TECHNIQUES OF OPTICAL FIBER BRAGG GRATING SMART SENORS AND INTELLIGENT MONITORING SYSTEMS OF INFRASTRUCTURES

Transcription

1 TECHNIQUES OF OPTICAL FIBER BRAGG GRATING SMART SENORS AND INTELLIGENT MONITORING SYSTEMS OF INFRASTRUCTURES OU Jinping ZHOU Zhi ABSTRACT For the purpose of long-term health monitoring of large civil infrastructures, the sensing principle of FBG sensors, FBG sensors encapsulation techniques, smart composite FRP-OFBG bars and the interface strain transfer mechanics have been studied and developed in this paper. The intelligent monitoring system based on FBG sensors has been developed, and the practical infrastructures, such as bridges and offshore platform under construction and in service have been monitored by FBG monitoring systems. The experimental and practical applications results show that FBG sensors are proper for long-term health monitoring of infrastructures. 1 INTRODUCTION Civil infrastructures, such as long-span bridges, high-rise buildings, large dams, nuclear power stations, offshore platforms and so on, will inevitably generate damage accumulation and resistance degradation subjected to coupling actions of environmental corrosion, material aging, long-term loading, fatigue and natural disaster hazards during long-term service, even collapse. Therefore, in order to assure structural safety, integrity, suitability and durability, lots of infrastructures in service are in great need of intelligent health monitoring systems to evaluate their safety and rehabilitate, further control their damages. Due to the recent disastrous lessons, more and more long-term heath monitoring systems have been added to infrastructures during construction [1~3]. However, infrastructures are large, long span and service for a very long time, so the durability and stability of the health monitoring systems are the key questions. Optical fiber sensor, especially optical fiber Bragg grating (FBG) shows distinguishing advantages: immunity to electromagnetic interference, high precision, durability, quasi-distribution, absolute measurement, compact size and so on. FBG has become one of the most prominent sensors for structural health monitoring. The first report of photosensitivity in optical fiber was attributive to Hill in 1978 [4], but the first practical demonstrations of grating formation was reported by Meltz [5]. Rapid progress has been made in both FBG sensor system developments and applications in recent years[6~1]. In this paper, the sensing principle of FBG sensors, FBG sensors encapsulation techniques, smart composite FRP-OFBG bars and the interface strain transfer mechanics have been studied and developed. The intelligent monitoring system based on FBG sensors has been developed, and the practical infrastructure, such as bridges and offshore platform under construction and in service have been monitored by FBG monitoring systems. SENSING PRINCIPLE OF FBG SENSORS

2 When light emitting from special broadband source passing through the FBG, a narrowband spectral component equaling the Bragg wavelength is reflected by the FBG, and this spectral component is missing in the transmission light. The basic principle of a FBG based monitoring system is to measure the shift of reflected Bragg wavelength, which is called Bragg wavelength(λ B ), depicted as expression (1) [9~1]. λ n Λ (1) B = eff where λ B, neff and Λ are central wavelength, effective refraction and the periodicity of FBG. Any perturbation that can change effective index ( n eff ) and periodicity (Λ) will result in a shift in Bragg wavelength. Strain and temperature are such measurands, on which structural monitoring has focused..1 STRAIN SENSING OF FBG The relationship between strain and the shift of FBG central wavelength is given as expression () dλb = ( 1 P) ε = K ε ε () λ b Where P = [ p1 υ ( p11 + p1 )] neff /, and p 11, p 1, υ and Kε are Pockel s coefficients of strain-optical tensor, Poisson ratio and strain sensitivity respectively. If the optical fiber core is silicon dioxide, the corresponding P is about 0., the strain sensitivity coefficient is about 1. pm / µε if the FBG wavelength is 1550 nm or so.. TEMPERATURE SENSING OF FBG The relationship between temperature and the shift of FBG central wavelength is given as (3) dλ 1 = ( α + ζ ) dt = K T dt (3) λ n dn 1 dλ where = ζ, = α and K T are thermal-optical tensor, thermal expansion dt Λ dt coefficient and temperature sensitivity respectively. If taken silicon fiber and 1550nm 6-1 wavelength into account, α + ζ / n is about , and the coefficient is about 10p m /. However, as for different FBGs, the strain and temperature sensitivity coefficients of FBG should be calibrated by experiments..3 TEMPERATURE COMPENSATIN FOR FBG STRAIN SENSORS The above data show that the wavelength shift induced by 1 is almost 10 times of that induced by 10 µε, which shows that temperature compensation is absolutely essential when FBGs are used for long-term strain monitoring. Many solutions to

