fiberoptic sensor

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INSTITUTE OF PHYSICS PUBLISHING MEASUREMENT SCIENCE AND TECHNOLOGY

Meas. Sci. Technol. 13 (2002) 15231534 PII: S0957-0233(02)38661-2

An evaluation of a novel plastic opticalfibre sensor for axial strain and bend

measurementsK S C Kuang1,3, W J Cantwell1 and P J Scully2

1 Materials Science and Engineering, Department of Engineering, University of Liverpool,

Brownlow Hill, Liverpool L69 3GH, UK2 Liverpool John Moores University, School of Engineering, James Parsons Building,

Byrom Street, Liverpool L3 3AF, UK

E-mail: [email protected]

Received 25 June 2002, in final form 24 July 2002, accepted for publication1 August 2002Published 4 September 2002Online at stacks.iop.org/MST/13/1523

AbstractThis paper reports the use of a low cost, intensity-based plastic optical fibresensor for curvature and strain measurements in samples subjected toflexural and tensile loading conditions respectively. This simple and robustsensor exhibits a high signal-to-noise ratio and excellent repeatability,rendering the system cost effective for operation in harsh environments. Inaddition, this inexpensive system offers a signal linearity and signal stabilitycomparable to that of an in-fibre Bragg grating sensor and other moresophisticated optical fibre sensor systems. Test results have shown that thesensor exhibits a highly linear response to axial strains of up to 1.2% andbending strains up to 0.7% offering a strain resolution of up to 20microstrain. Findings from a series of cyclic tests have demonstrated thatthe sensor response is highly repeatable, exhibiting only a very smallamount of hysteresis. The results also highlight the possibility of using thesensor for monitoring strain on either the tensile or compressive side/regionof a beam subjected to flexural loading.

Keywords: plastic optical fibre (POF), segmented POF sensor, strain sensors,smart structures, intensity modulation

(Some figures in this article are in colour only in the electronic version)

1. Introduction

Engineering structures experience a variety of loading regimesduring their lifetime of operational service. The structuralintegrity of surviving structures in the proximity of ademolished building has been a source of serious concern,particularly in the aftermath of an earthquake [1]. In recent

years, engineers have been exploring various options fordeveloping structures that have ahealth-monitoring capability.

Such structures are capable of providing vital informationconcerning the integrityof load-bearing engineering structuresthrough the use of embedded or surface-bonded sensors.

3 Author to whom any correspondence should be addressed.

Amongst the various methods available, optical fibre sensing

systems have attracted considerable attention and have been

widely demonstrated to be a highly promising technology for

structural health monitoring [24].

Fibre optic sensors offer many advantages over

conventionalstrainsensorsthese include theirinsensitivity to

electromagneticfields, lightweight and minimal intrusiveness.

Optical fibre sensing systems have been shown to be capableof measuring a variety of parameters including strain and

deflection. In highly loaded engineering structures such as

highway bridges, pedestrian footbridges, skyscrapers, aircraft

wingsand helicopter rotorblades, transverseloading canresult

in large bending strains, which can lead to the initiation of

0957-0233/02/101523+12$30.00 2002 IOP Publishing Ltd Printed in the UK 1523


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K S C Kuang et al

cracks and a reduction in fatigue life. In many applications,

the maximum deflection of the structure defines the safe

operational limit, and it is therefore important to have a

structurally integrated monitoring system capable of real-time

strain and bend measurements. The incorporation of a fibre

opticsensingsystem (FOSS) capable ofcontinuous acquisition

of strain and bending information would allow preventive

measures to be taken before the onset of catastrophic failure.Recent progress in the field of optical fibre sensors

has demonstrated the ability of such systems to perform

health-assessment monitoring in large civil structures such as

bridges and flyovers [57]. The use of fibre Bragg grating

(FBG) sensors for strain and bending measurements has been

attractingsignificant attention in recent yearsand thesesensors

have been shown to offer excellent potential for structural

health monitoringin a rangeof structures[812]. However, the

cost of implementing an FBG sensing systemfrequently limits

itswidespreaduse toonlywell funded andspecializedprojects.

In FBG-based optical systems, signal recovery requires

complicated demodulation techniques, frequently involving

costly specialist hardware such as optical spectrum analysers.

In contrast, inexpensive intensity-based optical fibre systems

for structural-health monitoring have been demonstrated andshown to be a viable option in the development of reliable and

cost-effective sensing systems [1315]. Recently, intensity-

based systems using tapered glass optical fibre sensors have

been considered for use in strain monitoring of load-bearing

structures [16, 17].

