Hybrid optical fiber sensor system based on fiber Bragg gratings and plastic optical fibers for health monitoring of engineering structures

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1 Hybrid optical fiber sensor system based on fiber Bragg gratings and plastic optical fibers for health monitoring of engineering structures K.S.C. Kuang, M. Maalej, S.T. Quek, Department of Civil Engineering, E1A #7-3, 1 Engineering Drive 2, National University of Singapore, Singapore ABSTRACT In this paper, packaged fibre Bragg grating (PFBG) sensors were fabricated by embedding them in 7mm x 1mm x.3mm carbon-fibre composites which were then surface-bonded to an aluminium beam and a steel I-beam to investigate their strain monitoring capability. Initially, the response of these packaged sensors under tensile loading was compared to bare FBGs and electrical strain gauges located in the vicinity. The effective calibration constant/ coefficient of the PFBG sensor was also compared with the non-packaged version. These PFBG sensors were then attached to an I-section steel beam to monitor their response under flexural loading conditions. These realistic structures provide a platform to assess the potential and reliability of the PFBG sensors when used in harsh environment. The results obtained in this study gave clear experimental evidence of the difference in performance between the coated and uncoated PFBG fabricated for the study. In another experimental set-up, bare FBG and POF vibration sensors were surface-bonded to the side-surface of a CFRPwrapped reinforced concrete beam which was then subjected to cyclic loading to assess their long-term survivability. Plain plastic optical fibre (POF) sensors were also attached to the side of the 2-meter concrete beam to monitor the progression of cracks developed during the cyclic loading. The results showed excellent long-term survivability by the FBG and POF vibration sensors and provided evidence of the potential of the plain POF sensor to detect and monitor the propagation of the crack developed during the test. Keywords: plastic optical fibres; POF, Fibre Bragg gratings; smart structures, optical fibre sensor, structural health monitoring, cracks, concrete structures, damage detection, strain monitoring. 1. INTRODUCTION Structural health monitoring is an emerging field which involves the detection, localization and assessment of these damages within the structures using strategically-placed sensors thus allowing engineers to estimate the remaining life of the structure readily and objectively. Civil infrastructures such as bridges and off-shore platforms, oil and gas pipelines, aircrafts, marine vessels and machineries are examples of the application domains where continuous health monitoring of their structural integrity constitutes an important element in ensuring their safe and efficient operations. Photonic technologies have, in recent years, enabled the development of a variety of optical fibre-based sensors. These optical fibre-based sensors offer significant advantages over their conventional electronic-based counterparts which include immunity to electromagnetic interference, multiplexing capability, long term strain monitoring, high measurement resolution, their being corrosion and spark-free, and the ease of embedding them in advanced composite materials. Fibre Bragg gratings, for example, offer excellent potential as sensors for structural health monitoring in view of their inherent signal stability and therefore their suitability for long-term strain-monitoring. In addition, this class of optical fibre sensor system offers the possibility of interrogating multiplexed sensors using a single channel offering ease in the handling of a network of sensors since minimal number of cables and acquisition channels will be required. In cveksck@nus.edu.sg; phone ; fax ; Smart Structures and Materials 26: Sensors and Smart Structures Technologies for Civil, Mechanical, and Aerospace Systems, edited by Masayoshi Tomizuka, et al., Proc. of SPIE Vol. 6174, 61742P, (26) X/6/$15 doi: / Proc. of SPIE Vol P-1

