J Opt DOI 10.1007/s12596-015-0266-5
RESEARCH ARTICLE
Bond monitoring in temperature dependent applications using Brillouin optical time domain analyser M. Kasinathan 1 & C. Babu Rao 1 & N. Murali 1 & T. Jayakumar 1 & Aleksander Wosniok 2 & Katerina Krebber 2
Received: 19 December 2014 / Accepted: 17 August 2015 # The Optical Society of India 2015
Abstract Adhesive bond has to be evaluated for its integrity over a range of temperature. Adhesive is being used to bond the sensors with structures. There is no validated technique to test its performance. In this paper, we propose a Brillouin Optical Time Domain Analyzer (BOTDA) based methodology to detect temperature-induced adhesive bond failure below room temperature using distributed fiber optic sensor. The differential coefficient of thermal expansion of the structure and fiber sensor can lead to bond failure at low temperature. Optical fiber impregnated in the structure will experience differential temperature/strain due to debond of the adhesive. This leads to the frequency and amplitude decomposition of the Brillouin spectra. This is a good indication for real-time monitoring of the integrity of a bond. Keywords Adhesive bond . Brillouin scattering . Fibre sensor . Spectral decomposition . Time domain
Introduction Brillouin Optical Time Domain Analyser (BOTDA) is used for various applications in measuring temperature and strain [1–3]. Temperature and strain sensors are used for structural health monitoring in the areas such as civil, mechanical and aerospace applications [4–6]. In this paper, we study the debond of adhesive used to bond the optical fiber to the * M. Kasinathan
[email protected] 1
Indira Gandhi Centre for Atomic Research, Kalpakkam, India
2
Federal Institute for Materials Research and Testing (BAM), Berlin, Germany
structure. The bonding of the fiber sensor to the structure/ host material is critical to transfer the strain truthfully to the fiber sensor which is coupled with the bonding material. Generally three approaches are being followed to attach the optical fiber sensor to the structures. They are soldering, laser welding and adhesive bonding. Among these approaches, adhesive bonding is preferred due to its light weight, better bonding capability and flexible curing methods for different materials [7]. High performance adhesives are used in the field of aeronautics, aerospace, automobiles and other areas like optics, electronics, electrical systems where high strength bonds are required [8]. The epoxy based materials are commonly used in room temperature applications and its performance is well documented. There are several high temperature bonding materials such as high content silica-filled ceramic epoxy used for application for 350 °C, ceramic cements up to 700 °C and pure aluminum oxide sprayed up to 1200 °C [9]. The silicone based adhesives are commercially available and used as structural adhesives. Based on elastomeric technology, silicone adhesives offer flexibility and high heat resistance, making them suitable for a wide range of applications such as electrical, electronic, automotive, aerospace and construction industries. It is a water repellent material that contains silicon as base material and can withstand upto 315 °C. Silicone adhesives are creamy & consistent and it will set immediately on air exposure (http://www.masterbond.com/techtips/why-usesilicone-adhesive). In order to test the adhesive bonds, a high intensity laser short pulse is used in laser based adhesion tests. It generates an elastic shock wave that debonds the weak adhesive bonds, making the defect visible for other Non-destructive Testing (NDT) techniques (Visual inspection, Thermography, Ultrasonic scan, holography etc.) [10]. Although often ignored as a NDT technique in certain cases visual inspection can be one of the most reliable and rapid technique available. This has been
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used for rapid screening of structure and the sensor, where a transparent adhesive is used to bond the sensor on the structure and the bonding region to be clearly visible. The visual inspection tests are commonly used as non-destructive technique in nature and can be performed only after prescribed test but not in real time. Non-destructive techniques like acoustic emission and ultrasonic inspection are used for detection of failures in bonds. In acoustic emission inspection, the transducer is mounted on the section of the inspection using compression mounts or couplants. The compression mount uses a spring or mounting mechanism which acts as an undesirable source of stress on the system and may lead to detection of false failure events. Acoustic emission cannot be used after the occurrence of the event/failure. Also it is not effective