Polymer-based optical fiber sensors for health monitoring of

Polymer-based optical fiber sensors for health monitoring of engineering structures K.S.C Kuang∗, S.T. Quek, M. Maalej Department of Civil Engineering, 1 Engineering Drive 2, E1A #07-03, National University of Singapore

ABSTRACT This paper describes the design of an extrinsic optical fibre sensors based on poly(methamethycrylate) for structural health monitoring applications. This polymer-based optical fiber sensor relies on the modulation of light intensity and is capable of monitoring the response of the host structure subjected to either static or dynamic load types. A series of mechanical tests have been conducted to assess the response of the plastic optical fiber (POF) sensor. The readings of the sensors attached to an aluminium bar were found to compare well to electrical strain gauge response. The POF sensors were also attached to rebar concrete beams and exhibited encouraging response under flexural loading. Static and cyclic loading tests were also performed and the sensor was shown to exhibit excellent strain linearity and repeatability. Free vibration tests on a cantilever beam set-up in which the POF sensor was surface-bonded to a composite beam were also conducted. The results obtained highlight the capability of the sensor to accurately monitor the dynamic response of the beam. Impulse-type dynamic response of the sensor was also conducted and the POF sensor demonstrated potential for detecting the various modal frequencies of the host structure. POF sensors were also attached to a series of impacted composite beams with varying degree of damage to assess their potential to detect and quantify the damage in the host structure. The results demonstrated the feasibility of using the sensor for structural health monitoring applications. Keywords: Plastic optical fibre, sensor, structural health monitoring, intensity-based sensor, dynamic loading, impulse loading, smart structures

1. INTRODUCTION Optical fibre sensors are currently attracting considerable attention from a variety of industry as they offer a number of significant advantages over existing electrical based sensors. These advantages include their immunity to electromagnetic interference, capability for distributed monitoring, spark-free, long-term monitoring without susceptibility to drift and multiplexing capability. Optical fibre sensors such as fibre Bragg gratings have been very popular due primarily to their long-term strain monitoring capability as they do not suffer from long-term drift. The wavelength-encoded information is not susceptible to fluctuation in optical power and need not be reinitialized in the event of power disruption. Their capability to measure local strains very precisely (up to 1 microstrain or better) is wellrecognized and their applications for structural health monitoring are well-documented in the literatures as seen in recent review papers [1,2]. Although FBG sensors offer a number of advantages, they also exhibit a number of limitations. FBG interrogation systems are costly and require trained personnel to handle both the acquisition system as well as the FBG sensors. Termination of the fibres requires care and the quality of cleave is important to obtain good data. In applications where the precise strain measurement is not required, other less sophisticated system may be more attractive. In addition, in frequency-based structural health monitoring, absolute strain measurement is not critical, hence other optical fibre schemes such as intensity-based optical fibre sensing offers a more cost-effective alternative.

∗

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[email protected]; phone 65 6874-4683; fax 65 6779-1635; www.nus.edu.sg

Smart Structures and Materials 2005: Sensors and Smart Structures Technologies for Civil, Mechanical, and Aerospace Systems, edited by Masayoshi Tomizuka, Proc. of SPIE Vol. 5765 (SPIE, Bellingham, WA, 2005) · 0277-786X/05/$15 · doi: 10.1117/12.599349

