor cable regions are placed in flexible tubes between prepreg layers. However, uses of this technique are limited to a planar structure like skin stiffener panels. Aircraft structures like wings, horizontal tails etc., which are box structures can have greater difficulty in above-said sensor packages.\u003c/p\u003e\u003cp\u003eThe above-mentioned issues can be resolved if proper installation methodology is worked out with proper bonding of sensors and its cables, routing scheme etc. This minimizes the risk of damaging of sensors during the final assembly of structures. This method also eases the certification process and acceptance. To develop the surface bonded packaged sensors, a novel technique is developed for sensor packaging which can be used for SHM of aircraft applications. In this packaging the FBG sensors are encapsulated with unidirectional (UD) carbon fiber followed by glass fabric using 5052 epoxy resin system. The sensor cable region is protected by Teflon and other tubing. This makes sensor cable very flexible and routing within structures becomes easy. The developed sensor packages can be bonded using suitable adhesives depending on the environmental condition. In this research, Hysol EA 934NA adhesive from M/s Loctite is used for bonding the sensor package. In order to demonstrate the strain transfer capability, reliability of the packaging technique and bonding methodology the packaged sensor is bonded on composite specimen and subjected to static and fatigue loading. An RSG is bonded to compare the strains during testing and comparison of test results. The temperature response of packaged sensor is compared with bare FBG sensors subjected to different temperature range.\u003c/p\u003e"},{"header":"2. Development of sensor package","content":"\u003cp\u003eThe major objectives for the design and development of the sensor package that is kept in consideration are (a) protection of sensing region and cable region of optical sensors from breaking during installation and further use, (b) retaining of optical characteristics of FBG sensor, (c) enough flexibility of sensor cable for installation on complex composite structures and (d) reusability of packaged sensors. Different schemes are explored to package the sensors based on their application. However, the common scheme in all the different methods, packaging of cable region of sensors remains the same. The cable region is covered with a Teflon tube and heat shrink tube. A thin layer of room-temperature curable silicon paste is applied on the tip of the cable region of sensor. The silicon paste further helps in preventing of epoxy resin filling in the tube and breaking of sensor cable. A Teflon tube is then inserted and placed carefully without braking of sensor. Another issue of these kind of sensor is breaking of cable near connector. This area is reinforced with heat shrink tubes. A heat shrink tube of nearly 0.5mm is placed from connector and terminated 10mm before sensor region. Further, another heat shrink tube of 1.5mm is placed covering partially on the connector boot and previously heat shrink tube. The schematic of placement of tubes is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003ePackaging of sensors are carried out by encapsulating the sensor between composite layers. A schematic of encapsulation scheme is depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. Initially the complete sensors (sensing region and cable region) is encapsulated between two glass fabric layers of 0.18mm thickness. This is carried out for better strength with enough flexibility which intern helps in installation on composite structure. However, an unacceptable reduction of power level (Fig.\u0026nbsp;3.a) is observed after packaging therefore this scheme is forbidden. The reduction in power is attributed mainly due to microbending of sensing region, a consequence effect of weaving architecture of glass fabric. In another trial a thin layer of glass fabric of 0.1mm thickness is used to cover the sensing region followed by 0.18mm thickness of glass fabric. In this case the loss is diminish to an acceptable limit (Fig.\u0026nbsp;3.b). However, high flexibility of design leads to breakage of sensor package frequently. In order to overcome the fragility and add some stiffness near the sensor, it is opined that a carbon unidirectional (UD) layer to be used. This would also help in complete elimination of sensor microbending and thereby eliminating the optical power loss. In the third trial sensing region is encapsulated between carbon UD fabric of 0.17mm thick followed by 0.18mm thick glass fabric. The carbon layer is used just to cover the sensing region along with tubing region of around 10mm. This would prevent slippage of tube within composite layers. In this the power loss was very minimum and power level remain almost equal in comparison to before packaging and after packaging (Fig.\u0026nbsp;3.c). In order to check the repeatability and power loss few more packaging is carried out in same fashion and it is found that the response of sensors packages is same with almost no power loss. slight shift in wavelength is observed in all the scheme of packaging, this is attributed to the vacuum pressure applied during solidification of epoxy resin system. This vacuum pressure has elongated FBG sensor that result in very minor shift in wavelength.\u003c/p\u003e"},{"header":"3. Experimental method","content":"\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e3.1. Characterization of packaged sensors\u003c/h2\u003e\u003cp\u003eA single FBG sensor from M/s Avensys Inc ITF Labs, Canada with wavelength of 1535nm is used for the experiment. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e shows the packaged sensors developed with fiber connector angle polished contact (FCAPC) type end connector. A glass/epoxy composite specimen of 220mm length, 25mm wide and 12.5mm thick specimen is used for mechanical characterization of packaged sensor. The packaged sensor is bonded to a composite specimen. Alongside of FBG sensor an RSG is also bonded for comparison of the test results. This is mainly to ascertain the functionality of packaged sensor. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003e depicts the specimen with bonded packaged sensor and RSG.