3 temperature compensation have been brought forward, which include dual-parameter measurement, long-period gratings, polarization maintenance, and so on. But Considering the operational convenience, we think that the method of dual FBGs is the best one. The basic sensing principle for temperature compensation can be interpreted as (4)~(6). If the coupling effect of strain and temperature sensing of FBG is neglected, the sensing equations for the FBGs, one for strain and temperature while the other for temperature only, can be given as: λ ε (4) B 1 / λ B1 = K ε + K T 1 T1 λ (5) B / λb = K T T From (4) and (5), we can get the strain with temperature compensation: ε = ( λ B 1 / λ B1 K T 1 λ B /( K T λ B )) / K ε (6) 3 ENCAPSULATION TECHNIQUES FOR FBG SENSORS Due to fragility of bare FBG, it is hard to directly apply bare FBGs in infrastructures without any protection, so it is necessary to develop encapsulation techniques for bare FBG strain sensors. Two kinds of encapsulation techniques are developed for bare FBG strain sensors and one for FBG temperature sensors[13~15][18], depicted as Figure 1 and Figure. Figure 1 Sketch for mental capillary encapsulation for FBG strain sensor Figure Sketch for mental slice encapsulation for FBG strain sensor

4 Figure 3. Sketch for steel capillary encapsulation for FBG temperature sensor Due to that the encapsulation layers will change the sensitivity of the original FBGs, the encapsulated FBG sensors must be calibrated before they are applied in practical structures. 4 FIBER REINFORCED POLYMER (FRP) -OFBG BARS FRP is now more and more accepted as a kind of important construction material. To make full use of FRP s strength properties and FBG s sensing properties, Prof. OU (00) has developed the fabrication technique of FRP-OFBGs bars and gotten the products, shown as Figures 4 and 5. FRP bar embedded with OFBGs can not affect its mechanical properties because OFBG diameter is by far smaller compared with FRP bars diameter[1][15][16]. Figure 4. OFBG-GFRP bars Figure 5. OFBG-GFRP bars The sensing tests are carried out using material test system (MTS). And the coefficient of CFRP-OFBG and GFRP-OFBG bars are 1. pm/µε or so, which shows that the FRP does not affect strain sensing properties of optical FBG. In order to test the sensing properties of FRP-OFBG bars in concrete structures, twelve concrete beams were built. The beams are subjected to third-point flexural bending testing, and the electric resistance strain gage is adhered to the concrete at the same height with FRP-OFBG bar. The load is applied to the beam step by step by means of one hydraulic jacks. The beams are instrumented with three linear variable differential transformer (LVDT) at supports and midspan to monitor deflection. Load, mid-span deflection, strain gage reading and wave length are recorded at the end of each step. The typical test results are shown in Figures 6 and 7.

5 Load (kn) Strain (με) FBG at 1/3 span FBG at abutment FBG at mid-span Extensometer Load (kn) Strain (με) FBG at mid-span FBG at 1/3 span FBG at abutment Extensometer Figure 6. Load-strain relationship of CFRP-OFBG beam Figure 7. Load-strain relationship of GFRP-OFBG beam The maximum strain of FRP-OFBG bar is 1000 µε. When concrete uncracked, the strain of FRP-OFBG agree well with concrete strain therefore they can work together. In load-unload loop the FRP-OFBG bar can work well. The FRP-OFBG bar can monitor the slip between FRP bar and concrete by means of the difference FRP strain and concrete strain. The results show that the smart FRP-OFBG composite bar is wonderful for its mechanical and sensing properties, so it is a kind of perfect material for civil engineering, which is not only structural material but also function material. As strain sensor, FRP-OFBG completely overcomes the difficulties of FBGs installation due to their fragility, and can be used as local damage detection sensors. 5 INTERFACE STRAIN TRANSFER MECHANICS OF FBG SENSOR[1] In Figure 8, based on the relationship of deformation for the cylindrical model of FBG sensor, considering the equilibrium equations of infinitesimal, the axial strain, ε x, in the fibre core region is given by c ( ) ( λx) ( ) λl ( ) () sh ε = c x ε h 0 1 (7) sh f Where ε h ( 0) is the axial strain of the host material at x = 0, and parameter λ is a function of the material properties and geometric factors of the adhesive layer, protective coating and bare optical fiber If the fibre is very short or long, λ can be given as λ 1 and λ : λ 1 =, 1 rh 1 rap Ecrc ln + ln Ga rap Gp rc Ecrc h z h λ = (8) 1 rh 1 rap G a ln r ap by + E S r + G p ln r c