2 principle, a single channel is capable of interrogating tens of multiplexed sensing units limited only by the intensity of the light source, bandwidth and dynamic range. In order that these fragile optical fibres survive the typical harsh engineering environment, the sensors require rugged packaging to improve the ease of handling during transportation and installation. Since optical fibres are suited for embedment in unidirectional fibre composite prepregs, these host materials are ideal for packaging purposes. Following a suitable curing regime (for a thermosetting epoxy system), the packaged fibre Bragg grating (PFBG) sensor can be readily attached to the flat surface of any test structure. However, when an FBG is packaged and attached to a structure with adhesive, the strain experienced by the grating of the fibre is expected to be less than that applied to the host structure. The reduction in the strain experienced by the grating is due to strain transfer inefficiency from the host structure to the grating region of the fibre through the intermediate mediums namely (a) the adhesive used to bond the packaged FBG to the structure, (b) the layers of composite packaging material and (c) the optical fibre coating (if not already removed prior to embedment). In the majority of FBG sensor studies reported, they have been used in its bare form (i.e. without packaging) [1-4]. Bare FBG sensors are highly susceptible to accidental fracture during handling and are not amenable for used in harsh engineering sites although under laboratory conditions they have been successfully surface bonded or embedded in composite materials to detect damage or to monitor the structural response of the host material. Also, in practice, prior to embedding the FBG in composite prepregs, the coating layer around the grating region of the optical fibre is removed in order to optimize the strain transfer from the composite host to the grating. This process of removing the recoat layer requires skill and care and is inconvenient to perform on site. In addition, this process can be time consuming where large number of sensors is used and the removal of the recoat layer further increases the difficulty in transporting them safely. It is interesting to note, however, that in a recent study by Leibid et al. [5] acrylate-recoated FBG fibres were embedded (with fibre coating intact) in composite materials and the sensor was shown to respond reliably under axial strain and vibration tests. The composite host used in their study was the load-bearing structure while in this present study, the composite material acts as a protective packaging and is attached to a steel I-beam. It is therefore of interest the compare the response of coated and uncoated packaged sensors and demonstrate the effects of packaging on the sensor functionality. The benefit of a rugged packaging is that it allows operators, given minimal instructions, to install and use these packaged sensors readily with virtually 1% sensor survivability. There are limited studies on the response of these packaged FBG sensors when used for strain monitoring applications and it would of be of interest to gain an insight into their performance. In this study, we have fabricated PFBGs with both the coating removed as well as with the coating intact to compare the difference in their strain response experimentally. The tests will provide clear experimental results of the effects of the coating on the strain response of the sensor under quasi-static loading. In this paper, four PFBG sensors have been fabricated and evaluated under tensile and flexural loading using standard loading rigs. Plastic-based optical fibre sensors, on the other hand, have also received considerable attention in recent years [6]. Plastic optical fibres have been available for a number of years as an alternative to glass-based fibres particularly for short distance data transmission such as that used in local area network, security, video, consumer electronics, industrial control, robotics and automobile applications. Other applications include signs and illumination and fibre-woven textiles. Plastic optical fibres based on polymethylmethacrylate are inexpensive (US$.13 per meter), exhibit high fracture toughness, are easy to terminate and offer ease of handling. In view of the advantages associated with plastic optical fibres, research has been carried out to assess their potential for smart structures applications and structural health monitoring. In the literature, plastic-based optical fibre sensors have been reported for a wide range of applications including detection/monitoring of cracks and curvature in concrete [7], humidity [8], dynamic flexural loading [9], biofouling [1] and wave and water flow conditions [11]. Compared to FBGs sensing scheme, plastic optical fibre (POF) sensors based on intensity-principles, offers several unique advantages which include their low development, installation, maintenance and running costs. In addition, their design simplicity, low price performance ratio and the relative ease of signal interrogation render them highly attractive for structural health monitoring, particularly where the cost of employing high density FBG sensor network may be inhibiting. Although POF intensity based system typically does not exhibit the high measurement resolution achievable using FBG sensors, this is not always necessary. As an example, the application of less expensive intensity-based POF sensing scheme would be ideal for the detection of crack in concrete structures where only monitoring of large change in Proc. of SPIE Vol P-2