\u003c/p\u003e\u003cp\u003eDuring service life of aircraft, it experiences subzero temperatures. In order to determine the compensation factor for FBG sensor, which is an important factor to convert the wavelength shift into a meaningful strain, FBG sensor is subjected to subzero temperature. Packaged sensor is first characterized in order to estimate the compensation factor and to check the functionality of packaging. Specimen bonded with packaged sensor is kept in refrigerator (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e6\u003c/span\u003e) where it is subjected to different temperature range to ascertain the compensation factor and functionality of package. A bare FBG sensor is also kept along with specimen in order to measure the wavelength shift due to temperature change in non-packaged form. A callibrated thermocouple is attached with specimen in order to analyze the response of sensors with temperature. Subsequent to subzero temperature calibration, specimen is also calibrated up to a temperature of 30\u0026deg;C. Further, same specimen is characterized for its mechanical performance. The mechanical characterization is mainly carried out to study parameters like strain response to static loading. Further, specimen is subjected to fatigue loading, wherein the longevity of bonding capability is ascertained. Figure\u0026nbsp;7 shows the specimen mounted in a universal testing machine for static and fatigue loading test.\u003c/p\u003e"},{"header":"4. Results and discussion","content":"\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e4.1. Temperature effect on packaged sensor and determination of compensation factor\u003c/h2\u003e\u003cp\u003eTemperature response of packaged sensor is studied from +\u0026thinsp;25\u0026deg;C to -18\u0026deg;C temperature range with understanding that the packaged would function properly in a normal environmental temperature. Packaged sensor bonded on the composite specimen is subjected to different temperature. FBG sensors are connected to FBG interrogator and thermocouple is connected the CA150 calibrator cum date indicator. For FBG, a baseline wavelength is recorded before switching on the refrigerator. Wavelength along with temperature against time is recorded simultaneously at definite temperature after stabilizing the temperature. Wavelength change for bare FBG and packaged FBG sensor is plotted against the temperature and depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e8\u003c/span\u003e. There is change in behavior of wavelength shift for bare FBG and packaged FBG sensor. However, the change is very marginal and within acceptable limit. This change is attributed to response of FBG sensor encapsulated and bonded on composite specimen. This experiment also helps in determining the compensation factor using bare FBG sensors in case where range of packaged FBG sensors are used for aircraft structure.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\u003ch2\u003e4.2. Mechanical characterization for static loading\u003c/h2\u003e\u003cp\u003eBased on the operational strain experienced by the spar (wing component) of a trainer aircraft, specimen is subjected to the loading range of -45kN to +\u0026thinsp;45kN in steps of 5kN. Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows the steps followed for static loading. The complete load makes it to one cycle of loading. The base line for FBG and RSG is taken at \u0026lsquo;zero\u0026rsquo; load condition. Strain are measured in steps of 5kN. Strain response from FBG sensor and RSG is compared to see if there are any anomalies in the test results. Figure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e9\u003c/span\u003e shows the load vs strain response from FBG and RSG. The strain response of FBG is in good agreement with RSG strain. RSG data shows good repeatability and strain during loading and unloading is almost same and thereby no deviation. However, in case of FBG strain the path for loading and unloading is not same. Though, the variation is negligible small change is path could be attributed to residual strain in the encapsulation material and bonding adhesives.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eLoading sequence for static test condition\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"2\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eLoading steps\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eLoads\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0 kN to +\u0026thinsp;45 kN\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e+\u0026thinsp;45 kN to 0 kN\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0 kN to -45 kN\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e-45 kN to 0 kN\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003e4.3. Mechanical characterization for fatigue loading\u003c/h2\u003e\u003cp\u003eSubsequent to completion of static characterization of specimen, same specimen is subjected to fatigue loading with R = -1 spectrum with the same loading range of -45kN to +\u0026thinsp;45kn. A proper gripping pressure for gripping of specimen is chosen based on the experience to prevent specimen crushing during testing and withstanding the repeated cycle for the required load spectrum. Specimen is loaded with frequency of 0.5Hz. Specimen is subjected to ten thousand cycles. FBG strain is measured continuously and recorded. Strains are also recorded from RSG sensor but at an interval of approximately one thousand cycles for 20 cycles. After ten thousand cycles, no visible damage is seen in the test specimen. The FBG and RSG is found intact with the specimen without any interface disbond. Strain response from FBG sensor and RSG sensor is plotted at different interval of time of about 20 cycles and shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e10\u003c/span\u003e and Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e11\u003c/span\u003e. A close view of one complete cycle is plotted and shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e12\u003c/span\u003e to access the variation in the strain. The maximum variation at highest load is around 10% which is very minimal and could be attributed to experimental error band and residual strain of encapsulating material and bonding adhesive. Further, the strain response at various load levels for each cycle is identical and reliable for both FBG and RSG sensor.