against static conditions [11]. Ultrasonic inspection can be used for identification of structural defects in the bond. The technique is based on the propagation of ultrasonic pulses of the medium, which is reflected on encountering a change in the impedance due to variation in the density of the material. The attenuated/ reflected signal provides information about the defects in the bonding material / bonding interface. However, the information is averaged behavior of the bond. Electrical resistance strain gauges are used to monitor failure of the adhesive at cryogenic temperature [8]. A discontinuity due to thermal expansion of aluminium/ composite joint and its bond was observed by strain gauges which are adhered to the corners of interfaces. The strain values are correlated with the failure of the bond. This behavior is observed at room temperature and also at cryogenic temperature. Low-temperature adhesives, call for reliable adhesion at room temperature and below. Also, it is desirable that the bonding will withstand a reasonable number of thermal cycles. Optical fiber sensors have earlier been used for monitoring bond failure of composites under the influence of mechanical loading [12, 13]. I.McKenzie et al. has demonstrated the monitoring of load transfer processes of composite structure repairs through an array of Fiber Bragg Gratings (FBG) for the in-situ monitoring of bonded repairs. In this an indication of the integrity of the repair in aircraft structures particularly, the monitoring of crack propagation beneath a repair [14]. P. V. Wnuk et al. has discussed about the process for mounting and packaging of FBG strain sensors in harsh environmental applications. FBG sensor bonding depends mainly on the optimum curing of adhesive, coefficient of thermal expansion and the adhesion properties of the constituent materials [9]. M. A. Uddin et al. reported on the inter-facial delamination due to uneven curing of ultraviolet adhesive in the fixing process of a fiber on a V-groove [15, 16]. In previous study, monitoring of the adhesive bond between temperaturedependent applications using FBG sensor, with spectral decomposition has successfully been used to detect the bond
failure of the structure [17]. Optical fiber can flip off from the structure of bond failure [18, 19]. Temperature effects on adhesive bond strength are performed in modulus for space craft [20]. Each temperature variance has an effect on the performance of the adhesive. Applications were discussed for high temperature and low temperature adhesives. The parametric analysis of the strain on the fiber for varying bond surface conditions has been studied by R. Bernini et al. [2]. In this paper, integrity testing of adhesive bond using application of BOTDA is discussed in detail. Adhesive bond failure detection from structure of fiber optic sensors using BOTDA is not reported. Also, we demonstrate the application of fiber optic sensor using BOTDA to evaluate the performance of the adhesive bond below room temperature in real time. This method compares the performance of the bonds for monitoring the frequency and amplitude of Brillouin spectra of optical fiber.
Experimental details Two single mode fibers (SMFs) are bonded with the surface of a stainless steel (SS) plate structure of the silicone adhesive (Fig. 1). The dimension of the SS plate structure is 100 cm length, 10 cm width and 1 mm thick. Prior to bonding, the surface is cleaned thoroughly using emery paper, washed and dried. After that the bond is allowed to cure for 24 h at room temperature. It is thus intended to monitor temperature through the BOTDA using fiber sensor. Controlled heating arrangements are made by using environment chambers upto 90 °C. The temperature is raised from −40 to 90 °C in steps of 10 °C by setting the temperature in the environment chamber. The temperature is allowed to stabilize before the Brillouin measurement is taken. The two bonds will be referred as bond-S1 and bond-S2 with the corresponding sensor fibers. The Brillouin frequency of SMF is 10.87 GHz at room temperature and that of the corresponding pump signal wavelength is 1.55 μm. Prior to bonding, each of the sensor fibers is tested for its temperature response to 100 °C. The acrylic coating of SMF withstand the temperature upto 100 °C and its Brillouin frequency shift also noticed. The block diagram of the experimental setup with BOTDA is shown in Fig. 2. Sensor fiber is connected to both the ports of the BOTDA, like a loop. These ports provide pulsed pump signal and continuous Stokes signal. The back scattered light
Fig. 1 Fiber sensor bonded onto a metal plate
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Metal plate
BOTDA From FOS
Fig. 2 Experimental set-up with BOTDA