For some time now, plastic optical fibres (POFs) have

attracted a considerable amount of focus for a number

of reasonsamongst these being their low cost, ease of

termination and coupling, and their relatively high resistance

to fracture. The cost of POFs is low and their use as

sensors requires no more than basic solid-state devices such as

light emitting diodes (LEDs) and photodiodes. The inherent

fracture toughness and flexibility of POFs makes them much

simpler to handle in field applications than their glass-based

counterparts. As the sensingprinciplerelies on the modulationof light intensity, sophisticated signal interrogation techniques

are not necessary. Presently, POFs are fast becoming a viable

alternative to glass-based fibres since advances in materials

and manufacturing have dramatically reduced transmission

losses, thereby increasing their suitability for use in local

area networks (LANs) as well as communications and sensing

applications [18]. The use of POFs for detecting transverse

cracks in composites has recently been demonstrated for smart

structural applications [19, 20]. Although the use of glassfibre

tapered sensors has been attracting some attention for use in

strain monitoring of load-bearing structures, research into the

use of POF as strain/bend sensors is still lacking.

The present study reports for the first time the use of an

inexpensive intensity-basedplastic opticalfibresensingsystem

for performing strain and bending measurements in loadedstructures. The system relies on monitoring the modulation

of light intensity as the sensor is subjected to flexural and axial

loading conditions. The findings of this study highlight the

potential offered by these sensors for monitoring bending and

axial strains.

y

W

NeutralAxis

ctr

L

x

Figure 1. A simply supported beam with a central concentratedload.

2. Beam theory background

The theoretical analysis of the deflection of an isotropic

beam subjected to out-of-plane loading is well documented

in standard mechanics texts [21] and it will be summarized

briefly here.Consider a simply supported beam subjected to a central

concentrated load as shown in figure 1. The engineers theory

of bending gives

y=

E

R=

M

I(1)

where is the in-plane longitudinal stress, y is the distance of

the plane from the neutral axis, E is the Youngs modulus of

the beam material, R is the radius of curvature of the plane, Mis the bending moment and I is the second moment of area of

the cross-section of the beam.

The second derivative of the deflection (), w.r.t. the

longitudinal distance from the applied load (x), is related tothe bending moment (M) as follows:

M= E Id2

dx2= E I

1

R

(2)

which for a centrally loaded beam gives

d2

dx2=

W L

4

W x

2

1

E I(3)

where the load

W= 48

L3

E Ictr

L is the distance between the supports and ctr is the beam

deflection at the mid-span.Substituting (2) and (3) into (1) and expressing the in-

plane strain () on the lower surface of the beam as a function

ofctr,

= 12y

L2ctr. (4)

Since the values of y and L are not varied in this study, thelongitudinal in-plane strain, , can be directly related to the

beam deflection at mid-span ctr. Alternatively, can be

expressed as

=

1

R

y. (5)

The above analysis shows that the in-plane strain (), thecurvature(1/R) andthe central deflection(ctr )ofthebeamare

proportionally related to each other. Therefore, if the response

of the POF sensor varies linearly with the central deflection

(direct measurement obtained from experiment), the sensor, inprinciple, can be used to provide a direct measure of the strain

and the beam curvature.

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An evaluation of a novel plastic optical fibre sensor for axial strain and bend measurements

Table 1. Specification of the ESKA CK40 POF.

Core Cladding

Material PMMA resin Fluorinated polymerDiameter (typical) 980 m 1000 mYoungs modulus 3.09 GPa 0.68 GPaPoissons ratio 0.3 0.3Refractive index 1.492 1.405

Yield strength 82 MPaTransmission loss (@ 650 nm) 200 dB km1

Maximum operating temperature 70 CApproximate weight 1 g m1

0.5 mm 0.25 mm

Segmented

region of POF

Fibre holder

Plastic

optical

fibre

Figure 2. Photograph showing the cross-section view of the

segmented POF.

3. Experimental methods

3.1. Details of the optical fibres and preparation of the sensor

The POF used in this research was a 1 mm diameter multi-

mode step-index fibre (ESKA CK40) supplied by Mitsubishi

Rayon Co., Ltd. Details of the POF used are shown in table 1.