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"5. Conclusion","content":"\u003cp\u003eDifferent scheme for FBG sensor packaging is worked out and encapsulating the FBG using 2 layers of carbon UD fabric followed by 2 layers of glass fabric is found to be most suitable. The sensor cable is protected using Teflon and heat shrink tube of different diameter. The compensation factor and functionality of the sensor packaging is determined by subjecting the test specimens at different test temperature. There was no evidence of sensor interface disbond for both FBG and RSG. Strain response from static testing showed very minor difference between RSG and FBG. Further, strain response from fatigue loading also showed very minor difference of maximum of 10% (~\u0026thinsp;150\u0026micro;ε) at maximum load (+\u0026thinsp;45kN \u0026amp; -45kN). This difference could be attributed to band of experimental error. The concept of FNG sensor packaging is established and can be surface bonded for any composite structures at the desired location.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u0026lsquo;Ethics, Consent to Participate, and Consent to Publish declarations: not applicable.\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eAugustin M.J. was responsible for developing the sensor packaging and experimental design. He also wrote the first draft of the manuscript.Kundan K. Verma handled the design and fabrication, package development, and graph plotting. He contributed to the initial proofreading and corrections.Saransh Jain served as the point of contact for the Department of Science and Technology (DST) in this project. He was responsible for structural testing, the test schedule, fabrication, and data interpretation.Pooja M.G. was the Project Graduate Trainee for this project, responsible for data acquisition, analysis, and developing codes for data analysis.Nitesh Gupta managed the overall project.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eThe authors are indebted to the DST, Govt. of India, for funding the research (Grant Ref.: SERB/F/11922/2018-2019) under Imprint-2 program. The author thanks their colleagues for support from CSIR-NAL, Bengaluru, for this research work.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eThe datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eAhmed O, Wang X, Tran M-V, Ismadi M-Z. 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Optical Engineering 2000;39:3068-75.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"discover-sensors","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"Learn more about [Discover Sensors](https://link.springer.com/journal/44397)","snPcode":"44397","submissionUrl":"https://submission.nature.com/new-submission/44397/3","title":"Discover Sensors","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Discover Series","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Composite, Fiber Bragg Grating, Optical sensor, Strain, Temperature","lastPublishedDoi":"10.21203/rs.3.rs-7319127/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7319127/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe use of fibre optic sensors for structural health monitoring has been persuaded by the aircraft industry and research organizations for more than three decades. The Fiber Bragg Grating (FBG) sensors because of their inherent advantages are considered the best choice for the aircraft industry for structural health monitoring applications. However, these sensors are fragile in nature and are still being used in lab-scale experiments. To make the sensor robust without altering its sensitivity a methodology is developed to package the sensors using a glass-carbon/epoxy composite. Different arrangements are established for the protection of the sensing region and the non-sensing region of the sensor. Sensor packaging is carried out in such a way that the sensor quality and its strain transfer capability are unaltered. Based on spectrum comparison and ease of installation, the sensing region of FBG is packaged between composite layers, and the non-sensing region is protected using Teflon and other flexible tubes. The temperature and strain response of the sensors are studied experimentally and compared with bare FBG sensor and resistance strain gauge (RSG) respectively. These studies showed that packaging has the same response to strain and temperature and is very reliable.\u003c/p\u003e","manuscriptTitle":"Development and Characterization of Fibre Bragg Grating Sensor Packaging for Aircraft Structural Health Monitoring Applications","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-09-02 10:17:10","doi":"10.21203/rs.3.rs-7319127/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-09-22T14:02:22+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-09-16T10:41:42+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-09-08T19:20:05+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-09-05T12:50:49+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-08-28T11:37:42+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"172181153524332912326731723307748031629","date":"2025-08-28T04:55:12+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"225813502908147586996350362833719839272","date":"2025-08-25T10:40:34+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"74036104534282182365803437337774193707","date":"2025-08-25T09:59:20+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"109514550703417373509752716880330177238","date":"2025-08-25T09:06:42+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-08-25T08:41:40+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-08-20T08:10:37+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-08-20T07:34:57+00:00","index":"","fulltext":""},{"type":"submitted","content":"Discover Sensors","date":"2025-08-20T07:31:35+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"discover-sensors","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"Learn more about [Discover Sensors](https://link.springer.com/journal/44397)","snPcode":"44397","submissionUrl":"https://submission.nature.com/new-submission/44397/3","title":"Discover Sensors","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Discover Series","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"930aa7b8-3a5d-418e-840f-a7a1ef5ebd5a","owner":[],"postedDate":"September 2nd, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2025-11-13T05:53:56+00:00","versionOfRecord":[],"versionCreatedAt":"2025-09-02 10:17:10","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7319127","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7319127","identity":"rs-7319127","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}