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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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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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(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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An evaluation of a novel plastic optical fibre sensor for axial strain and bend measurements
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
Instron Machine Data
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
Acquisition Time (ms)
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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An evaluation of a novel plastic optical fibre sensor for axial strain and bend measurements
3700
3800
3900
4000
4100
4200
4300
0 50000 100000 150000 200000 250000 300000 350000 400000
Acquisition Time (ms)
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)
Segmented region of POF
Segmented region of POF
(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
Instron Machine 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
Acquisition Time (ms)
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.
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