is collected by this system through pump port after the three wave interaction. The Brillouin frequency shift depends on the velocity of the acoustic signal, which is produced by strong pump pulses of electrostriction principle. The Brillouin measurements of the two bonded SMFs are obtained using a Fiber Optic Distributed Temperature and Strain Analyser (DITEST STA-R, OMNISENS). The BOTDA is configured to follow 1 m spatial resolution of a sampling interval of 50 cm. The temperature is measured and the corresponding Brillouin frequency and amplitude are obtained using BOTDA. The basic principle of temperature/strain measurement using Stimulated Brillouin Scattering (SBS) is extensively discussed in the literature [1, 21, 22]. In this study, BOTDA works on the principle of Optical Time Domain Reflectometer (OTDR) is used [4]. The Brillouin frequency shift is given by νB ¼
2nν a λi
ð1Þ
where n is the refractive index of the fiber, νa is the velocity of the acoustic wave and λi is the wavelength of the laser input. Since n and νa are dependent on temperature or strain, these parameters are measured in terms of νB. The relation between the temperature (T) and the strain (ε) is given in the literature [1]. The spatial resolution ‘dz’ in BOTDA usually depends on the incident pulse width τ, as with conventional OTDR and is determined by dz ¼ cτ=2
ð2Þ
where c is the velocity of light in optical fiber. The measured temperature/strain magnitude is the average magnitude in the dz region. In contrast with OTDR, dz is limited not only to τ but also by νB, it depends on the optical wavelength [22].
Fig. 3 Frequency profiles for bond intact (bond - S1)
bond-S1 is found to vary monotonically from a function of temperature (Fig. 5) and the other bond-S2 shows a discontinuity at 30 °C (Fig. 6). At 30 °C, the Brillouin frequency profile is split into many frequencies. Below this temperature, the measured Brillouin parameters such as frequency and amplitude are exhibiting random behavior. These parameter drops and subsequent random behavior are attributed as debonding of the fiber sensor from the metal plate. However, the frequency decomposition has utilized for bond-S2 at 30 °C, (Fig. 7). The frequency decomposition has continued to be present at −40 °C. The origin of splitting of the frequencies is explained in the following way. The bond fails at lower temperatures, due to different coefficient of thermal expansion of SS plate and the bond material. Thus the adhesive and the fiber sensor impregnated in the structure experience differential expansion of the effect of temperature/strain. As a result, fiber on the SS plate is effectively divided into groups of debonding in bonding sections, each with different debonding/bonding spacing. Each debonding with specific spacing gives a specific frequency. Also, the frequency split
Effect of bond failure and analysis As the temperature of the SS plate structure is varied from −40 to +90 °C, the Brillouin frequencies have been observed to undergo changes due to temperature variations [1]. The bondS1 is healthy (subjected to one temperature cycle) and the bond-S2 is failed when subjected to 3rd temperature cycle. Figure 3, shows the frequency profile of the bond-S1 as a function of temperature. The frequency profile of failed bond-S2 is shown in Fig. 4 as a function of temperature. The
Fig. 4 Frequency profiles for bond failure (bond - S2)
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Fig. 5 Brillouin frequency shift vs Temperature oC for Bond-S1 Fig. 7 Brillouin frequency shift vs Temperature OC for Bond - S2
in debonding is lower than the bonding fiber frequencies (Fig. 7). The higher number of debonding on the bond, the more number of such groups, which contributes to increase the number of smaller frequencies. It can be generalized that if ‘n’ frequencies are observed, then there are at least (n-1) debonds that have developed in the SS plate. It has been reported that a change in the slope of the strain curve is an indication of the failure of a bond [8]. A Brillouin scattering is a measure of the change in temperature and change in strain on distributed fiber optic sensor in terms of Brillouin frequency and Brillouin amplitude [6]. The frequency (Fig. 7) and amplitude (Fig. 8) of failure bond-S2, clearly shows a drastic slope change and which are corresponding to frequency/amplitude decomposition from 30 °C and downwards. The frequency/amplitude decomposition is explained as follows, The BOTDA is configured to monitor the frequencies of specified spatial resolution (1 m). It is not configured to detect closely spaced multiple frequencies. The uncertainty of