In order to improve the bend sensitivity of the POF, a

segment of the POF cross-sectional profile was removed overa predetermined length by abrading the POF surface with a

razor blade. Care was taken to prevent the blade from cutting

into the fibre by positioning the blade vertically or tilting ittowards the direction of abrasion. This simple procedure

was found to offer reasonable repeatability although a jig is

being designed to improve the efficiency and effectivenessof the process. This technique for sensitizing the opticalfibre to bending differs from other methods such as chemical

tapering[22], intermittentetching[23]and radial grooving[24]

inthat thesensitized regionof thesensor is limited toa segmentof the POF, rather than the entire cross-section of the fibre.

Figure 2 shows a micrograph of the cross-section of a typical

segmented POF sensor, clearly highlighting the region that has

been removed. The sensor is designed to be sensitive to thedirectionof bend/curvature, makingit possible todeterminethe

extent and direction of bend with respect to the initial position

by examining the shift in signal intensity.

3.2. Specimen preparation

Two types of three-point bend sample were investigated in

this study. In the first, a sensor was surface bonded to a

plain rectangular specimen. In the second, the sensor wassurface bonded to a plastic specimen based on an H-section

profile. The H-section was selected to prevent the crosshead

from crushing the top surface of the sensor during the flexural

test. The sensor was therefore protected as the crosshead rests

on the upper surface of the profile. In the plane rectangular

specimens, however, the segmented POF was only bonded

to the lower surface of the specimen and therefore no suchprotection was necessary.

When bonding the sensor to the specimen, the adhesive

was applied in between the POF and the host specimen. Sincethe procedure to remove the segment was carried out after the

curing of the adhesive, the segmented surface was clear of

any adhesive residue. No mechanical degradation of the POF

surface was observed with the application of the adhesive and

the optical transmission property was not noted to be affected

after the bonding process.Care was taken when preparing the specimen to ensure

that the segmented surface of the POF was oriented such that

itfacedaway from thehostspecimen. This was done inorderto

maximize the bend sensitivity of the POF. Figures 3(a) and (b)

show the specimen configuration employed during tests on the

three-point bend test specimens.

Tests were also undertaken to evaluate the ability of the

POFs to monitor in-plane strains. In these specimens, the

sensor was bonded to an aluminium alloy (Al-2024-T3) dog-

bone specimen. Since it is likely that the sensor will be less

sensitive to in-plane loads than pure bending, the sensing

section of the POF was curved slightly when bonding it to

the aluminium alloy. The segmented surface of the POF waspositionedsuchthatit facedout ofthe curve(i.e.on theconcave

surface). It is clear that in thehighly curved fibre light will tend

to escape in the bend region. The application of an axial strainwill decrease the fibre curvature, effectively reducing the light

lossdue to fibre bend which in turn should result in an increase

in light transmission through the fibre. During preparation of

the tensile specimens, masking tape was used to secure the

position and shape of the sensor. The sensor was then bonded

to the aluminium alloy using a cyano-acrylate-based adhesive.

Figure 3(c) shows a schematic of the specimen used for the

tensile tests.

3.3. Experimental set-up

The experimental arrangements for both the three-point bend

and tensile tests are shown in figure 4. A standard voltage

supply was used to power the ultra-high luminescent LED

(centred at 612 nm). The detector and data acquisition system

consisted of a light-dependent resistor (LDR) and a low costcommercial data acquisition system from Pico Technology

which automatically records voltage changes across the LDR

as the light intensity varies. The data acquisition system offersup to a 16-bit resolution analogue to digital conversion (ADC)

and offers up to eight input channels. The resolution of the

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K S C Kuang et al

(a) Specimen A

(b) Specimen B

(c) Specimen C

Segmented region of POF

x

x

x-x

220 mm

40 mm

3 mm

Adhesive

Segmented region of POF x

x

x-x220 mm

20 mm

4 mm

2mm

Adhesive

Segmented region of POFAdhesive

60 mm

16 mm

Figure 3. Schematic drawings of three three-point bend (A and B) and tensile test (C) specimens showing the location and configuration ofthe POF sensors.

Instron MachineAcquisition system

Basiccircuitry

SignalAmplifier

486-based Personal computer

PicotechADC-16

Standard powersupply

Plastic opticalfibre sensor

LED andoptical fibreadapter

LDR

(a)

486-based Personal computer

Standard

power

supply

Instron Machine

Acquisition system

Picotech

ADC-16

Plastic optical

fibre sensor

bonded toaluminium

specimenLED and

optical fibre

adapter

LDR Basic

circuitry

Signal

Amplifier

(b)

Figure 4. Experimental set-up for (a) the three-point bend test, (b) the in-plane tensile test.

