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Investigation on packages of fiber Bragg grating for use as embeddable strain sensor in concrete structure
Sensors and Actuators A 157 (2010) 77–83
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Investigation on packages of fiber Bragg grating for use as embeddable strain sensor in concrete structure P. Biswas a , S. Bandyopadhyay a,∗ , K. Kesavan b , S. Parivallal b , B. Arun Sundaram b , K. Ravisankar b , K. Dasgupta a a b
Fiber Optics Laboratory, Central Glass and Ceramic Research Institute, Council of Scientific and Industrial Research (CSIR), Kolkata, India Experimental Mechanics Laboratory, Structural Engineering Research Centre, Council of Scientific and Industrial Research (CSIR), Chennai, India
a r t i c l e
i n f o
Article history: Received 20 August 2009 Received in revised form 28 October 2009 Accepted 28 October 2009 Available online 10 November 2009 Keywords: Fiber Bragg grating (FBG) Sensor packaging Embeddable sensor Extrinsic Fabry-Perot interferometer (EFPI) Health monitoring
a b s t r a c t The paper covers detailed investigation on encapsulation and packaging of fiber Bragg grating (FBG) for strain sensing of concrete structures in embeddable form. Non-uniform strain distribution due to imperfect curing of the epoxy and its effect on the FBG spectrum has been studied experimentally and correlated with theoretical simulation. For a specific package, an optimal curing condition has been found and shown to have good repeatability. The successfully packaged sensor has been embedded in a concrete structure and the response has been found to be linear. Response of the sensor under static loading condition is compared with surface mountable electrical resistance strain gauge (ERSG) and embeddable type EFPI (extrinsic Fabry Perot interferometer) fiber optic sensor. The sensor has also been tested under dynamic loading of the structure. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Over the last few years the use of fiber Bragg grating (FBG) sensors for sensing and monitoring of structures in civil engineering applications has seen a rapid growth [1–6]. FBG sensors are characterized by a very good long-term stability and a high reliability. These sensors are self-referencing, free from EMI, can be used for multi-point sensing through a single optical channel and finally they can be an integral part of the structure by embedding the sensor during the construction of the structures. In structural health monitoring (SHM) it is essential that placement of the sensors are appropriately chosen so that the measured strains and/or vibrations provide valuable information about the integrity of the structural system. Concrete structures are generally large in dimension and geometrically complex and in this respect, to get meaningful data, it is often required that the sensors be embedded into the concrete structure at proper location. FBGs, even with protective coatings like polyimide or acrylate resin, are brittle in nature and suitable protective housing is necessary for embedding in concrete structures. It is desirable that the design of the protective housing be optimal to have efficient strain transfer from the host to the sensor with minimum pos-
∗ Corresponding author. Tel.: +91 33 24838083; fax: +91 33 24730957. E-mail addresses: [email protected] (P. Biswas), [email protected] (S. Bandyopadhyay). 0924-4247/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2009.10.020
sibility of debonding from the structure and also the structural integrity is not compromised with sensor deployment. Among various propositions a specific housing design [7], was shown to have good application potential in concrete structure. Packaged EFPI and FBG sensors in those optimally designed housings were shown to have excellent linear strain transfer from the host structure to the sensors [8]. It is important to note that, in addition to a perfect housing design, embedding of fiber optics components like EFPI, FBG in the housing is also an important issue because it influences the overall performance of the sensor. Any void or non-uniform shrinkage of the epoxy consequently induces non-uniform strain on the FBG and causes substantial distortion like occurrence of ripples in the corresponding wavelength spectra and/or broadening of FBG spectra and this leads to increased system noise and false measurements with the requirement of special techniques to demodulate the sensor signal [9]. Imperfect bonding may also affect the strain transfer characteristics from the host to the sensor. These effects related to shrinkage of epoxy depends on number of factors starting from epoxy type, bonding length and bonding thickness and therefore require precisely case specific knowledge before the packaged sensors can be used with utmost reliability. Useful works which focused these issues in relevant applications are reported elsewhere [10–14]. Our aim is to extend this study for a specific housing suitable for embedding in concrete structure. Earlier work [8], however though described the design and characterization of packaged FBG sensors
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P. Biswas et al. / Sensors and Actuators A 157 (2010) 77–83
for embedding in concrete structure, did not cover the studies on the encapsulation of FBG sensors which certainly has importance for fabrication of the package in a reliable and reproducible way. This paper presents a detailed study on the influences of curing of the epoxy on the wavelength spectrum of the FBG embedded in a similar housing. It is interesting to see that even for a particular type of epoxy all curing schedules as prescribed by the manufacturer are not suitable to retain the original spectral characteristics of FBG. The nature of degradation of the spectrum is correlated with theoretically computed spectrum simulating similar non-uniform strain distribution due to shrinkage of epoxy during curing. Packaged sensors have been produced with good reproducibility with similar residual stresses associated with post-cure shrinkage of the epoxy. Strain transfer characteristics have been found through static and dynamic loading of a specimen concrete cylinder embedded with the packaged FBG.