3 light intensity due to fibre damage is required. POF sensor system is a highly cost effective technology which could be used to complement the more costly FBG scheme and their combined use to monitor strain and damage (i.e. cracks) in the form of a hybrid system offers a potentially attractive option for structural health monitoring applications. In addition, POF sensor can be designed to measure vibration and strain as reported in [12]. 2.1 Sensor fabrication and health monitoring principle 2. METHODOLOGY Packaged FBG sensors All the bare FBG fibres were initially supplied as acrylate-recoated FBGs. Two types of PFBG sensors were fabricated to compare the effects of the presence of the fibre coating on the strain response of the PFBGs. In the first type of PFBG sensor, the acrylate coating layer around the grating region of the fibre was stripped and then inserted between two layers of carbon-epoxy unidirectional composite Fibredux 913 prepregs (Hexcel Composites) and placed in a mould. The processing cycle includes curing the prepregs for 1 hour at 12 o C (7 bar) with an initial heating rate of between 2-8 o C/min. The final cured PFBG external dimensions were 7 mm x 1 mm and the nominal thickness of the packaging was.3 mm. In the other type of PFBG sensors, FBGs with coatings intact were fabricated in the same method. The ability of FBG sensors to measure structural strain involves monitoring the normalized shift in the peak wavelength of the sensor as a measure of strain experienced by the host structure. This is expressed as λ/λ Β =κ ε, where, λ/λ Β is the normalized shift of the Bragg wavelength, κ is the optical strain gauge coefficient or calibration constant and ε is the strain experience by the sensor/host structure. When embedded in a composite packaging, it is important to calibrate the sensor to take into account the inefficiency in strain transfer through the composite medium and adhesive. To ensure optimal transfer of strain, the thickness of the prepreg and the applied adhesive layer should be as thin as possible. The stiffness of the adhesive should be as high as possible to minimize the loss of strain due to longitudinal shear effects within the adhesive. For optimum strain transfer, the calibration constant of the packaged member should approximate that of the theoretical optical strain gauge coefficient of the bare fibre sensor which has a typical value of.78 x 1-6 ε -1 [13] Plastic optical fibre sensors The POF sensor used in this study is based on a 1 mm diameter multimode fibre. The core is made of pure PMMA and the cladding material from fluorinated PMMA. Table 1 shows the properties of the POF used. A good quality cleave can be obtained at the end faces of the POF using a sharp razor blade or using commercially available plastic fibre cutter. These plastic-based fibres were used as supplied for crack detection applications and this is done by attaching them to the side-surface of a concrete beam under flexural load. The principle of crack detection using optical fibres is straightforward- the scheme relies on the monitoring for abrupt decrease in optical intensity in the event of crack propagation across the POF sensor. It was expected that when a crack propagates through the POF sensor, the fibre is damaged and is accompanied by the light leakage at the crack location which results in a significant decrease in the optical intensity. In addition, the light leakage also serves as a visual damage locator. It is clear that the optimum positioning of the POF sensor at the right location on the host structure is critical to the success of the sensing scheme and hence an insight of the possible crack location (area with the highest stress concentration) on the structure will prove useful. This is not a unique limitation of the POF scheme; since as in the case with any other structurally-integrated on-line monitoring scheme, optimal placement of sensors is important and necessary consideration. Being relatively inexpensive, the application of these POF sensors is not constrained by cost as is normally encountered when using other more costly sensors and therefore POF sensors can be deployed widely on all load bearing structural members as supplements to those sensors attached to critical locations where high stress concentration are expected. In addition to the plain POF sensor, a POF-based vibration sensor was attached to the concrete beam to assess its performance under cyclic loading. Details of the POF design can be found in [12]. 2.2 Experimental program Flexural and tensile tests on metallic structures Proc. of SPIE Vol P-3