tagging the frequencies results in an abnormal change in the slope. However, the response to the frequency of bond-S1 remains linear with no frequency decomposition upto 90 °C (Fig. 5). This could be attributed to the relaxation of residual tensile stress present in the SS plate. Since, Brillouin frequency and amplitude behavior is same, the explanation is applicable to amplitude as well. These bonds are subject to a visual inspection to confirm the intactness of bond-S1 and bond-S2. Bond-S2 (Fig. 9b) has debonded from the surface of the SS plate and bond-S1 is intact (Fig. 9a). This clearly indicates that bond-S2 could not withstand thermal cycles and failed. This test confirms that slope change is not an indication of the failure of a bond. Since, there is no discontinuity in the slope of bond-S1, the bond is still intact. The relaxation of residual stress present in the SS plate may contribute to slope change, which may lead to false indication of the failure of a bond. It is shown that the
Fig. 6 Brillouin amplitude vs Temperature oC for Bond - S1
Fig. 8 Brillouin amplitude vs Temperature oC for Bond - S2
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Bond intact
(a) Fiber intact on the structure
Bond Failure
(b) Debond Fiber from the structure frequency and amplitude decomposition are the indications of temperature-induced failure of the bond than a change in slope. Especially, the fiber debond length (ΔL) region in the bonded fiber length (L0) are divided among the initial fiber bond length ΔL/L0 yield to the temperature/strain induced debonding. The Brillouin frequency/amplitude measured by BOTDA in the debond location is much lower as compared to the actual frequency/amplitude at room temperature. The spatial resolution limits the actual frequency/ amplitude of measured parameters by BOTDA. If the debonding region, smaller than the spatial resolution of the BOTDA, the measured frequency/amplitude is lowered by approximately the ratio of the debonding length of the spatial resolution [23]. In spite of the limitation of the spatial resolution, the different rate of change of frequency/ amplitude with temperature at the debond region and the normal region of the bond with the SS plate is clearly revealed by the BOTDA measurements. Naked eye visual inspection of the bonds structure is preferred over examination of a photograph as identifying marks on the bonds, which are intact with structure and under the debonding becomes easier.
detection of Brillouin spectral decomposition. This technique is more reliable compared to monitor the change in the slope for failure of the bond. The results are verified with Brillouin parameters, i.e. Frequency shift and amplitude. Simultaneous analysis of temperature induced debond through Brillouion frequency/amplitude is obtained. It is might not be detected if the frequency profile as a function of temperature was solely studied. This type of analysis could be recommended as a procedural routine testing for fiber deployed in operating temperatures. In practice, the number of bonds should be studied for a given composition / preparation and procedure of bonding for reliable evaluation.
References 1.
2.
3.
Conclusions 4.
Temperature induced bond failure is detected using the proposed BOTDA technique on the basis of Brillouin spectral decomposition. The fiber debond has been observed using BOTDA based on the results of temperature cycles. Adhesive debond has been detected on distributed fiber sensors and its temperature dependence also has been noticed. It demonstrated that the random decomposition of Brillouin frequency shift and amplitude attributed to the failure of the bond. In addition to that the visual inspection also supported the debond
5.
6.
X. Bao, J. Dhilwayo, N. Heron, D.J. Webb, D.A. Jackson, Experimental and theoretical studies on a distributed temperature sensor based on Brillouin scattering. J. Lightwave Technol. 13(7) (1995) R. Bernini, A. Minardo, L. Zeni, Accurate high-resolution fiberoptic distributed strain measurements for structural health monitoring. Sensors Actuators A 134, 389–395 (2007) T. Gogolla, Krebber, Fiber sensors for distributed temperature and strain measurements using Brillouin scattering and frequency domain methods. SPIE Vol.3105.0277-786X/97. X. Bao, F. Ravet, L. Zou, Distributed Brillouin sensor based on Brillouin scattering for structural health monitoring. Optical Fiber Communication Conference, 2006 and the 2006 National Fiber Optic Engineers Conference. OFC 2006. IEEE, Anaheim, CA, 510 March 2006 M. Basjar Rao et al., Structural health monitoring using strain gauges, PVDF film and Fiber Bragg Grating sensors: a comparative study. Proc-National Seminar on Non-Destructive Evaluation, Hyderabad, Dec 7–9, (2006) M. Nikles, Fiber optic distributed scattering sensing system: perspectives and challenges for high performance applications. Proc.of SPIE Vol.66190D-1, (2007)
J Opt 7. 8.