In a Bragg grating a permanent periodic change of the refractive index in the fibre core is induced for the fundamental purpose to reflect a narrow spectral component of a broadband light source. The reflected light is centered on the Bragg wavelength B , which is related to the inscribed grating period B , and the effective refractive index neff through the Bragg condition as (1)
where B is the period of the grating and neff is the effective mode index. A simple shift in Bragg wavelength without any degradation of the spectrum shape implies the occurrence of position independent axial strain or temperature change around the grating and in this case the change in B or neff due to the external perturbation remains uniform along the grating length. The shift of Bragg wavelength may be represented as B = (1 − peff )εz + [{(1 − peff )}˛ + ] T B where peff =
1 2 n (p 2 eff 12
B 1 = 1 − n2eff (p12 − (p11 + p12 ) εz 2 B
(2)
− (p11 + p12 )
The parameter peff is the effective strain-optic constant, where p11 and p12 are photoelastic constants and is the Poisson’s ratio of the fiber, ˛ is the thermal expansion coefficient of material and the parameter is related to thermo-optic coefficient of the same. An applied strain shifts the Bragg wavelength through expansion or contraction of the grating periodicity and through the photoelastic effect. Temperature influences the Bragg wavelength through thermal expansion coefficient of the fiber material by changing the grating periodicity and also through thermal dependence of the refractive index i.e. the thermo-optic coefficient of the material. If
(3)
When the applied strain in the sensing region is non-uniform then the condition expressed in Eq. (1) becomes position dependent and may be expressed as B (z) = 2B (z)neff (z)
(4)
The grating period and the effective index under these circumstances may be written as [15]: B (z) = 0 [1 + (1 − peff )εz (z)]
ıneff (z) = ıneff 1 + a cos
2. FBG spectrum under uniform and non-uniform strain
B = 2B neff
it is assumed that that the temperature remains constant or it is compensated by any means the shift in B is then linearly proportional to the applied axial strain εz and the relationship as described in Eq. (1) takes simpler form as
2 z 0 [1 + (1 − peff )εz (z)]
(5) (6)
where ıneff is the mean induced change in neff and 0 is the initial grating period at zero strain. Coupled mode theory is in general used to compute the spectral characteristics of the FBG for any position dependent period and effective index variation as induced by any arbitrary strain field εz (z) where the transverse strain ε(x,y) applied to the fiber is assumed as ε(x,y) = −vεz (z) in our simulation and is valid for many useful application. Grating modeling software Opti-grating (version 4.2) from Optiwave Inc. is used for our theoretical simulation and to understand non-uniform strain fields those arise due to curing of epoxy during encapsulation of FBG and are elaborated in the following section. 3. FBG encapsulation and development of packaged sensor Stainless steel housing based on the design as described [7] was manufactured and packaged FBG sensors e.g. sample #1 to sample #4 were prepared. A few samples have been shown in Fig. 1a and b. The length between two flanges, flange diameter and flange thickness are of 70 mm, 12 mm and 5 mm, respectively. The inner diameter of the tube is 3.5 ± 0.2 mm with a wall thickness of 0.5 mm. Apodized gratings with physical length of about 5 mm were inscribed on hydrogen loaded fiber using a 248 nm UV laser exposure with a phase mask. The reflectivity of grating was ∼50 ± 5% with >35 dB peak to side-lobe suppression. Grating zones were recoated with polyimide and were annealed at 150 ◦ C for 12 h before packaging into steel housing. The strain transfer from host material to grating zone depends on the properties of intermediate epoxy layer and therefore, selection of epoxy is an important issue in photonic packaging especially for sensor applications. For the present study EPO-TEK® 353ND was selected which is a two
Fig. 1. (a) Sensor housing and (b) packaged sensor.
P. Biswas et al. / Sensors and Actuators A 157 (2010) 77–83
79
Table 1 Curing schedule considered in the experiment. Sample #
Curing time (min)
1 2
25 30
3 4
Curing temperature (◦ C)
Initial Bragg wavelength (nm)
Final Bragg wavelength (nm)
90 85
1553.28 1549.38
2
150
1546.87
4
130
1549.44
1545.88 1542.88 1538.86 1539.14 (split spectrum) 1542.08
Fig. 2. Peak wavelength excursion of FBGs embedded in the housing against time during curing of epoxy.
Fig. 4. FBG spectrum after curing of epoxy of sample #3. Inset shows the spectrum of the same sample before curing.
component, high temperature epoxy commonly used for similar applications. A suitable heating oven was designed to accommodate the epoxy filled housing and curing of the epoxy. Adequate precaution was taken during resin infusion and filling up of housings to avoid trapping of air bubbles which is a likely source that induces non-uniform strain along the length of the FBG during curing of the epoxy and degrades the FBG spectrum. Different cure schedules i.e. a set of temperature and corresponding curing time as available in the data sheet of the manufacturer of the epoxy has been considered for different samples and are shown in Table 1. The cure monitoring was performed to observe change in the spectrum of FBG during the cure process of epoxy. Fig. 2 shows the peak wavelength excursion of FBGs against curing time for sample #1 and sample #2 where curing was done for a longer time at low temperature. The oven was preheated to the curing temperatures considered for respective experiments when the housing was placed. At point ‘A’ FBG starts sensing the temperature of the oven and as usual there is a red shift of the peak wavelength of the FBG. During section ‘A’ to ‘B’ no compressive
strain was detected as the epoxy seems to be still in a liquid stage. After certain time (region from ‘B’ to ‘C’) a fast blue shift of the peak was observed which arises due to compressive strain (∼0.6% as calculated from the wavelength shift) on FBG due to hardening of the epoxy. The region ‘C’ to ‘D’ represents the stabilization of the epoxy which is accompanied by a released strain at the end of the cure regime and due to this the peak wavelength experiences a red shift before stabilization. The housing was then taken out from the oven to room temperature which is reflected by the decrease of wavelength from the point ‘D’ onward. Fig. 3 shows the spectrum of the respective FBGs in pre- and post-cured condition. The measured spectra have found to have undergone minimum change in terms of bandwidth and spectral quality. It may be appreciated that the epoxy introduces compressive stress on the FBG that results in a residual curing strain equivalent to a peak wavelength shift of approximately −6.5 to 7 nm. Samples #3 and #4 which were cured at higher temperatures for shorter time duration also show wavelength excursion almost similar in nature but appreciable spectrum degradation was observed. Figs. 4 and 5 show the post-cured FBG
Fig. 3. FBG spectra of samples #1 and #2 (a) before curing of epoxy and (b) post-cured.