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Polymer optical fibres have recently been attracting attention as intensity-based sensors for structural health monitoring. Research using polymer optical fibres has shown their potential as versatile and highly cost-effective sensors for structural health-monitoring [3-6]. Their strain, curvature and deflection monitoring capability make them ideal sensors for mechanical systems. POF when used as an intensity-based sensor offers an in-expensive and effective method to monitor strain and detect crack in structures. POF offers a number of significant advantages including their high fracture resistance and excellent flexibility, making them much less susceptible to damage and fracture when operating in harsh engineering environments compared to other glass-based fibre optics systems. POF also offer excellent chemical resistance and non-flammability, immunity to electro-magnetic interference, and lower overall system cost with the availability of inexpensive light source (from visible to near IR) and high speed detectors. Although the strain sensitivity of POF sensors based on the present design is less than FBGs, the lower cost of the former allows them to be more extensively deployed i.e. in a larger scale and used in sections of structures where strain measurement accuracy is less critical. In applications where they are used as simple crack sensors, they low cost and effectiveness render them highly attractive over FBG sensors. In modal-based structural health monitoring (e.g. analysis of eigen-frequencies) where only the dynamic response of a structure is required, the susceptibility to signal drift over extended time-frame inherent to intensity-based systems is circumvented. Furthermore, the use of reference techniques, signal perturbation due to factors other than the parameter of interest may be accounted for (e.g. fluctuation in light source intensity, temperature or losses due to micro-bending of the fibre). In view of these attractive features, this paper presents the details of an intensity-based POF sensor design. Although the sensor presented in the current paper adopts a similar strain sensing principle to those presented by earlier works [7, 8], the present design offers significant advantages in terms of strain sensitivity and simplicity in sensor construction. The study initially outlines the sensor fabrication method followed by a summary of the experimental work performed. The results the experimental assessment of the sensor is then discussed.

2. METHODOLOGY 2.1 Sensor Fabrication The POF sensor used in this study is based on a 1 mm diameter multimode fibre. The core is made of super pure PMMA and the cladding material from fluorinated PMMA. 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. The housing of the sensor consists of two polytetrafluoroethylene (PTFE) sleeves and a PTFE outer tube as shown schematically in Fig. 1. PTFE material was chosen in view of its excellent fracture toughness, almost universal chemical resistance, high temperature resistance (up to 250oC), self-lubricating property, dielectric property and weather resistance. PTFE tubes of various standard diameter sizes are readily available commercially. In contrast to sensor construction reported by other workers [7, 8], the present sensor fabrication procedure requires neither precision boring nor heating for the purpose of sensor construction. The PTFE can be cut to the desired length readily and the housing constructed easily. In order to increase the light attenuation resulting for a given increase in the longitudinal distance between the cleaved ends, a selection of homogeneous liquid solution with varying opacity was introduced into the cavity of the space within the housing (See Table 1). The liquid solution was injected carefully into the housing using a syringe to avoid air bubbles from forming in the housing. It will be shown that by simply introducing a suitable solution into the cavity, the sensor sensitivity to changes in the separation of the cleaved ends (i.e. with applied strain) can be improved significantly versus a sensor design based on air-gap. Clearly in order to cater for compressive load, an initial gap may be incorporated between the cleaved surfaces of the optical fibres. In this paper, three types of liquid solution were selected for this study and for each sensor type (i.e. air-filled and liquid-filled) will be evaluated in terms of sensitivity and strain response. Table 1 summarizes the types of sensor configurations used in this study.

2.2 Experimental Program A series of quasi-static loading tests were conducted to evaluate and compare the strain response of the liquid-filled sensors and air-filled sensor. These sensors were surface-bonded to a rectangular aluminium alloy beam and subjected to tensile loading using a universal testing machine at a loading rate of 1mm/min (see Fig.2). The gauge length of the