ADC system allows for the detection of voltage changes assmall as 40 V in electrical signal. To increase the ADC

data acquisition rate, the PicoLogTM acquisition software has

been set to a 13-bit resolution conversion. The software

was also configured to a sampling rate of 10 Hz during

all quasi-static tests. The data from the optical fibre were

automatically collected by the computer at this sampling rateand displayed graphically in real time. Both the flexural

and tensile tests were conducted using a servo-hydraulic

Instron (model 4505) universal testing machine. Crosshead

displacement rates between 1 and 40 mm min1 were used

during these experiments.

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3400

3500

3600

3700

3800

3900

4000

4100

4200

0 2 4 6 8 10 12 14 16 18

Crosshead Displacement (mm)

POFLightIntensity

(mV)

0

2

4

6

8

10

12

14

16

Flexura

lLoa

d(N)

POF Sensor Data

Instron Machine Data

Figure 5. Typical POF sensor signal response under flexural loading where the sensor was bonded to the bottom (tensile) surface of thebeam.

Table 2. Estimation of errors in the bending strain measurement.

at low I/I, at high I/I,Strain-gauge (e.g. I/I= 0.02) SD as % (e.g. I/I = 0.16) SD as %coefficient () error () error

1.803 105 1109 0.3 8874 0.31.802 105 1110 0.3 8879 0.31.796 105 1114 0.3 8909 0.31.808 105 1106 0.3 8850 0.31.801 105 1110 0.3 8884 0.31.795 105 1114 0.3 8914 0.3

Standarddeviation, SD 3 24

The strain-optic coefficients of each sensor determined

under the three-point bend and unidirectional tensile loadingconditions were compared to highlight any drift in the

sensitivity of thesensor. Inorderto evaluate thereproducibility

of thesensorsignal, boththe flexureandtensile testsspecimens

were loaded, unloaded and reloaded repeatedly for a number

of cycles.

4. Results and discussion

4.1. Three-point bend experiments

In order to evaluate the functionality of the POF sensor for

bend/displacement measurements, the modulation of the POF

light intensity was systematically compared to the Instron

loaddisplacement data. Figure 5 shows the variation of the

POF signal during a flexural test for the plain rectangular

specimen (i.e. specimen A), highlighting the linear decreasein light intensity with linearly increasing load and central

displacement. Since the central displacement of the beam

can be directly related to the beam curvature and flexural

strain, it is possible to use the sensor to directly monitor these

parameters as shown in figure 6. It is clear from the plot

that the POF sensor exhibits a linear response with increasing

specimen curvature and, by inference, bending strain. Thesensor also exhibits a highly stable response showing no signs

of any spurious data commonly encountered in intensity-based

systems. Although a sampling frequency of 10 Hz was used in

figure 6, an acquisition rate of 1000 Hz can be achieved using

thelow cost ADCsystememployed in this study. Thehigh rate

of data acquisition offers the potential for monitoring dynamic

events. The absence of any requirements for complex signal

demodulation procedures makes it possible to achieve these

high data sampling rates.

The repeatability of the POF signal under flexural loading

was found to be very encouraging and the results of six

tests are summarized in figure 7. Here, the sensor signal is

plotted against flexural strain to determine the sensor strain-

gauge coefficient. The computed values of this coefficient

are included in the figure. The repeatability of the sensor

signal can be clearly seen from the graph with all six tracescollapsing on the one curve although slight variations in the

strain-gauge coefficient are apparent. However, the variations

in the coefficients result in a maximum error of approximately

0.3% in the strain values, as shown in table 2. The inset

in figure 7 shows a magnified plot of the first test cyclea

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K S C Kuang et al

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

Curvature, I/R (m-1

)

NormalisedLossinLightIntensity,

I/I

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

FlexuralStrain(micr

ostrain)

POF Signal

Instron Data

Figure 6. Plot showing the proportional change in POF sensor signal with changes in beam curvature and strain under flexural loading. Thesensor was bonded to the bottom (tensile) surface of the beam.