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Fig. 5. FBG spectrum after curing of epoxy in sample #4. Inset shows the spectrum of the same sample before.
spectra of samples #3 and #4 along with the initial spectra of the respective FBGs before packaging. The distortions in FBG spectra substantiate the creation of a nonuniform strain field around the encapsulated FBG which certainly arises due to shrinkage of epoxy during curing. It is also interesting to note that the non-uniformity in induced strain due to fast curing of epoxy at higher temperature is more or less unpredictable as has been seen from two drastically different spectra of samples #3 and #4 though the cure temperature and the time was roughly comparable. As for example, significant broadening (∼4 times w.r.t. the original) along with a split has been observed in the post cured spectrum of sample #3. Formation of chirp in the grating region due to a strain gradient is the reason for the broadening. Presence of simple linear strain gradient cannot explain the origin of the splitting of the spectrum of the FBG. Characteristic ripples often may be observed for a strong un-apodized chirped grating but is absent when the grating is apodized. However, in case of a Moiré grating [16] where the Bragg grating is modulated by a low spatial frequency sinusoidal envelope, a split of similar nature in the spectrum is observed. The situation therefore demands a strain distribution on the FBG of the type as shown in Fig. 6. FBG spectrum of a 5 mm apodized FBG upon imposition of a position dependent strain field εz (z) having a peak to peak strain variation of 1000 ε as shown in Fig. 6 has been computed. It has been found that the assumed strain field induces a position dependent period variation of ±0.26 nm and average index variation of ±1.6 × 10−4 of the FBG around their initial values and is shown in Fig. 7. The resulting spectrum as shown in Fig. 8 is broadened due to the induced chirp
Fig. 6. Position dependent strain as considered for sample #3.
Fig. 7. Variation of grating period and average index along the length of the grating due to the imposed strain as shown in Fig. 6.
Fig. 8. Simulated FBG spectrum for a position dependent strain as shown in Fig. 6.
with a symmetric split in the middle of the spectrum as observed experimentally. The degradation observed in sample #4 (Fig. 5) is comparatively more familiar and easy to interpret. Any form of ripples or side lobes in the higher wavelength side of an FBG spectrum indicates a depression in the average index in the middle of the FBG. For our simulation it has been assumed that a compressive Gaussian strain field as shown in Fig. 9 induces a position dependent average index change and generates chirping in the grating period as well. The computed chirp and average index variation for a peak compressive strain of −1000 ε at the middle of the FBG is shown in Fig. 10. The
Fig. 9. Position dependent strain as considered for sample #4.
P. Biswas et al. / Sensors and Actuators A 157 (2010) 77–83
Fig. 10. Variation of grating period and average index along the length of the grating due to the imposed strain as shown in Fig. 9.
Fig. 11. Simulated FBG spectrum for a position dependent strain as shown in Fig. 9.
resulting spectrum for this type of strain field is shown in Fig. 11 where significant degradation of the FBG spectrum on the higher wavelength side may be observed and is similar to as has been found in sample #4. 4. Performance test of the packaged sensor embedded in concrete The strain response of the packaged sensors (samples #1 and #2) was tested by embedding the sensors in concrete and by applying compressive loads at a constant rate in a universal testing machine (UTM). Prior to the embedding of sensors in the concrete, thermal response and stability of the sensors were evaluated by applying consecutive temperature cycles from room temperature (∼25 ◦ C) to 100 ◦ C and the result obtained is shown in Fig. 12. The thermal response has been found to be linear with no hysteresis with a sensitivity of ∼15.4 pm/◦ C and similar to that of Stainless Steel 316 (Austenitic), the material which is used for fabricating the housing. Concrete cylinders were prepared by embedding one FBG sensor along with a procured EFPI optical sensor (make FISO) having a housing of similar type. Strain developed onto the FBG sensor during curing of concrete was also recorded. It is encouraging to observe very similar results for both samples #1 and # 2 in two different concrete cylinders of same material type which justifies perfect bonding of the housing in the concrete material. During curing the wavelength shift of the FBG sensors against curing time is shown in Fig. 13. Before placing the concrete cylinder in the UTM four electrical resistance strain gages (ERSG) of 60 mm gauge
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Fig. 12. Thermal response of one packaged sensor (sample #1).
Fig. 13. Wavelength shift of the FBG sensors against curing time during curing of the concrete.
length were bonded diametrically opposite on the outer surface of the concrete cylinder. FBG, EFPI and ERSG sensors were simultaneously monitored using Micron Optics sm130, FISO demodulator and strain gauge data logger, respectively. The experimental set-up is shown in Fig. 14. A maximum compressive load of 250 kN was applied to the concrete cylinder. Strain response was recorded at every 50 kN load step. Strain data as sensed by embedded FBG sensors were found to have good linearity against applied load and also have close matching with the other two types of the sensors used. Typical comparison is shown in Fig. 15. A preliminary characteristic of the FBG sensor under dynamic loading condition was also found satisfactory. Fig. 16 shows measured response of the FBG sensor when loading of the specimen was varied between 50 kN and 10 kN at 5 Hz. The measured strain (∼67 ε peak to peak) under this dynamic loading condition matches exactly with the strain measured during static load testing. 5. Discussion and conclusion Embedding FBG sensors in concrete structure requires suitable protective housing where a perfect bond between the housing and the surrounding concrete needs to be ensured for a faithful transfer of structural strain to the sensor. In this respect error free bonding of FBG sensor in the protective housing is also immensely important for reliable strain measurement. This paper has outlined the major aspects of encapsulation of FBG in a properly designed housing in regard to the curing of the bonding material and its effect on the FBG spectrum. It may be noted for a packaged sensor with ∼70 mm gauge length FBG of any arbitrary length may be cho-
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sen for encapsulation. However, as the cumulative induced strain developed on the FBG due to the shrinkage of the epoxy is length dependent and also possibility of degradation of reflection spectrum increases as well with the length of the FBG, in the present work a 5 mm Gaussian apodized FBG whose effective length is nearly 2 mm (considering the strength of apodization ∼0.4) has been considered to minimise effect of shrinkage induced strain on the FBG. It has been shown that rapid curing of thermally curable epoxy induces non-uniform strain gradient along the FBG and the sensors turn out to be unusable due to significant degradation of the spectrum. The strain fields that degrade the spectrum for the particular housing and the epoxy considered in this work can well be interpreted by simulating the spectrum using standard couple mode theory taking some specific strain fields in account. Preliminary experimentation shows sensor response is linear without any hysterisis under static and dynamic loading. More detailed performance test with these sensors like cycling test under compressive load upto elastic limit of the specimen, low cycling under high strain etc are underway.