6 Adherent Coating Fibre core Adherent r Host material l f r h r c r ap x Host material Adherent Coating Fibre u h ( x) u c ( x) a( x) p( x) Figure 8. Relationship of deformation for cylindrical model of optical fiber strain sensing nth ith nd 1st Fibre r 0 r 1 r r i r n load (kn) FBG measurement result FBG error modificaiton result practical value Strain ( με ) Figure 9. Sketch of n layers coating for optical fiber Figure 10. Comparison between the results from error modification and the original Furthermore, in Figure.9, under the n-embedment layers the united form of λ is given by by + E E S r 0r0 h z n λ 1 =, λ n = (9) n 1 ri 1 ri E r 0 0 i= 1 Gi r i 1 i= 1 Gi ri 1 Based on the definition of average strain, the error rate η and the modification coefficient k of FBG sensor is obtained as ( λl f) 1 sh( λl ) ch η =, λl f f 1 k = (10) 1 η Figure 10 shows that the results of experiments after modified are consistent with the accurate solution. The data are from the experiment of FBG-OFBG concrete beams mentioned above.

7 6 INTELLIGENT MONITORING SYSTEMS BASED ON FBGS AND THEIR APPLICATION IN CIVIL INFRASTRUCTION 6.1 INTELLIGENT MONITORING SYSTEMS BASED ON FBG SENSORS The intelligent monitoring system based on FBG sensors include FBG sensors, optical coupler(optical switch), FBG interrogator, transmission cable, jumpers and relative software[1], which shown as figure 11(a)~(c). computer FBG Interrogator Jumper Jumper Optical switch ( Coupler). Optical switch (Coupler) Optical coupler. Transmission cable Transmission cable. FBG1 FBG FBGn FBGn (a) Construction of intelligent monitoring system based on FBG sensors (b) Practical devices for monitoring system based on FBG sensors (c) Software of intelligent monitoring system based on FBG sensors

8 Figure 11. Intelligent monitoring system based on FBG sensors 6. FBG SENSORS APPLICATION IN BRIDGES[15] Due to that the bridges is the key part of transportation, lots of large-span bridges are under construction in developing countries as well as China. In order to avoid disastrous tragedy to happen and with the development of structural health monitoring, the bridge owners realize the importance of adding structural health monitoring system to the bridges under construction. OU (003) have developed several structural health monitoring systems based on FBG sensors to be applied in the several large-span bridges, shown as figure 1~19, to monitor the performance of the bridges under construction and in service. Figure 1. Fulan river bridge in Heilongjiang Figure 13. Niutoushan Bridge in Heilongjiang Figure 14. Songhuajiang River Bridge in Heilongjiang Province Figure 15. Dongying Yellow River Bridge in Shandong Province Figure 16. Binzhou Yellow River Bridge in Shandong Province Figure 17. Maocaojie Bridge in Hunan Province