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0 1000 2000 3000 4000 5000 6000 7000 8000

Flexural Strain (microstrain)

Norma

lise

dLoss

inLightIntensi

ty,

I/I

0

5

10

15

20

Flexura

lLoa

d(N)

Test Cycle 1

Test Cycle 2

Test Cycle 3

Test Cycle 4

Test Cycle 5

Test Cycle 6


I/I1=1.803x10-5

I/I2=1.802x10-5

I/I3=1.796x10-5

I/I4=1.808x10-5

I/I5=1.801x10-5

I/I6=1.795x10

-5

4340.28

4320.98

0.075

0.076

0.077

0.078

0.079

0.08

4200 4250 4300 4350 4400 4450 4500

Flexural Strain (microstrain)

NormalisedLossinLight

Intensity,

I/I

Test Cycle 1

Figure 7. Plot of six POF signals against strain to assess signal repeatability and to determine the strain-gauge factor.

strain resolution of approximately 20 microstrain is evident(based on a 13-bit ADC conversion and a strain-gauge factorof 1.803 105 1). Although the strain resolution can beimproved (i.e. setting the ADC to 16-bit conversion), this isoften unnecessary in structural monitoring applications.

Figure 8(a) shows the POF signal response when acrosshead displacement rate of 10 mm min1 was appliedduring the loading and unloading phases of the beam. Infigure 8(b), the test machine was configured to allow thecrosshead to return to zero load after each cycle using themachines fast auto-return setting. It is clear that in bothcases the POF sensor successfully monitored the loading andunloading of the beam, exhibiting excellent repeatability withnoobvious sign of hysteresis at theend ofthe test. Thestability

of the optical signal is evident throughout the test and this doesnot exhibit any significant noise.

The flexural tests were extended to investigate the effectof increasing the crosshead displacement rate on the ability ofthe sensor to monitor the response of the beam and to evaluatethe stability of the signal. In this part of the study, crossheaddisplacement rates between 10 and 40 mm min1 were used.Figure 9 (crosshead displacement rate= 40 mmmin1) showsa typical set of results from this series of tests. At the endof each cycle, the beam was unloaded for several seconds inorder to observe possible signal drift resulting from any POFstress/strain relaxation or material fatigue. It is clear from theplots that the POF did not suffer from any observable short-term relaxation or fatigue effects. A plateau in the optical

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Figure 8. Plot of a typical POF signal during cyclic flexural loading. (a) Crosshead displacement rate of 10 mm min1 applied during theloadingand unloading phases. (b) Crosshead displacement rates of 10 mm min1 during loading and automatic fast return during unloading.The sensor was bonded to the bottom (tensile) surface of the beam.

signal is clearly evident at the end of each cycle, highlighting

the systems overall stability and excellent signal-to-noise

ratio.

Figure 10 shows the results of tests on the H-section beam

in which the POF sensor was attached to the compressive

region of the centrally loaded beam. The ability of the sensor

to monitor compressive strains is clearly demonstrated in this

plot. It is clear that the optical signal increases with increasing

load and crosshead displacement instead of decreasing as

previously observed in the case where the sensor was on thelower (tensile surface)of thebeam. It isapparentfrom figure10

that the POF signal was equally well behaved, exhibiting a

repeatable response under compressive loading.

The ability of POFs to monitor both tensile and

compressive strain in a centrally loaded beam is illustrated

in figure 11. Here, the sensor was initially tested in the tensile

configuration by placing the sensor on the lower surface of the

beam. A crosshead speed of 10 mm min1 and a maximum

central deflection of 10 mm was applied and the machine

was set to return automatically to zero during the unloading

phase of the test. Following this, the beam was inverted in

order that the sensor would experience a compressive loading

regime (i.e. the sensor was now on the top surface of the

beam). The same crosshead displacement rates and central

deflection were applied in order to permit comparisons to bemade between the two loading conditions (the two plots were

intentionally separated to prevent any crossing over of the

data points and to improve clarity of presentation). Figure 11

shows a superimposed plot of the sensor response under both

conditions, from which it can be observed that the sensor

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3550

3600

3650

3700

3750

3800

3850

3900

3950

4000

4050

4100

0 50000 100000 150000 200000 250000 300000

Acquisition Time (ms)

POFIntensi

ty(mV)

Crosshead Displacement Rate : 40 mm/min

Figure 9. Typical plot of POF signal under cyclic flexural loading showing stability and responsiveness of POF sensor.

3900

3950

4000

4050

4100

4150

4200

0 100000 200000 300000 400000 500000 600000 700000


POFIntensi

ty(mV)

Loading

Unloading

Figure 10. Plot of POF signal under cyclic flexural loading with the sensor bonded to the upper surface (compression region) of the beam.