Acknowledgements
Fig. 14. Experimental set-up to study strain response of packaged FBG and EFPI sensors embedded in concrete.
The authors acknowledge the Council of Scientific and Industrial Research (CSIR), India, for funding the research programme during 11th five year plan through network project (NWP026). The authors are thankful to all the technical staffs for their contribution during experiments and specially to Mr. Bipalb Mitra for developing Labview based data logging software.
References
Fig. 15. Strain response of embedded FBG, EFPI and surface mounted ERSG.
Fig. 16. Strain response of embedded FBG sensor under dynamic loading condition.
[1] W. Chung, S. Kim, N.S. Kim, H. Lee, Deflection estimation of a full scale prestressed concrete girder using long-gauge fiber optic sensors, Construction and Building Materials 22 (2008) 394–401. [2] C. Li, Y.G. Zhao, H. Liu, Z. Wan, C. Zhang, N. Rong, Monitoring second lining of tunnel with mounted fiber Bragg grating strain sensors, Automation in Construction 17 (2008) 641–644. [3] W. Zhang, J. Gao, B. Shi, H. Cui, Health monitoring of rehabilitated concrete bridges using distributed optical fiber sensing, Computer-Aided Civil and Infrastructure Engineering 21 (2006) 411–424. [4] P. Moyoa, J.M.W. Brownjohn, R. Suresh, S.C. Tjin, Development of fiber Bragg grating sensors for monitoring civil infrastructure, Engineering Structures 27 (2005) 1828–1834. [5] Y.B. Lin, C.L. Pan, Y.H. Kuo, K.C. Chang, J.C. Chern, Online monitoring of highway bridge construction using fiber Bragg grating sensors, Smart Materials and Structures 14 (2005) 1075–1082. [6] Y.B. Lin, T. Lin, C.C. Chen, J.C. Chiu, K.C. Chang, Online health monitoring and safety evaluation of the relocation of a research reactor using fiber Bragg grating sensors, Smart Materials and Structures 15 (2006) 1421–1428. [7] G.F. Fernando, A. Hameed, D. Winter, J. Tetlow, J. Leng, R. Barnes, G. Mays, G. Kister, Structural integrity monitoring of concrete structures via optical fiber sensors: sensor protection systems, Structural Health Monitoring 2 (2) (2003) 123–135. [8] J.S. Lenga, R.A. Barnes, A. Hameed, D. Winter, J. Tetlow, G.C. Mays, G.F. Fernando, Structural NDE of concrete structures using protected EFPI and FBG sensors, Sensors and Actuators A 126 (2006) 340–347. [9] H.W. Lee, H.-J. Park, J.H. Lee, M. Song, Accuracy improvement in peak positioning of spectrally distorted fiber Bragg grating sensors by Gaussian curve fitting, Applied Optics 46 (12) (2007) 2205–2208. [10] M. Harsch, J.K. Kocsis, F. Herzog, Influence of cure regime on the strain development in an epoxy resin as monitored by a fiber Bragg grating sensor, Macromolecular Materials and Engineering 292 (2007) 474–483. [11] J.R. Lee, et al., Single-mode fibre optic Bragg grating sensing on the base of birefringence in surface-mounting and embedding applications, Optics & Laser Technology 39 (2007) 157–164. [12] R. Montanini, L. D’Acquisto, Simultaneous measurement of temperature and strain in glass fiber/epoxy composites by embedded fiber optic sensors: I. Cure monitoring, Smart Materials and Structures 16 (2007) 1718–1726. [13] C.-C. Cheng, Y.-L. Lo, B.S. Pun, Y.M. Chang, W.Y. Li, An investigation of bonding-layer characteristics of substrate-bonded fiber Bragg grating, Journal of Lightwave Technology 23 (11) (2005) 3907–3914.
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Biographies P. Biswas received the B.Tech. and M.Tech. degree in Applied Physics from University of Calcutta in the year 1998 and 2000, respectively. He worked as a design engineer for packaging of photonic component in Celetron India from 2001 to 2004. From 2004 till to date he is in Central Glass and Ceramic Research Institute, Kolktata, India. His current research interest includes optical fiber Bragg grating and long period grating for application in acousto-ultrasonic measurement and detection of biological and chemical species. S. Bandyopadhyay received the B.Sc. and M.Sc. Tech. degree in Physics and Applied Physics from University of Calcutta in the year 1987 and 1990, respectively. He received Ph.D. Tech. in Applied Physics from the same university in 1998. He was then with Central Scientific Instruments Organisation, a National laboratory of Council of Scientific and Industrial Research (CSIR), India. From 2002 till to date he is in Central Glass and Ceramic Research Institute, Kolktata, India, which is another sister laboratory of CSIR. His current research interest includes optical fiber and optical fiber based sensors for ultra high temperature application and structural health monitiring. K. Kesavan is a scientist in Structural Engineering Research Centre, Chennai. He obtained his B.E. (civil) from Anna University, Chennai and M.Tech. (Structural Engineering) from Indian Institute of Technology, Chennai. He has been working in the area of experimental stress analysis since 1995. His current areas of interest include condition monitoring of structures, existing stress evaluation in prestressed concrete structures and health monitoring of civil engineering structures using fiber optic sensors. He has contributed more than 50 technical papers in journals and seminars and more than 100 technical reports.