9 Figure 18. Luoguo Bridge in Panzhihua of Sichuan Province 6.3 FBG SENSORS APPLICATION IN OFFSHORE PLATFORM[15] Offshore platform is the key infrastructures for oil exploitation, which serve in extremely bad environments with wind, wave, ice and ocean current, even corrosion, so the life of the offshore platform are very short, 0 years or so. It is in great need to real-time evaluate the safety and predict the remaining life of the platform. Under the support of 863 High Tech. Research Program, OU and Duan (00) have developed a large real time structural health monitoring system to monitor the global and local performances of the platform, such as acceleration, strain, temperature and crack and so on, and even the environment load mentioned above by the ocean weather observation station. According to the monitoring information and the designed model, the safety state of the platform can be given real time. CB3A located in Bohai Bay is the application demonstration of the project achievement, show as Figure 19, where the intelligent monitoring system based on FBG sensors are applied. Figure 19. CB3A platform in Bahai bay 7 CONCLUSIONS This paper has discussed the sensing principle of FBG sensors, FBG sensors encapsulation techniques, smart composite FRP-OFBG bars and the interface strain transfer mechanics. The intelligent monitoring system based on these sensors has been developed, and the practical infrastructure, bridges and offshore platform under construction and in service monitored by FBG monitoring systems are also briefly

10 introduced. From the research results and experience, the techniques of FBG sensors is proper for long-term health monitoring system, especially for the civil infrastructures. ACKNOWLEDGEMENT This paper is supported by the Chinese 863 High Tech. Research Project (Granted No.001AA6003 and 00AA ), NFSC( Granted No ) and Chinese Postal doctoral foundation. REFERENCES 1. Ou J.P Damage accumulation and safety evaluation for important large infrastructures. 1st century s Chinese mechanics-9th science association reports of forum for youth scientist. Beijing, Tsinghua university press: 179~189,. Ou J.P., Guan X. Ch State of art for intelligent structure and system in civil engineering. Earthquake Engineering and Engineering Vibration,19(): 1~8,. 3. Housner G W Structure Control: Past, Present, and Future. Journal of Engineering Mechanics, 13(9): 897~ Hill K O Photosensitivity in optical fiber waveguides application to reflection filter fabrication. App. Phys. Lett., 3(10): Meltz G., Morey W.W., Glenn W.H Formation of Bragg gratings in optical fibers by a transverse holographic method. Optical letter, 14 (15): Rao, Y.J Recent progress in applications of in fiber Bragg grating sensors. Optics and lasers in Engineering, 31: Tennyson,R.C., T. Coroy G.Duck et al. Fiber optic sensors in civil engineering structures. J. Civ. Eng.: , 8. Nellen, P.M., Rolg Bronnimann, Fank A Structurally Embedded fiber Bragg gratings: Civil engineering applications. SPIE, Vol.3860: Nellen, P.M., P. Anderegg, Rolf Br Bronnimann Application of fiber optical and resistance strain gauges for long-term surveillance of civil engineering structures, SPIE, VOL.3043: Fuh, P. L. r, S. Spammer Fiber optic sensors in the Waterbury Bridge. SPIE, 3489:14-19, 11. Sein J., Eric Udd, W. Schulz Health monitoring of an Oregon historical bridge with fiber grating strain sensors. SPIE. Vol. 3671: Zhou Z Optical Fiber Smart BRAGG Grating Sensors and Intelligent Monitoring System in Civil Infrastructures, Doctor Dissertation, Harbin Institute of Tech., Harbin, China. 13. Zhou Z., Wu Zh.J. Ou J. P. 00. Technique and Application of In-situ Monitoring for Concrete Structures with FBG Sensors, Pacific Science Review, Far East Technical University of Russian,Vlasivostok,Vol Ou J.P. Zhou Z.,Wu Zh. J The sensing properties and practical application in civil infrastructures of optical FBGs. SPIE., Ou J. P Some recent advance of intelligent monitoring system for civil infrastructures in mainland China. Proceeding of the first international conference on structural health monitoring and intelligent Infrastructures, 13~15, November, 003, Tokyo, Japan. A.A. Balkema Publishers, Netherlands: Zhou Z. OU J. P. And Wang B Smart FRP-OFBG bars and their application in reforced concrete beams. Proceeding of the first international conference on structural health monitoring and intelligent Infrastructures, 13~15, November, 003, Tokyo, Japan. A.A. Balkema Publishers, Netherlands: Zhou Z., Ou J. P Embedded FBG Strain Sensors and its Bridge Monitoring Technique. IMAC1. Florida, USA, 003, 18. Ou J.P. Zhou Z. Zhao X. F Encapsulation Techniques for FBG and Smart Monitoring for Bridges with FBG Sensors, Proceeding of the 4th International Workshop on Structural Health Monitoring at Stanford University, September 15~17,003, Stanford University.

11