Table 3. Estimation of errors in the axial strain measurement.

at low I/I, at high I/I,Strain-gauge (e.g. I/I= 0.01) SD as % (e.g. I/I = 0.05) SD as %coefficient () error () error

0.382 105 2618 2.7 13 089 2.70.362 105 2762 2.6 13 812 2.60.379 105 2639 2.7 13 193 2.70.361 105 2770 2.6 13 850 2.60.376 105 2660 2.7 13 298 2.7

Standarddeviation, SD 71 357

is more sensitive to bending when located on the tensile

surface of the beam. The schematic drawing in figure 12

illustrates a possible reason for this effect. It is likely that

the POF loses light to the environment through evanescent

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3700

3800

3900

4000

4100

4200

4300

0 50000 100000 150000 200000 250000 300000 350000 400000


POFIntensity(mV)

POF on compression region of beam POF on tensile region of beam

Figure 11. Superimposed plot of POF signal showing the difference in signal sensitivity when the sensor was attached to the either thecompressive or tensile region of the beam under cyclic flexural loading.

(a)

Light ray

(c)



(b)

Figure 12. Schematic drawing of POF sensor shape during bending.

Load

Load Load

Load

Load Load

Segment orientation sensitive to compression loading Segment orientation sensitive to tensile loading

POF sensor POF sensor

Figure 13. Schematic drawing of POF sensor showing the influence of POF segment orientation on bending direction sensitivity.

penetration and as a result of a significant reduction in the

number of propagation modes that experience total internal

reflection. When deformed as shown in figure 12(b), the

sensor readily loses light since its out-of-plane movement

effectively enlarges the sensitized (de-cladded) region. When

the sensor is deformed in the opposite direction as shown

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K S C Kuang et al

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0.05

0 2000 4000 6000 8000 10000 12000 14000

Strain (microstrain)

NormalisedChangeinLig

htIntensity,

I/I

0

1000

2000

3000

4000

5000

6000

Load(N)

POF Data


Figure 14. Typical POF sensor signal response under an axial load.

0

0.01

0.02

0.03

0.04

0.05

0 2000 4000 6000 8000 10000 12000 14000

Strain (microstrain)

NormalisedChangeinLightIntensity,

I/I

Test Cycle 1 Test Cycle 2

Test Cycle 3 Test Cycle 4

Test Cycle 5

I1/I = 0.382x10-5

I2/I = 0.363x10-5

I3/I = 0.379x10-5

I4/I = 0.361x10-5

I5/I = 0.376x10-5

Figure 15. Plot showing the POF signal response during six tensile loading cycles.

in figure 12(c), the effective area of the sensitized region is

reduced, increasing the number of propagation modes that

undergo total internal reflection, resulting in increasing light

intensity, as was observed in figure 11. The difference in the

sensor response (i.e. decreasing and increasing light intensity

in sensor configurations (b) and (c) in figure 12 respectively)

implies that the response/sensitivity of the sensor to bending

is influenced by the circumferential position of the segment.It may be expected that when the segmented region is located

at an angle between the two maximum positions, the bend

sensitivity willbe lesswithrespectto eachsensor configuration

(figures 12(b) and (c)). This result clearly demonstrates

the importance of the rotational alignment of the sensitized

region with respect to the loading direction. Figure 13illustrates the optimum position of the sensitized region forbend measurements.

4.2. Tensile experiments

Thepossibility ofusingPOF sensors tomonitor axialstrainwasstudied using the specimen geometry outlined in figure 3(c).The concept of curving the sensor in the orthogonal directionto the direction of loading appears to offer potential formonitoring axial strain. Figure 14 illustrates the excellentlinearityof thePOFresponsewhich is wellbehaved throughoutthe loading regime, highlighting also the stability of the POFsignalunder theseconditions. As thespecimenwasloaded, the

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0.92

0.94

0.96

0.98

1

0 20000 40000 60000 80000 10000 20000


Norma

lise

dLoss

inLigh

tIntensi

ty,

I/I

Figure 16. Typical POF sensor signal response under tensile loading (5 mm min1 crosshead displacement rates).

curvature of the sensing region decreases, improving the lighttransmission efficiency in the fibre. Comparing these results

with the response obtained during the three-point bend tests,

there appears to be a marginally higher degree of data scatter

in the POFs response to axial strain. This is not surprising

since bending the POF, as in the three-point bend tests, would

lead to a greater degree of fibre deformation than in a tensile

configuration, leading to greater loss of light in the former

configuration, for a given crosshead displacement. Clearly,

for the tensile specimen to lose the same amount of light as the

flexural specimen, the extent of straightening of the curved

POF (i.e. the horizontal displacement of the sensitized region)

has to be of the same magnitude as the vertical displacement

experienced in the three-point bend configuration. The lower

strain sensitivity of the sensor in the tensile specimen results

in a lower signal-to-noise ratio, leading to greater scatter in thedata. Nevertheless, the excellent strain response of the POF

sensor is clearly evident.