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S. Parivallal obtained his B.E. (civil) from PSG College of Technology, Coimbatore and M.E (Structural Engineering) form Govt. College of Technology, Coimbatore. He has been with Structural Engineering Research Centre since 1994. His area of interest includes experimental Mechanics such as strain gage instrumentation, condition monitoring of structures, existing stress evaluation in prestressed concrete structures and smart civil engineering structures using fiber optic sensors. He has contributed more than 50 technical papers in journals and seminars and more than 100 technical reports. B. Arun Sundaram obtained his B.E. (civil) and M.E. (Structural Engineering) from Crescent Engineering College, Chennai and he has been with Structural Engineering Research centre since 2008. His current areas of interest include remote health monitoring of civil engineering structures, experimental stress analysis. He has contributed about 4 technical papers in journals and seminars and about 5 technical reports. K. Ravi Sankar is a deputy Director in Structural Engineering Research Centre (SERC), Chennai. He obtained his Bachelor’s degree in Civil Engineering in and Master’s degree in Structural Engineering from P.S.G. College of Technology, Coimbatore, and Ph.D. in Structural Engineering from Anna University, Madras. He has been working in the area of experimental stress analysis since 1980. He has vast experience in experimental techniques for stress analysis such as strain gages (fiber optics, vibrating wire and electrical resistance types), photo elasticity, holography, high sensitivity moire, etc., including on-site measurements and investigation of critical engineering structures. He has published around 100 technical papers and 200 technical/research reports. K. Dasgupta received M.Tech. degree in Microwave & Communication from Indian Institute of Technology, Kanpur in1978. He worked as a design engineer in Avionics Design Group (Hindustan Aeronautics Ltd., India) from 1978 to1981. He also worked as a senior engineer in Development Consultant Pvt. Ltd., India from 1982 to 1986. From 1986 till to date he is in Central Glass and Ceramic Research Institute, Kolkata, India. He actively worked for the development of EDFA and coordinated several National and International projects successfully in the area of fiber optic sensors. He has published about 100 papers in national and international journals/conferences. Presently he is the head of fiber optics laboratory of CGCRI. His current interest is towards development of FBG interrogation system.
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Investigation on packages of fiber Bragg grating for use as embeddable strain sensor in concrete structure P. Biswas a , S. Bandyopadhyay a,∗ , K. Kesavan b , S. Parivallal b , B. Arun Sundaram b , K. Ravisankar b , K. Dasgupta a a b
Fiber Optics Laboratory, Central Glass and Ceramic Research Institute, Council of Scientific and Industrial Research (CSIR), Kolkata, India Experimental Mechanics Laboratory, Structural Engineering Research Centre, Council of Scientific and Industrial Research (CSIR), Chennai, India
a r t i c l e
i n f o
Article history: Received 20 August 2009 Received in revised form 28 October 2009 Accepted 28 October 2009 Available online 10 November 2009 Keywords: Fiber Bragg grating (FBG) Sensor packaging Embeddable sensor Extrinsic Fabry-Perot interferometer (EFPI) Health monitoring
a b s t r a c t The paper covers detailed investigation on encapsulation and packaging of fiber Bragg grating (FBG) for strain sensing of concrete structures in embeddable form. Non-uniform strain distribution due to imperfect curing of the epoxy and its effect on the FBG spectrum has been studied experimentally and correlated with theoretical simulation. For a specific package, an optimal curing condition has been found and shown to have good repeatability. The successfully packaged sensor has been embedded in a concrete structure and the response has been found to be linear. Response of the sensor under static loading condition is compared with surface mountable electrical resistance strain gauge (ERSG) and embeddable type EFPI (extrinsic Fabry Perot interferometer) fiber optic sensor. The sensor has also been tested under dynamic loading of the structure. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Over the last few years the use of fiber Bragg grating (FBG) sensors for sensing and monitoring of structures in civil engineering applications has seen a rapid growth [1–6]. FBG sensors are characterized by a very good long-term stability and a high reliability. These sensors are self-referencing, free from EMI, can be used for multi-point sensing through a single optical channel and finally they can be an integral part of the structure by embedding the sensor during the construction of the structures. In structural health monitoring (SHM) it is essential that placement of the sensors are appropriately chosen so that the measured strains and/or vibrations provide valuable information about the integrity of the structural system. Concrete structures are generally large in dimension and geometrically complex and in this respect, to get meaningful data, it is often required that the sensors be embedded into the concrete structure at proper location. FBGs, even with protective coatings like polyimide or acrylate resin, are brittle in nature and suitable protective housing is necessary for embedding in concrete structures. It is desirable that the design of the protective housing be optimal to have efficient strain transfer from the host to the sensor with minimum pos-
∗ Corresponding author. Tel.: +91 33 24838083; fax: +91 33 24730957. E-mail addresses: [email protected] (P. Biswas), [email protected] (S. Bandyopadhyay). 0924-4247/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2009.10.020
sibility of debonding from the structure and also the structural integrity is not compromised with sensor deployment. Among various propositions a specific housing design [7], was shown to have good application potential in concrete structure. Packaged EFPI and FBG sensors in those optimally designed housings were shown to have excellent linear strain transfer from the host structure to the sensors [8]. It is important to note that, in addition to a perfect housing design, embedding of fiber optics components like EFPI, FBG in the housing is also an important issue because it influences the overall performance of the sensor. Any void or non-uniform shrinkage of the epoxy consequently induces non-uniform strain on the FBG and causes substantial distortion like occurrence of ripples in the corresponding wavelength spectra and/or broadening of FBG spectra and this leads to increased system noise and false measurements with the requirement of special techniques to demodulate the sensor signal [9]. Imperfect bonding may also affect the strain transfer characteristics from the host to the sensor. These effects related to shrinkage of epoxy depends on number of factors starting from epoxy type, bonding length and bonding thickness and therefore require precisely case specific knowledge before the packaged sensors can be used with utmost reliability. Useful works which focused these issues in relevant applications are reported elsewhere [10–14]. Our aim is to extend this study for a specific housing suitable for embedding in concrete structure. Earlier work [8], however though described the design and characterization of packaged FBG sensors
78
P. Biswas et al. / Sensors and Actuators A 157 (2010) 77–83
for embedding in concrete structure, did not cover the studies on the encapsulation of FBG sensors which certainly has importance for fabrication of the package in a reliable and reproducible way. This paper presents a detailed study on the influences of curing of the epoxy on the wavelength spectrum of the FBG embedded in a similar housing. It is interesting to see that even for a particular type of epoxy all curing schedules as prescribed by the manufacturer are not suitable to retain the original spectral characteristics of FBG. The nature of degradation of the spectrum is correlated with theoretically computed spectrum simulating similar non-uniform strain distribution due to shrinkage of epoxy during curing. Packaged sensors have been produced with good reproducibility with similar residual stresses associated with post-cure shrinkage of the epoxy. Strain transfer characteristics have been found through static and dynamic loading of a specimen concrete cylinder embedded with the packaged FBG.