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sensor was fixed at approximately 50mm although this value can be changed accordingly to suit the test/application requirements. A repeatability test was also performed to assess the reliability of the sensor under a load-unload condition. An electrical strain gauge was attached to the aluminium beam as a reference. A strain-free POF was also connected to the sensor system to monitor for possible fluctuation in the light source intensity. The static response of the sensors were further evaluated by attaching them to concrete specimens and tested in a threepoint bend configuration. The scaled concrete beams (100mm x 100mm x 400mm) were cyclically-loaded in flexure by progressively increasing loads in each subsequent cycle until failure occurred. The crosshead displacement used was 0.5 mm/min. The optical fibre response was recorded continuously during the loading. An electrical strain gauge was attached alongside the POF sensors and the central vertical displacement of the beam was recorded using a displacement transducer. The dynamic response of the POF sensor was evaluated using a simple cantilever configuration. The POF sensor was attached to a carbon fibre reinforced composite beam on one surface and collocated with an electrical strain gauge on the opposite surface. The beam has a dimension of approximately 1.3mm x 5.0 mm x 250.0 mm. The beam was excited by applying an initial deflection at the free end and allowed to vibrate freely at its resonant frequency. The signal from the optical fibre sensor was collected simultaneously with the strain gauge data using a digital oscilloscope and compared to assess the accuracy of the POF sensor. Another test includes subjecting the POF sensor to an impulse-type dynamic loading to evaluate its response to localized impact on the host structure. A carbon composite beam was also used where the POF sensor was surface-bonded and collocated with a piezofilm sensor. The piezofilm sensor offers excellent dynamic sensing capability and is used to validate the impulse signal acquired from the POF sensor [9]. In this test, the cantilever beam was impacted using a steel rod in order to obtain higher order modes of vibration. The signal of the POF sensor and the piezofilm were recorded via the digital oscilloscope. In view of their potential in acquiring the dynamic response of the host structure, a preliminary study was undertaken to assess the ability of the sensor to monitor the changes in the modal frequencies as a means of detecting damage in a number of impacted composite beams. These beams composed glass fibre epoxy composite beams measuring approximately 13mm x 5mm x 250 mm. The locating of impact was fixed at 125mm from the edge of the beam

3. RESULTS AND DISCUSSIONS 3.1 Quasi-static uni-axial test on aluminium specimen The results of the quasi-static loading test of the aluminium beam specimen with four POF sensors are shown in Fig. 3 (a). The strain response of each sensor (both air-filled and liquid-filled) was plotted against the electrical strain gauge exhibiting a high degree of strain linearity. It is also evident from the plot that the liquid-filled POF sensors are more sensitive for a given applied strain. Since air is almost optically transparent compared to the other liquid solutions investigated, the transmission attenuation resulting from the separation between the fibre cleaved ends is therefore significantly lower. In contrast, the sensors filled with solutions B and C were capable of greater light absorption leading to higher transmission attenuation. It is evident from the test results that by introducing a high opacity liquid medium in the sensor cavity, an improvement in strain sensitivity over the air-filled type can be achieved. On the other hand, however, the results also highlight the working range of the two different types of POF sensors (i.e. air-filled and liquid-filled). Comparing the sensor with Solution C to the air-filled POF sensor, it is clear that the latter offers a greater dynamic range and can be applied in situations where the applied strain is expected to be large (>1% strain). A series of repeated quasi-static loading and unloading tests were also performed. Fig. 3 (b) shows the typical response of the POF sensor (with Solution B) for a test consisting of five loading and unloading cycles. A high degree of overlapping of data points can be clearly observed in the plot highlighting the reliability of the POF sensor under cyclic loading conditions. For structural health monitoring applications, the repeatability of the results suggest the possibility of using the sensor for detecting changes in the stiffness of the structure. Monitoring of structural stiffness as a means of

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assessing the health of the structure represents an attractive avenue for this intensity based sensor as monitoring of stiffness changes in structures does not require high strain measurement resolution, hence the inherent weakness of possible long term signal drift may be avoided.