4 These PFBG sensors (type one with the coating intact while type two with the coating removed) were attached to the bottom of a 2-meter steel I-beam subjected to a three-point bend loading condition. A 35 mm gauge length electrical strain gauges (ESG) were attached at the side of the two PFBG sensors to serve as reference a shown in Figure 2. An additional ESG was attached for good measure to cater for contingencies. All the sensors including the ESG was bonded to the polished surface of the I-beam using a typical cyanoacrylate-based adhesive (CN-glue) for ESG applications supplied by Tokyo Sokki Kenkyujo Co. Ltd. (Japan). Prior to testing the I-beam, two bare FBG sensors (with coating removed) were surface-bonded to an aluminium beam prepared for tensile test. One of the FBGs was attached to the beam using CN-glue while the other FBG was attached using a fast-curing 3-ton epoxy and was then loaded onto a tensile testing machine. A conventional electrical strain gauge was attached alongside the FBG sensors. The aim of the test was to compare for differences in the strain response of the FBG sensors for the two types of adhesive used. The calibration constants obtained for both sensors were compared to check for any significant difference in the strain transfer efficiency of these adhesives. In addition, two PFBG sensors were surface bonded to a unidirectional carbon fibre (Fibredux 913) composite beam made from the same material as the packaging. One of the PFBG sensors was fabricated with the FBG fibre coating intact while the other PFBG with the coating removed. Here, a 3-ton epoxy adhesive was used to attach the PFBG sensors to the host specimen. In this specimen, the electrical strain gauge was attached on the opposite surface. Figure 1 shows the tensile composite specimen used in the test Three point bend and cyclic flexural test on concrete beams In this study, CN-glue was used to bond the fibres to the concrete surface. The surfaces of the concrete specimens were initially ground using sand papers to remove surface unevenness. The prepared surface was then dusted and cleaned with a damp cloth to remove any residual dust to ensure good adhesion between the fibre and the concrete surface. The intended location of the POF sensor on the concrete specimen was marked out on the beam and was temporarily held in position by securing the ends of the fibre to the surface of the concrete specimen using masking tapes. The POF sensor was then bonded to the surface of the concrete. The adhesive was carefully applied to the interface between the fibre and the surface of the concrete along the entire length (2 mm) of which cracks were expected to occur. 3. RESULTS AND DISCUSSIONS 3.1 Tensile tests on bare fibres and PFBGs. Figure 2 shows the response of the two FBG sensors attached to the aluminium beam using CN-glue and a 3-ton epoxy adhesive. The beam was loaded in tension. From the plot, it is clear that both sensors exhibit excellent linearity during loading and unloading of the specimen. There is no significant difference in the values of the calibration constants for both configurations and these values approach the theoretical value of.78 x1-6 ε -1. Further comparison of the response of the sensors was made by loading the specimen in the manner shown in Figure 3. The respective calibration constant obtained earlier was applied and the plot in Figure 3 clearly shows that both adhesives result in no observable difference in terms of the strain response of the FBG sensors. The strain responses of the PFBG sensors surface-bonded to the unidirectional carbon composite specimen are shown in Figure 4. Both PFBG sensors exhibit excellent linearity over the entire loading regime tested. However, the marked deviation of the calibration constant corresponding to the coated PFBG sensor is also evident highlighting the strain transfer inefficiency through the acrylate coating of the optical fibre. This result provides experimental evidence of the effect on the strain response of the sensor by keeping acrylate coating intact when embedding in the composite packaging. 3.2 Flexural test of I-beam using attached PFBG sensors. Two PFBG sensors were attached to the base of a steel I-beam as shown in Figure 5. One of the PFBG sensors was fabricated with the acrylate coating removed while in the other PFBG sensor the acrylate coating remained intact. The I- beam was loaded under a three-point bend set-up and the strain responses of the PFBGs were compared to the conventional electrical strain gauge. Initially, the calibration constants of these PFBG sensors were obtained experimentally. Figure 6 shows the respective strain response of the two PFBGs. The linearity of the loading curves for both sensor configurations is excellent. As before, the coated PFBG sensor shows significant reduction in the strain-optic Proc. of SPIE Vol P-4