In a Bragg grating a permanent periodic change of the refractive index in the fibre core is induced for the fundamental purpose to reflect a narrow spectral component of a broadband light source. The reflected light is centered on the Bragg wavelength B , which is related to the inscribed grating period B , and the effective refractive index neff through the Bragg condition as (1)
where B is the period of the grating and neff is the effective mode index. A simple shift in Bragg wavelength without any degradation of the spectrum shape implies the occurrence of position independent axial strain or temperature change around the grating and in this case the change in B or neff due to the external perturbation remains uniform along the grating length. The shift of Bragg wavelength may be represented as B = (1 − peff )εz + [{(1 − peff )}˛ + ] T B where peff =
1 2 n (p 2 eff 12
B 1 = 1 − n2eff (p12 − (p11 + p12 ) εz 2 B
(2)
− (p11 + p12 )
The parameter peff is the effective strain-optic constant, where p11 and p12 are photoelastic constants and is the Poisson’s ratio of the fiber, ˛ is the thermal expansion coefficient of material and the parameter is related to thermo-optic coefficient of the same. An applied strain shifts the Bragg wavelength through expansion or contraction of the grating periodicity and through the photoelastic effect. Temperature influences the Bragg wavelength through thermal expansion coefficient of the fiber material by changing the grating periodicity and also through thermal dependence of the refractive index i.e. the thermo-optic coefficient of the material. If
(3)
When the applied strain in the sensing region is non-uniform then the condition expressed in Eq. (1) becomes position dependent and may be expressed as B (z) = 2B (z)neff (z)
(4)
The grating period and the effective index under these circumstances may be written as [15]: B (z) = 0 [1 + (1 − peff )εz (z)]
ıneff (z) = ıneff 1 + a cos
2. FBG spectrum under uniform and non-uniform strain
B = 2B neff
it is assumed that that the temperature remains constant or it is compensated by any means the shift in B is then linearly proportional to the applied axial strain εz and the relationship as described in Eq. (1) takes simpler form as
2 z 0 [1 + (1 − peff )εz (z)]
(5) (6)
where ıneff is the mean induced change in neff and 0 is the initial grating period at zero strain. Coupled mode theory is in general used to compute the spectral characteristics of the FBG for any position dependent period and effective index variation as induced by any arbitrary strain field εz (z) where the transverse strain ε(x,y) applied to the fiber is assumed as ε(x,y) = −vεz (z) in our simulation and is valid for many useful application. Grating modeling software Opti-grating (version 4.2) from Optiwave Inc. is used for our theoretical simulation and to understand non-uniform strain fields those arise due to curing of epoxy during encapsulation of FBG and are elaborated in the following section. 3. FBG encapsulation and development of packaged sensor Stainless steel housing based on the design as described [7] was manufactured and packaged FBG sensors e.g. sample #1 to sample #4 were prepared. A few samples have been shown in Fig. 1a and b. The length between two flanges, flange diameter and flange thickness are of 70 mm, 12 mm and 5 mm, respectively. The inner diameter of the tube is 3.5 ± 0.2 mm with a wall thickness of 0.5 mm. Apodized gratings with physical length of about 5 mm were inscribed on hydrogen loaded fiber using a 248 nm UV laser exposure with a phase mask. The reflectivity of grating was ∼50 ± 5% with >35 dB peak to side-lobe suppression. Grating zones were recoated with polyimide and were annealed at 150 ◦ C for 12 h before packaging into steel housing. The strain transfer from host material to grating zone depends on the properties of intermediate epoxy layer and therefore, selection of epoxy is an important issue in photonic packaging especially for sensor applications. For the present study EPO-TEK® 353ND was selected which is a two
Fig. 1. (a) Sensor housing and (b) packaged sensor.
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Table 1 Curing schedule considered in the experiment. Sample #
Curing time (min)
1 2
25 30
3 4
Curing temperature (◦ C)
Initial Bragg wavelength (nm)
Final Bragg wavelength (nm)
90 85
1553.28 1549.38
2
150
1546.87
4
130
1549.44
1545.88 1542.88 1538.86 1539.14 (split spectrum) 1542.08
Fig. 2. Peak wavelength excursion of FBGs embedded in the housing against time during curing of epoxy.
Fig. 4. FBG spectrum after curing of epoxy of sample #3. Inset shows the spectrum of the same sample before curing.