To assess the repeatability of the POF response, a tensile

specimen was unloaded after each test and subsequently

reloaded. The results following six such tests are summarized

in figure 15. It is evident from the plot that the POF sensors

response to strain is highly reproducible, resulting in only

small variations in the strain-gauge coefficient (see inset in

figure15). Table 3 presents estimationsof theerrorsin theaxial

strain measurements in order to assess the significance of the

previously reported variations in the strain-gauge coefficients.

From the table, it is clear that the variation in the strain-gauge

coefficient is insignificant, withstrainreadings beingwithin an

error of 3%. This clearly highlights the excellent repeatability

of the POF sensors used in this study.

The ability of the sensor to monitor repeated loading and

unloading of the tensile specimen was also assessed. Here

crosshead speeds of 1 and 5 mm min1 were used. Figure 16

shows a typical sensor response during a cyclic tensile load-

ing/unloading test (at 5 mm min1). The graph clearly in-

dicates that the POF sensor was well behaved and shows no

observable deviation in the response of the sensor. The excel-lent linearity, repeatability and stability of the system clearly

demonstrate the potential of this simple inexpensive sensing

scheme. To enhance the sensitivity of the sensor to strain,

more than one segmented curved region may be introduced

into the POFsince it is likely that the magnitude of change

in the light intensity (and therefore strain sensitivity) will in-

crease with the number of active sensing regions employed.

Work is currently being planned to evaluate this concept.

5. Conclusions

This research has investigated the use of a novel POF sensor

for monitoring the mechanical response of structures when

loaded in tension and flexure. The sensor offers a number

of advantages including ease of fabrication, high strain andbend sensitivity, strain linearity and a high signal-to-noise

ratio. This investigation has demonstrated the potential of this

inexpensive segmentedopticalfibresystemfor measuringaxial

strainand monitoringflexuralparameters suchas curvatureand

bending strain. Ithas been shownthatthe sensorexhibits a high

signalsensitivity to bending loads and that it can be configured

in such a way as to render it sensitive to in-plane axial loads.

This can be achieved by simply curving the sensing region

of the POF in the appropriate orientation with respect to the

direction in which the strain is to be measured. A further step

in this research would be to study multiplexing possibilities

(i.e. having the bend sensing and the axial-strain elements in

a single POF), although some difficulties may be encountered

in discriminating the two types of physical perturbation due to

cross sensitivity of the POF sensor.

Following both flexural and tensile tests, the sensor wasfound to offer excellent signal linearity without suffering any

reductionin strain sensitivityunder theloading regimesconsid-

ered (up to 0.7% strain in the pure bending specimen and 1.2%

strain in tension). Repeatability tests have confirmed that the

signal was well behaved and highly reproducible in both test

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K S C Kuang et al

configurations. The findings of an analysis performed to eval-uate the accuracy of the sensor measurement has shown that

POFs are capable of strain measurements to within a standard

deviation of0.3% forflexuralloadingand 3% foraxialloading.

The results of a series of repeated loading and unloading

flexural tests are also encouraging. Here no significant

hysteresis was observed and the response of the sensor was

wellbehaved. The results also showed that the sensor linearitywas maintained throughout the tests.

No attempt has been made to characterize the effects

of varying the various optical fibre sensor design parameters

(e.g.segment length, deptheffects)on sensorsensitivity. It can

be expectedthat witha longer segmentedregion, thesensitivity

of the sensor will increase as a result of a higher tendency for

light to escape when the sensor is bent. Increasing the depth

of the segment would expose the POF core further, resulting

in a greater loss of light at the segmented surface; however,

the significance of an increase in the segment depth on the

sensorsensitivityis unclear at this stage. It is important tonote

that with an increase in the depth and length of the segment,the transmitted light intensity would decrease accordingly as

a result of a greater loss of light over the segmented region.