5 calibration constant. In addition, it can be observed that the coated PFBG sensor exhibits considerable hysteresis although the final strain reading appears to return to its initial condition. In order to investigate further the strain response of the PFBG sensors, the I-beam was loaded progressively from rest and then held at specific load level momentarily. Figure 7 shows the result of the flexural loading test. It is interesting to observe the response of the coated PFBG. Although the coated PFBG sensor exhibit good linearity when loaded undirectionally as observed in the previous test result, Figure 7 clearly shows that the coated PFBG sensor exhibited a high degree of stress relaxation which appeared particularly evident during the holding phases of the loading regime. This may result from slippages which occur at the interfaces of the acrylate coating and the grating core and/or the coating medium and the fibre composite layer leading to the loss of strain at the grating region of the fibre. Although the coated PFBG sensor exhibits good strain linearity over the initially loading region it is also evident that as the loading progressed the response became increasingly non-linear. Clearly, the effect of the acrylate coating is detrimental to the performance of the sensor and this can be observed in terms of the reduced strain sensitivity relative to the uncoated PFBG as well as the non-linear response of the sensor with respect to the applied strain, particularly at higher level of loading. The uncoated FBG sensor on the other hand exhibits excellent strain linearity over the entire loading range and overlaps the electrical strain gauge readings at every stage of the loading. Although the final strain reading of the coated PFBG sensor approaches zero strain, a large hysteresis loop (characteristic of viscoelastic materials) is observed as illustrated in Figure 8. Here, the strain readings of the PFBG sensors are plotted against the electrical strain gauge readings. The inset shows a magnified view of the coated PFBG trace. It is evident from the inset that in both the loading and unloading curves, when the load is held stationary at each predetermined stage the strain reading of the coated PFBG sensor drifts from its initial value. At each stationary phase during the loading stage, significant decrease in the sensor strain reading can be observed. A similar observation is made during the unloading stage here, the strain values drifted towards the opposite direction to that witnessed during the loading cycle (i.e. increase in strain values). These observations provide clear experimental evidence of the stress relaxation by the sensor as a result of the slippage at the acrylate-coating interface. The uncoated PFBG sensor on the contrary exhibits excellent monitoring capability throughout the entire loading test as evidenced by the linear-elastic strain response and absence of signal drift at each load-holding phase yielding very little or no hysteresis. In view of the results obtained, it is thus expected that the uncoated PFBG sensor is capable of performing as well as its bare version, in terms of their potential for strain-monitoring applications. 3.3 Cyclic flexural test In this set-up, FBG sensor was employed together with plain POF to monitor the response of a 2-meter reinforced concrete beam retrofitted with carbon fibre composites at the base of the beam. One bare FBG sensor for strain monitoring and four plastic optical fibres for crack detection were attached to the side surface of the beam which was cyclically loaded up to 4 thousand cycles in flexure at a vibration frequency of 2 Hz. Here, the hybrid optical fibre sensing concept involves integrating the key features of each sensor scheme in a synergistic manner by exploiting the strengths and benefits associated with each type of fibre. The strain monitoring capability of the FBG-based sensors is well-known and hence employed for precise and absolute measurement of strain. Plastic optical fibres, being inexpensive, may be employed more extensively with little cost constrains and being sufficiently sensitive to cracks [7], they offer the potential for monitoring crack propagation when several POF are used in parallel as in this test set-up. Although an unpackaged FBG sensor (bare and with the acrylate coating removed) was used in this particular test, it is reasonable