component, high temperature epoxy commonly used for similar applications. A suitable heating oven was designed to accommodate the epoxy filled housing and curing of the epoxy. Adequate precaution was taken during resin infusion and filling up of housings to avoid trapping of air bubbles which is a likely source that induces non-uniform strain along the length of the FBG during curing of the epoxy and degrades the FBG spectrum. Different cure schedules i.e. a set of temperature and corresponding curing time as available in the data sheet of the manufacturer of the epoxy has been considered for different samples and are shown in Table 1. The cure monitoring was performed to observe change in the spectrum of FBG during the cure process of epoxy. Fig. 2 shows the peak wavelength excursion of FBGs against curing time for sample #1 and sample #2 where curing was done for a longer time at low temperature. The oven was preheated to the curing temperatures considered for respective experiments when the housing was placed. At point ‘A’ FBG starts sensing the temperature of the oven and as usual there is a red shift of the peak wavelength of the FBG. During section ‘A’ to ‘B’ no compressive
strain was detected as the epoxy seems to be still in a liquid stage. After certain time (region from ‘B’ to ‘C’) a fast blue shift of the peak was observed which arises due to compressive strain (∼0.6% as calculated from the wavelength shift) on FBG due to hardening of the epoxy. The region ‘C’ to ‘D’ represents the stabilization of the epoxy which is accompanied by a released strain at the end of the cure regime and due to this the peak wavelength experiences a red shift before stabilization. The housing was then taken out from the oven to room temperature which is reflected by the decrease of wavelength from the point ‘D’ onward. Fig. 3 shows the spectrum of the respective FBGs in pre- and post-cured condition. The measured spectra have found to have undergone minimum change in terms of bandwidth and spectral quality. It may be appreciated that the epoxy introduces compressive stress on the FBG that results in a residual curing strain equivalent to a peak wavelength shift of approximately −6.5 to 7 nm. Samples #3 and #4 which were cured at higher temperatures for shorter time duration also show wavelength excursion almost similar in nature but appreciable spectrum degradation was observed. Figs. 4 and 5 show the post-cured FBG
Fig. 3. FBG spectra of samples #1 and #2 (a) before curing of epoxy and (b) post-cured.
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Fig. 5. FBG spectrum after curing of epoxy in sample #4. Inset shows the spectrum of the same sample before.
spectra of samples #3 and #4 along with the initial spectra of the respective FBGs before packaging. The distortions in FBG spectra substantiate the creation of a nonuniform strain field around the encapsulated FBG which certainly arises due to shrinkage of epoxy during curing. It is also interesting to note that the non-uniformity in induced strain due to fast curing of epoxy at higher temperature is more or less unpredictable as has been seen from two drastically different spectra of samples #3 and #4 though the cure temperature and the time was roughly comparable. As for example, significant broadening (∼4 times w.r.t. the original) along with a split has been observed in the post cured spectrum of sample #3. Formation of chirp in the grating region due to a strain gradient is the reason for the broadening. Presence of simple linear strain gradient cannot explain the origin of the splitting of the spectrum of the FBG. Characteristic ripples often may be observed for a strong un-apodized chirped grating but is absent when the grating is apodized. However, in case of a Moiré grating [16] where the Bragg grating is modulated by a low spatial frequency sinusoidal envelope, a split of similar nature in the spectrum is observed. The situation therefore demands a strain distribution on the FBG of the type as shown in Fig. 6. FBG spectrum of a 5 mm apodized FBG upon imposition of a position dependent strain field εz (z) having a peak to peak strain variation of 1000 ε as shown in Fig. 6 has been computed. It has been found that the assumed strain field induces a position dependent period variation of ±0.26 nm and average index variation of ±1.6 × 10−4 of the FBG around their initial values and is shown in Fig. 7. The resulting spectrum as shown in Fig. 8 is broadened due to the induced chirp
Fig. 6. Position dependent strain as considered for sample #3.
Fig. 7. Variation of grating period and average index along the length of the grating due to the imposed strain as shown in Fig. 6.
Fig. 8. Simulated FBG spectrum for a position dependent strain as shown in Fig. 6.
with a symmetric split in the middle of the spectrum as observed experimentally. The degradation observed in sample #4 (Fig. 5) is comparatively more familiar and easy to interpret. Any form of ripples or side lobes in the higher wavelength side of an FBG spectrum indicates a depression in the average index in the middle of the FBG. For our simulation it has been assumed that a compressive Gaussian strain field as shown in Fig. 9 induces a position dependent average index change and generates chirping in the grating period as well. The computed chirp and average index variation for a peak compressive strain of −1000 ε at the middle of the FBG is shown in Fig. 10. The
Fig. 9. Position dependent strain as considered for sample #4.
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Fig. 10. Variation of grating period and average index along the length of the grating due to the imposed strain as shown in Fig. 9.
Fig. 11. Simulated FBG spectrum for a position dependent strain as shown in Fig. 9.
resulting spectrum for this type of strain field is shown in Fig. 11 where significant degradation of the FBG spectrum on the higher wavelength side may be observed and is similar to as has been found in sample #4. 4. Performance test of the packaged sensor embedded in concrete The strain response of the packaged sensors (samples #1 and #2) was tested by embedding the sensors in concrete and by applying compressive loads at a constant rate in a universal testing machine (UTM). Prior to the embedding of sensors in the concrete, thermal response and stability of the sensors were evaluated by applying consecutive temperature cycles from room temperature (∼25 ◦ C) to 100 ◦ C and the result obtained is shown in Fig. 12. The thermal response has been found to be linear with no hysteresis with a sensitivity of ∼15.4 pm/◦ C and similar to that of Stainless Steel 316 (Austenitic), the material which is used for fabricating the housing. Concrete cylinders were prepared by embedding one FBG sensor along with a procured EFPI optical sensor (make FISO) having a housing of similar type. Strain developed onto the FBG sensor during curing of concrete was also recorded. It is encouraging to observe very similar results for both samples #1 and # 2 in two different concrete cylinders of same material type which justifies perfect bonding of the housing in the concrete material. During curing the wavelength shift of the FBG sensors against curing time is shown in Fig. 13. Before placing the concrete cylinder in the UTM four electrical resistance strain gages (ERSG) of 60 mm gauge
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Fig. 12. Thermal response of one packaged sensor (sample #1).