3.2 Quasi-static three-point bend test on concrete specimens The load-displacement plot of the test is shown in Figure 4. The response of the POF sensors and electrical strain gauge is shown in Figure 5. It is evident from the figure that the POF sensor is capable of monitoring the flexural loading of the beam. The electrical strain gauge was damaged at the first crack of the beam and was rendered useless as evident by the data response of the strain gauge in Figure 5. The electrical strain gauge failed at a crosshead displacement of approximately 0.3 mm. The crack which developed across the electrical strain gauge is clearly visible in Fig. 6(a) limiting its usefulness for structural health monitoring in the vicinity of surface cracks. The POF sensors, however, did not appear to be significantly affected by the crack and continued to monitor the loading process even after severe crack damage has taken place as shown in Fig. 6(b). Since the POF sensors were attached to the beam at the extremities of their gauge lengths, the propagation of crack across the sensors has insignificant detrimental effect on their measurement capability. Based on the results in Fig.5, it is clear that the sensor with Solution C offers the highest strain sensitivity compared to the rest of the POF sensors. The air-type POF sensor and sensor with Solution A, however, demonstrated the widest dynamic range although the sensor with Solution A evidently offers superior strain response/sensitivity compared to the air-filled type. The strain responses for both Solutions B and C were initially high but suffered from a small dynamic range as evident by the tapering of POF response at the later stage of the test. Fig.7 (a)-(d) shows a comparison of the limits of linearity for each of the sensor. All the sensors exhibit excellent linear response (as indicated by the R2 values) for the range considered. Based on this analysis, Solution A sensor offers the best overall response in terms of signal linearity and dynamic range. Although the air-type sensor also demonstrated good linearity over a wide dynamic range, its strain sensitivity was significantly lower than all the liquid-filled type sensors considered.

3.3 Dynamic loading tests A typical POF sensor free vibration response when subjected to a dynamic loading based on the cantilever beam set-up is shown in Fig. 8 (a). Here, the POF sensor with Solution B was used for dynamic loading experiments. The electrical strain gauge and the POF sensor data exhibit a high degree of correlation. It is clear from the figure that the POF sensor exhibits excellent capability in monitoring the sinusoidal free vibration of the host structure. In the impulse-type dynamic loading test, an end mass of 13g was attached to the carbon-fibre reinforced epoxy composite beam at approximately 135mm from the free end of the specimen. The typical dynamic response of the POF sensor is shown in Fig. 8 (b). The inset shows an amplified version of the signal for a specific time range. To validate the results, fast-Fourier transforms of the POF sensor vibration data and the data from a collocated piezofilm sensor were compared. The transform operation converts the time-domain data of each sensor to a frequency-domain data as shown in Fig. 9(a) and (b) respectively. Fig. 9 (a) shows conclusively the ability of the POF sensor to respond to the three modes of vibration. In terms of structural health monitoring applications, the ability to monitor the modal response of the structure can be utilized for modal-analysis in detection of damage in structures [10, 11]. The results obtained from a preliminary study to assess the ability of the sensor to detect the changes in the modal response of a number of impact-damaged composite beams are shown in Fig. 10. The composite beams used in the study is shown in Fig.11- the impact-induced damage is visible in these specimens to provide a visual confirmation of the progressive damage sustained by each specimens which were impacted between 1 to 5 Joules. It is clear from the results that the sensor is capable of detecting the modal vibration signature of the beams with varying degree of damage and therefore, in principle, able to detect and quantify impact-induced damage in these beams. However, the technique may not be effective if the change in the three fundamental modes is not sufficiently significant (e.g. in this case, it is not able to discriminate the difference in damage level between 4 and 5 Joules impact). Since the sensitivity of modal response increases at higher modes of vibration, this technique may still be feasible if the sensor is sufficiently sensitive



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in detecting the low-energy higher vibration modes. Nonetheless, the results obtained clearly suggest the potential of the sensor in damage monitoring in composite beams in particular in the detection of barely visible impact damages (BVID).

4. SUMMARY and CONCLUSIONS In this paper, the design and fabrication procedure of a polymer-based optical fiber sensor was revealed. A series of quasi-static tensile tests conducted clearly demonstrated the significant improvement in strain sensitivity of the liquidtype sensor over the air-filled type while exhibiting a high degree of signal repeatability and strain linearity. Quasi-static three-point bend tests on scaled steel-reinforced concrete beams highlighted the susceptibility of electrical strain gauges to failure in the presence of surface hairline crack while the POF sensors were able to monitor the loading process even after severe beam cracking had occurred. The various POF sensors demonstrated difference sensitivity and dynamic working range- a careful selection of the appropriate solution (i.e. degree of liquid opacity) is necessary for optimal performance in each application. Cantilever beam free vibration tests and impulse-type loading tests were conducted and the results demonstrated the capability of the POF sensor to monitor up to the fourth mode of vibration and to detect vibration frequency exceeding 500 Hz. Preliminary study conducted to assess the ability of the POF sensor to detect impact-damaged composite beams has been encouraging. Further studies are underway to increase the sensitivity of the sensor to higher modes of vibration in order improve its damage detection resolution.