to deduce that based on the results obtained earlier, a PFBG version would yield similarly encouraging results under a flexural-type loading. In addition, a POF intensity-based vibration sensor [12] was surface bonded to the concrete specimen and compared to the FBG results to assess its capability to monitor the flexural response of the beam. A photo of the optical fibre sensors attached to the side surface of the specimen is shown in Figure 9. Figure 1 shows the plot of the FBG sensor and the POF vibration sensor readings highlighting the encouraging response of the POF-based sensor. Both types of sensors survived 4 thousand cycles and did not show any evidence of degradation in their performance and demonstrated the same typical response as that shown in Figure 1 when readings were taken towards the end of the cyclic loading. Furthermore, this result also demonstrated the long term adhesion quality of the glue used in this study. The POF system, being inexpensive and rugged, offers an attractive alternative to FBG sensing scheme in many less demanding applications where absolute strain monitoring may not be required e.g. where monitoring of vibrating frequency or changes in the frequency. Proc. of SPIE Vol P-5

6 The plain POF sensors used for crack detection purpose also yielded encouraging results. Figure 11 shows the response of the four plain POF sensors demonstrating their potential for monitoring the crack growth in concrete specimens as a result of cyclic flexural loading. Whenever the crack intersects the POF sensor, a significant drop in the signal intensity occurs. It can be observed from the photo (Figure 9) that at each location where the crack intersects the POF, significant light leakage is evident. This allows the operator to determine the location and growth of crack readily. While the use of cynoacrylate-based glue allows the fibre to be securely attached to the surface of the specimen, it also embrittles the fibre rendering it sensitive to cracks. The leakage of light observed suggests the presence of stress concentration at the intersection of the crack and the POF which led to the damage of the cladding layer and in turn resulted in a significant decrease in the light intensity. It may also be observed that a slight jump in the undamaged POF readings appears to accompany each significant intensity reduction in the sensing fibre. This anomaly was traced and was found to be due to a trivial circuit connection error in the photodetector system fabricated for the test. 4. SUMMARY and CONCLUSIONS In this study the concept of a hybrid optical fibre sensing system which involves the use of FBG and POF sensors for monitoring of strain and crack propagation respectively have been demonstrated. Initially, the performance of packaged FBG sensor (coated and uncoated) was assessed and following a series of mechanical tests it was shown that the uncoated PFBG sensor offers excellent strain response. The tests also highlighted significant stress relaxation in the acrylate-coated PFBG sensor and hence not suitable for structural health monitoring. Better results may be obtained using stiffer polyimide FBGs without removal of the coating prior to packaging and further tests are needed to demonstrate this. A flexural cyclic test was also conducted and the results gave clear evidence of the potential of the plain POF sensors to monitor crack propagation in addition to providing operators a visual indication of the location and extent of crack growth based the light coming off the POF sensors at the crack site. Also, the cyclic test demonstrates the long term survivability of the optical sensing system and adhesive employed in the study. A cyclic test involving the uncoated PFBG is presently being planned to study its long term fatigue performance. ACKNOWLEDGEMENTS The work presented here is supported by the NUS Academic Research Fund (grant no: R ). The provision of FBGs from SIF-U Pte Ltd is acknowledged. REFERENCES 1. Y Okabe, N Tanaka and N Takeda, Effect of fiber coating on crack detection in carbon fiber reinforced plastic composites using fiber Bragg grating sensors, Smart Materials and Structures, Vol.11, 22, pp KSC Kuang, MP Whelan, R Kenny, WJ Cantwell and PR Chalker, Residual strain monitoring of fibre metal laminates and impact response of fibre Bragg gratings, Smart Materials and Structures, Vol. 1, 21, pp MA Davis, AD Kersey, J Sirkis and EJ Friebele. Shape and vibration mode sensing using a fiber optic Bragg grating array Smart Materials and Structures, Vol.5, 1996, pp DC Betz, G Thursby, B Culshaw and WJ Staszewski Acousto-ultrasonic sensing using fiber Bragg gratings Smart Materials and Structures, Vol.12, 23, pp S Lebid, W Habel and W Daum, How to reliably measure composite-embedded fibre Bragg grating sensors influenced by transverse and point-wise deformations?, Measurement Science and Technology, Vol.15, 24, pp D Kalymnios, PJ Scully, J Zubia J and H Poisel, POF Sensors Overview, Proceedings of 13 th International Plastic Optical Fibres Conference, Nurnberg, Germany, September 27-3, 24, pp Proc. of SPIE Vol P-6