Fig. 13. Wavelength shift of the FBG sensors against curing time during curing of the concrete.
length were bonded diametrically opposite on the outer surface of the concrete cylinder. FBG, EFPI and ERSG sensors were simultaneously monitored using Micron Optics sm130, FISO demodulator and strain gauge data logger, respectively. The experimental set-up is shown in Fig. 14. A maximum compressive load of 250 kN was applied to the concrete cylinder. Strain response was recorded at every 50 kN load step. Strain data as sensed by embedded FBG sensors were found to have good linearity against applied load and also have close matching with the other two types of the sensors used. Typical comparison is shown in Fig. 15. A preliminary characteristic of the FBG sensor under dynamic loading condition was also found satisfactory. Fig. 16 shows measured response of the FBG sensor when loading of the specimen was varied between 50 kN and 10 kN at 5 Hz. The measured strain (∼67 ε peak to peak) under this dynamic loading condition matches exactly with the strain measured during static load testing. 5. Discussion and conclusion Embedding FBG sensors in concrete structure requires suitable protective housing where a perfect bond between the housing and the surrounding concrete needs to be ensured for a faithful transfer of structural strain to the sensor. In this respect error free bonding of FBG sensor in the protective housing is also immensely important for reliable strain measurement. This paper has outlined the major aspects of encapsulation of FBG in a properly designed housing in regard to the curing of the bonding material and its effect on the FBG spectrum. It may be noted for a packaged sensor with ∼70 mm gauge length FBG of any arbitrary length may be cho-
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sen for encapsulation. However, as the cumulative induced strain developed on the FBG due to the shrinkage of the epoxy is length dependent and also possibility of degradation of reflection spectrum increases as well with the length of the FBG, in the present work a 5 mm Gaussian apodized FBG whose effective length is nearly 2 mm (considering the strength of apodization ∼0.4) has been considered to minimise effect of shrinkage induced strain on the FBG. It has been shown that rapid curing of thermally curable epoxy induces non-uniform strain gradient along the FBG and the sensors turn out to be unusable due to significant degradation of the spectrum. The strain fields that degrade the spectrum for the particular housing and the epoxy considered in this work can well be interpreted by simulating the spectrum using standard couple mode theory taking some specific strain fields in account. Preliminary experimentation shows sensor response is linear without any hysterisis under static and dynamic loading. More detailed performance test with these sensors like cycling test under compressive load upto elastic limit of the specimen, low cycling under high strain etc are underway.
Acknowledgements
Fig. 14. Experimental set-up to study strain response of packaged FBG and EFPI sensors embedded in concrete.
The authors acknowledge the Council of Scientific and Industrial Research (CSIR), India, for funding the research programme during 11th five year plan through network project (NWP026). The authors are thankful to all the technical staffs for their contribution during experiments and specially to Mr. Bipalb Mitra for developing Labview based data logging software.
References
Fig. 15. Strain response of embedded FBG, EFPI and surface mounted ERSG.
Fig. 16. Strain response of embedded FBG sensor under dynamic loading condition.
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Biographies P. Biswas received the B.Tech. and M.Tech. degree in Applied Physics from University of Calcutta in the year 1998 and 2000, respectively. He worked as a design engineer for packaging of photonic component in Celetron India from 2001 to 2004. From 2004 till to date he is in Central Glass and Ceramic Research Institute, Kolktata, India. His current research interest includes optical fiber Bragg grating and long period grating for application in acousto-ultrasonic measurement and detection of biological and chemical species. S. Bandyopadhyay received the B.Sc. and M.Sc. Tech. degree in Physics and Applied Physics from University of Calcutta in the year 1987 and 1990, respectively. He received Ph.D. Tech. in Applied Physics from the same university in 1998. He was then with Central Scientific Instruments Organisation, a National laboratory of Council of Scientific and Industrial Research (CSIR), India. From 2002 till to date he is in Central Glass and Ceramic Research Institute, Kolktata, India, which is another sister laboratory of CSIR. His current research interest includes optical fiber and optical fiber based sensors for ultra high temperature application and structural health monitiring. K. Kesavan is a scientist in Structural Engineering Research Centre, Chennai. He obtained his B.E. (civil) from Anna University, Chennai and M.Tech. (Structural Engineering) from Indian Institute of Technology, Chennai. He has been working in the area of experimental stress analysis since 1995. His current areas of interest include condition monitoring of structures, existing stress evaluation in prestressed concrete structures and health monitoring of civil engineering structures using fiber optic sensors. He has contributed more than 50 technical papers in journals and seminars and more than 100 technical reports.
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S. Parivallal obtained his B.E. (civil) from PSG College of Technology, Coimbatore and M.E (Structural Engineering) form Govt. College of Technology, Coimbatore. He has been with Structural Engineering Research Centre since 1994. His area of interest includes experimental Mechanics such as strain gage instrumentation, condition monitoring of structures, existing stress evaluation in prestressed concrete structures and smart civil engineering structures using fiber optic sensors. He has contributed more than 50 technical papers in journals and seminars and more than 100 technical reports. B. Arun Sundaram obtained his B.E. (civil) and M.E. (Structural Engineering) from Crescent Engineering College, Chennai and he has been with Structural Engineering Research centre since 2008. His current areas of interest include remote health monitoring of civil engineering structures, experimental stress analysis. He has contributed about 4 technical papers in journals and seminars and about 5 technical reports. K. Ravi Sankar is a deputy Director in Structural Engineering Research Centre (SERC), Chennai. He obtained his Bachelor’s degree in Civil Engineering in and Master’s degree in Structural Engineering from P.S.G. College of Technology, Coimbatore, and Ph.D. in Structural Engineering from Anna University, Madras. He has been working in the area of experimental stress analysis since 1980. He has vast experience in experimental techniques for stress analysis such as strain gages (fiber optics, vibrating wire and electrical resistance types), photo elasticity, holography, high sensitivity moire, etc., including on-site measurements and investigation of critical engineering structures. He has published around 100 technical papers and 200 technical/research reports. K. Dasgupta received M.Tech. degree in Microwave & Communication from Indian Institute of Technology, Kanpur in1978. He worked as a design engineer in Avionics Design Group (Hindustan Aeronautics Ltd., India) from 1978 to1981. He also worked as a senior engineer in Development Consultant Pvt. Ltd., India from 1982 to 1986. From 1986 till to date he is in Central Glass and Ceramic Research Institute, Kolkata, India. He actively worked for the development of EDFA and coordinated several National and International projects successfully in the area of fiber optic sensors. He has published about 100 papers in national and international journals/conferences. Presently he is the head of fiber optics laboratory of CGCRI. His current interest is towards development of FBG interrogation system.