ACKNOWLEDGEMENTS The work presented here is supported by the NUS Academic Research Fund (grant no: R-264-000-172-112).

REFERENCES

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1.

Kuang K.S.C., and Cantwell W.J., “Use of conventional optical fibers and fiber Bragg gratings for damage detection in advanced composite structures: A review”, Applied Mechanics Reviews, ASME, Vol.56, 2003, pp.493-513.

2.

Zhou G.., and Sim L.M., “Damage detection and assessment in fibre-reinforced composite structures with embedded fibre optic sensors- review”, Smart Materials and Structures, Vol.11, 2002, pp.925-939.

3.

Kuang K.S.C., Akmaluddin, Cantwell W.J., and Thomas C., “Crack Detection and Vertical Deflection Monitoring in Concrete Beams using Plastic Optical Fibre Sensors”, Measurement Science and Technology, Vol.14, 2003, pp.205-216.

4.

Kuang K.S.C., and Cantwell W.J., “The use of plastic optical fibre sensors for monitoring the dynamic response of fibre composite beams” Measurement Science and Technology, Vol.14, 2003, pp.736–745.

5.

Kuang K.S.C., and Cantwell W.J., “Plastic optical fibre and shape memory alloy for damage assessment and damping enhancement of composite materials” Measurement Science and Technology, Vol.14, 2003, pp.1305– 1313.

6.

Takeda N., Kosaka T., and Ichiyama T., “Detection of transverse cracks by embedded plastic optical fiber in FRP laminates”, Proceedings of the SPIE, Vol.3670, 1999, pp.248-255.

7.

Lee D.C., Lee J.J., Kwon I.B., and Seo D.C., “Monitoring of fatigue damage of composite structures by using



embedded intensity-based optical fiber sensors”, Smart Materials and Structures, Vol.10, 2001, pp.285-292. 8.

Badcock R.A., and Fernando G.F., “An intensity-based optical fibre sensor for fatigue damage detection in advanced fibre-reinforced composites”, Smart Materials and Structures, Vol.4, 1995, pp.223-230.

9.

Kuang K.S.C., Quek S.T. and Cantwell W.J., “Use of polymer-based sensors for monitoring the static and dynamic response of a cantilever composite beam” Journal of Materials Science, Vol.39, 2004, pp.3839-3843.

10. Adams, R.D., Cawley P., and Pye C. J., “A vibration testing for non-destructively assessing the integrity of the structures”, Journal of Mechanical Engineering Science, Vol.20, 1978, pp.93-100. 11. Salawu O. S., “Detection of structural damage through changes in frequency: a review” Engineering Structures, Vol.19, 1997, pp.718–723.

TABLE AND ILLUSTRATIONS

Table 1 Details of the plastic optical fibre sensors used in this study

Liquid-filled type sensor

Liquid ID

Description of liquid

Solution A

A translucent epoxy-based liquid

Solution B

A high-opacity yellow liquid

Solution C

A high-opacity black liquid (black dye added)

Air-filled type sensor

Air

N.A.