9.
10.
11.
12.
13.
14.
15.
G.Z. Xiao, Z. Zhang, C.P. Grover, Int. J. Adhes. Adhes. 24(4), 313 (2004) Strain gauges indicate differential-CTE-induced failures, goddard space flight center, Greenbelt, Maryland Sunday, NASATech Brief, April 01–2007. P.V. Wnuk, A. Mendez, S. Furguson, T. Graver, Process for mounting and packaging of fibre Bragg grating strain sensors for use in harsh environment applications. Smart Structures Conference, SPIE paper 5758-6, (2005) B. Ehrhart, B. Valeske, C.-E. Muller, C. Bockenheimer, Methods for the quality assessment of adhesive bonded CFRP structures - a resume, 2nd international symposium on NDT in aerospace 2010 We.5.B.2, License: http://creativecommons.org/licenses/by-nd/3.0 L. Goglio, M. Rossetto, Ultrasonic testing of adhesive bonds of thin metal sheets, Corso Duca degli Abruzzi, 24610129 Torino, Italy. S. Takeda, Y. Yamamoto, Y. Okabe, N. Takeda, Debonding monitoring of composite repair patches using embedded small diameter FBG sensors. Smart Mater. Struct. 16, 763–770 (2007) D.C. Seo, J.J. Lee, I.B. Kwon, Monitoring of fatigue crack growth of craced thick aluminum plate repaired with a bonded composite patch using transmission type extrinsic Fabry-Perot interferometric optical fiber sensors. Smart Mater. Struct. 11, 917–924 (2002) I. McKenzie, R. Jones, I.H. Marshall, S. Galea, Optical fibre sensors for health monitoring of bonded repair systems. Compos. Struct. 50, 405–416 (2000) M.A. Uddin, H.P. Chan, K.W. Lam, Y.C. Chan, P.L. Chu, K.C. Hung, T.O. Tsun, Delamination problems of UV-cured adhesive
bonded optical fiber in V-groove for photonic packaging. IEEE Photon. Technol. Lett. 16(4) (2004) 16. M.A. Uddin, H.P. Chan, T.O. Tsun, Y.C. Chan, Uneven curing induced interfacial delamination of UVadhesive-bonded fiber array in V-groove for photonic packaging. IEEE J. Lightwave Technol. 24(3), 1342 (2006) 17. S. Sosamma, M. Kasinathan, C. Pandian, C. Babu Rao, T. Jayakumar, N. Murali, B. Raj, Bond monitoring in temperaturedependent applications by FBG spectra decomposition. Insight 52(11) (2010) 18. M.A. Uddin, W.F. Ho, H.P. Chan, J, Achieving optimum adhesion of conductive adhesive bonded flip-chip on flex packages, Mater. Sci. – Mater. Electron. 18(6), 655 (2007) 19. K.W. Lam, M.A. Uddin, H.P. Chan, Reliability of adhesive bonded optical fiber array for photonic packaging. J. Optoelectron. Adv. Mater. 10(10), 2539–2546 (2008) 20. C.E. Ojeda, E.J. Oakes, J.R. Hill, D. Aldi, G.A. Forsberg, Temperature effects on adhesive bond strengths and modulus for commonly used spacecraft structural adhesives, http://trs-new.jpl. nasa.gov/dspace/bitstream/2014/42025/1/11-0955.pdf 21. M. Nikles, L. Thevenaz, P.A. Robert, Brillouin gain spectrum characterization in single-mode optical fibers. J. Lightwave Technol. 15(10) (1997) 22. X. Bao, J. Smith, A.W. Brown, Temperature and strain measurements using the power, line-width, shape, and frequency shift of the Brillouin loss spectrum. Proc. SPIE 4920, 311–322 (2002) 23. X. Bao, D.J. Webb, D.A. Jackson, Temperature non-uniformity in distributed temperature sensors. Electron. Lett. 29, 976–977 (1993)