7 7. KSC Kuang, Akmaluddin, WJ Cantwell and C Thomas, Crack Detection and Vertical Deflection Monitoring in Concrete Beams using Plastic Optical Fibre Sensors, Measurement Science and Technology, Vol.14, 23, pp S Muto, O Suzuki, T Amano and M Morisawa, A plastic optical fibre sensor for real-time humidity monitoring, Measurement Science and Technology, Vol.14, 23, pp KSC Kuang and WJ Cantwell, The use of plastic optical fibre sensors for monitoring the dynamic response of fibre composite beams, Measurement Science and Technology, Vol. 14, 23, pp YM Wong, PJ Scully, R Bartlett, KSC Kuang and WJ Cantwell, Plastic Optical Fibre Evanescent Field Sensors for Environmental Monitoring; Biofouling and Strain Applications, Strain: International Journal of Strain Measurement Vol.39, 23, pp KSC Kuang, M Maalej and ST Quek, Web-enabled wireless structural health monitoring system based on plastic optical fibre sensors for offshore-engineering applications, Proceedings of the 19th Asian Technical Exchange and Advisory Meeting on Marine Structures, National University of Singapore, Singapore, 5-7 Dec 25, pp KSC Kuang, ST Quek and M Maalej, Assessment of an extrinsic polymer-based optical fibre sensor for structural health monitoring, Measurement Science and Technology, Vol.15, 24, pp KO Hill and G Meltz, Fibre Bragg grating technology fundamentals and overview, Journal of Lightwave Technology, Vol.15, 1997, pp TABLE AND ILLUSTRATIONS Core Cladding Material PMMA resin Fluorinated polymer Diameter (typical) 98 µm 1 µm Young s modulus 3.9 GPa.68 Gpa Poisson s ratio.3.3 Refractive index Yield strength 82 MPa Transmission loss (@ 65 nm) 2 db/km Maximum operating temperature 7 o C Approximate weight 1 g/m Table 1 Specification of the ESKA CK4 plastic optical fibre. Figure 1: Unidirectional CFRP specimen with two surface-bonded packaged FBG sensors Proc. of SPIE Vol P-7

8 Normalised Shift FBG y =.751x R 2 = ESG Reading (microstrain) (a) Normalised Shift FBG y =.767x R 2 = ESG Reading (microstrain) (b) Figure 2: Comparison of the calibration constant of the FBG sensors using (a) CN-glue (b) 3-ton epoxy adhesive FBG Reading (microstrain) FBG with CN Glue FBG with 3-ton Epoxy Time (sec) Figure 3: Plot showing the response of the FBG sensors bonded to an aluminium specimen using two different adhesives. Proc. of SPIE Vol P-8

9 I S Normalised Wavelength Shift y =.75x R2 =.99 PFBG (uncoated) on CFRP beam y =.44x R2 =.99 PFBG (coated) on CFRP beam Strain (microstrain) Figure 4: Plot showing the response of the PFBG sensors bonded to carbon fibre composite tensile specimen. Figure 5: Photograph of the bottom/tensile surface of the steel I-beam showing the two PFBG sensors and conventional electrical strain gauges. Proc. of SPIE Vol P-9

10 .14.8 Normalised FBG shift y =.712x R 2 = Loading Unloading Normalised FBG shift y =.386x R 2 = Loading Unloading ESG Strain (microstrain) ESG Strain (microstrain) (a) (b) Figure 6: Plot showing the response of the PFBG sensors (a) with coating removed and (b) coating intact. These sensors were surfacebonded to the bottom surface of the I- beam. Uncoated PFBG Strain (microstrain) Uncoated PFBG Elec.Strain Gauge Coated PFBG Coated PFBG Strain (microstrain) Time (sec) Figure 7: Plot showing the response of the PFBG sensors (a) with coating removed and (b) coating intact. These sensors were surfacebonded to a carbon fibre composite tensile specimen Proc. of SPIE Vol P-1

11 PFBG (microstrain) PFBG (microstrain) Electrical Strain Gauge (microstrain) Coated PFBG Uncoated PFBG Electrical Strain Gauge (microstrain) Figure 8: Plot showing the response of the PFBG sensors highlighting the large hysteresis loop corresponding to the stress relaxation observed in the coated PFBG sensor. Figure 9: Photo showing the various optical fibre sensors attached to the side-surface of a 2-meter concrete beam under a flexural loading condition. Proc. of SPIE Vol P-11

12 POF Intensity Change FBG Strain (microstrain) Time (sec) Figure 1: Plot showing the vibration response of the POF-based sensor compared to the FBG sensor Normalised POF Intensity POF Crack Sensor 1 POF Crack Sensor 2 POF Crack Sensor 3 POF Crack Sensor Time (mins) Figure 11: Plot showing the response of the POF crack sensors monitoring the propagation of the crack during the cyclic flexural loading Proc. of SPIE Vol P-12