Clearly, thishas to be taken intoconsiderationwhen fabricating

the sensor. A study is currently being carried out to examinethis area of sensor development. Further investigations to

characterize the axial strain sensitivity of the sensor as a

function of initial fibre curvature and length of curved segmentwould also be needed.

References

[1] Stansfield K (ed) 2001 The structural engineer J. Inst. Struct.Eng. 79 5

[2] Kageyama K, Kimpara I, Suzuki T, Ohsawa I, Murayama Hand Ito K 1998 Smart marine structures: an approach to themonitoring of ship structures with fiber-optic sensors Smart

Mater. Struct. 7 4728[3] Loewy R G 1997 Recent developments in smart structureswith

aeronautical applications Smart Mater. Struct. 6 R1142[4] Kuang K S C, Kenny R, Whelan M, Cantwell W J and

Chalker P R 2001 Residual strain measurement and impactresponse of optical fibre Bragg grating sensors in fibremetal laminates Smart Mater. Struct. 10 33846

[5] Grossman B, Cosentino P, Doi S, Kumar G and Vergese J1994 Development of microbend sensors for pressure, load,and displacement measurements in civil engineering Proc.SPIE 2191 11225

[6] Culshaw B, Michie C, Gardiner P and McGown A 1996 Smartstructures and applications in civil engineering Proc. IEEE84 7886

[7] Everall L, Gallon A and Roberts D 2000 Optical fibre strainsensing for practical structural load monitoring Sensor Rev.20 11319

[8] Patrick H J, Chang C C and Vohra S T 1998 Long period fibregratings for structural bend sensing Electron. Lett. 3417735

[9] Dong X Y, Meng H Y, Liu Z G, Kai G Y and Dong X Y 2001Bend measurement with chirp of fiber Bragg grating Smart

Mater. Struct. 10 111113

[10] Gwandu B A L, Shu X W, Liu Y, Zhang W, Zhang L andBennion I 2002 Simultaneous measurement of strain andcurvature using superstructure fibre Bragg gratings Sensors

Actuators A 96 1339[11] Ye C C, Wei C, Khaliq S, James S W, Irving P E and

Tatam R P 2000 Bend sensing in structures usinglong-period optical fibre gratings Proc. SPIE 4073 31115

[12] Kuang K S C, Kenny R, Whelan M P, Cantwell W J andChalker P R 2001 Embedded fibre Bragg grating sensors inadvanced composite materials Composite Sci. Technol. 61137987

[13] Hofer B 1987 Fibre optic damage detection in compositestructures Composites 18 30916

[14] Badcock R A and Fernando G F 1995 An intensity-basedoptical fibre sensor for fatigue damage detection inadvanced fibre-reinforcedcomposites Smart Mater. Struct.4 22330

[15] Lee D C, Lee J J, Kwon I B and Seo D C 2001 Monitoring of

fatigue damage of composite structures by using embeddedintensity-basedoptical fiber sensors Smart Mater. Struct. 1028592

[16] Martin A, Badcock R, Nightingale C and Fernando G F 1997A novel optical fiber-based strain sensor IEEE Photon.Technol. Lett. 9 9824

[17] Arregui F J, Matas I R and Lopez-Amo M 2000 Optical fiberstrain-gauge based on a tapered single-mode fiber Sensors

Actuators A 79 906[18] Bartlett R J, Chandy R P, Eldridge P, Merchant D F, MorganR

and Scully P J 2000 Plastic optical fibre sensors and devicesTrans. Instrum. Meas. Control 22 43157

[19] Takeda N, Kosaka T and Ichiyama T 1999 Detection oftransverse cracks by embedded plastic optical fiber in FRPlaminates Proc. SPIE3670 24855

[20] Takeda N 2002 Characterization of microscopic damage incomposite laminates and real-time monitoring by embeddedoptical fiber sensors Int. J. Fatigue 24 2819

[21] Hearn E J 1988 Mechanics of Materials vol 1, 2nd edn

(Oxford: Pergamon) ch 4, 5[22] Merchant D F, Scully P J and Schmitt N F 1999 Chemical

tapering of polymer optical fibre Sensors Actuators A 7636571

[23] Glossop N D W 1989 An embedded fiber optic sensor forimpact damage detection in compositematerials PhD ThesisUniversity of Toronto Institute for Aerospace Studies

[24] Philip-Chandy R, Scully P J and Morgan R 2000 The design,development and performance characteristics of a fibreoptic drag-force flow sensor Meas. Sci. Technol. 11 N315

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