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Fig. 1. Schematic drawing of the extrinsic plastic optical fibre sensor

10 mm

Fig. 2. Photos showing the static test set-up and a close-up view of the optical fibre sensors

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Normalised POF Signal Loss

0.60

0.50

0.40

Solution C

y = 0.0003x 2 R = 0.9735

Solution B

y = 0.0002x 2 R = 0.9975

Solution A

y = 0.000019x 2 R = 0.9623

Air

y = 0.000012x 2 R = 0.9640

0.30

0.20

Solution C

Solution B

0.10

Solution A Air

0.00 0

500

1000

1500

2000

Electrical Strain Gauge (microstrain)

(a)

0.3 Test Cycle 1

Normalised POF Signal Loss

0.25

Test Cycle 2 Test Cycle 3

0.2

Test Cycle 4 Test Cycle 5

0.15 0.1

0.05 0 0

500

1000

1500

2000


(b) Fig. 3. (a) Plot comparing the typical strain response of the four optical fibre sensors during a quasi-static loading test. (b) Plot showing the typical results of a repeatability test for liquid-filled type (Solution B) sensor



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35.00 30.00

Load (kN)

25.00 20.00 15.00 10.00 5.00 0.00 0.00

1.00

2.00

3.00

4.00

5.00

Displacement (mm) Fig. 4. Figure showing the load-displacement plot for the concrete beam cyclic flexural test

1.2

2000

1600 1400

0.8

1200

Air

0.6

1000 800

Electrical Strain Gauge

0.4

600 Solution A

0.2

Strain (microstrain)

Normalised POF Signal (V)

1800 1.0

400

Solution B

200

Solution C

0.0

0 0

1

2 3 4 Crosshead Displacement (mm)

5

Fig. 5. Plot summarizing the strain response of the four optical fibre sensors and an electrical strain gauge attached to the bottom side of a concrete beam during a quasi-static cyclic test.

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POF sensors

Crack across ESG

1.2

1

1

0.8

POF Reading (V)

POF Reading (V)

Fig. 6. (a) Photograph showing the crack line across the electrical strain gauge at approximately 0.3 mm beam central displacement (b) Photograph showing the widening of the crack after several loading cycles – the damage of the electrical strain gauge is evident while the POF sensors continued to monitor the loading process.

0.8 0.6 y = -0.00005x + 1.09468 0.4

2

R = 0.99197

0.2

0.6 0.4

y = -0.00013x + 0.94356 2

R = 0.99481

0.2

0

0 0

2000

4000

6000

8000

10000

0

1000


2000

(a)

4000

5000

(b) 2.5

0.5 0.49 0.48 0.47 0.46 0.45 0.44 0.43 0.42 0.41 0.4

POF Reading (V)

POF Reading (V)

3000


y = -0.00001x + 0.48938

y = -0.00154x + 2.10836

2

2

R = 0.99547

1.5 1 0.5

2

R = 0.99780

0 0

2000

4000

6000


(c)

8000

10000

0

500

1000

1500


(d)

Fig.7 Plot showing the limit of linear response of the POF sensor with (a) Solution A (b) Solution B (c) Air (d) Solution C



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1500

1.1

5

500

0.9 0

0.85 0.8

-500

POF Signal (Arbitrary U nit)

0.95

4.8 POF Signal (Arbitrary Unit)

1000 1


Normalised POF Intensity

5 4.8

1.05

4.6

4.6 4.4 4.2 4 3.8

4.4

3.6 0.2

0.25

0.3

0.35

0.4

Time (second)

4.2 4 3.8

0.75

~ 54 Hz

0.7

-1000 0

0.05

0.1

0.15

Time (sec)

(a)

0.2

0.25

0.3

3.6 0

0.2

0.4

0.6

0.8

1

Time (second)

(b)

Fig. 8 (a) Plot showing the free vibration response of a plastic optical fibre sensor surface-bonded to a fibre-reinforced composite cantilever beam (b) showing the response of the surface bonded POF signal during an impact-type excitation on a cantilever beam

(a)

(b)

Fig. 9 (a) The fast-Fourier transform of (a) the POF signal shown in Fig. 3 (b) and (b) the collocated piezofilm sensor signal.

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Fig. 10 Plot showing the FFT analysis of the dynamic response acquired by the optical fibre sensor for three composite beams impacted at 2, 4 and 5 Joules

Fig. 11 Photograph showing the surface-attached POF sensors and the damaged induced by impact loads.



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