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APPLICABILITY OF AN OPTIMIZED FIBER OPTIC SMART STRUCTURE

Yu Fan and Mojtaba Kahrizi

Department of Electrical and Computer Engineering, Concordia University1455 Maisonneuve Blvd. West, Montreal, Quebec, H3G 1M8, Canada

Abstract

In recent years, fiber optic smart structure is widely

studied because of its intrinsic benefits [1][2]. One of themost striking evolvements is the technology, of which

FBG (Fiber Bragg Grating) sensor systems are embedded

in fiber reinforced composite materials, so that any

ambience induced responses of the host structures can be

monitored in real time, consequently proper actuationsare initiated. Thus a neural system is realized.

This work reports an entire optimization process of

the FBG sensors embedded in Graphite/Epoxy compositematerial. Moreover, performance of the sensor system

was observed and the applicability is discussed.

Due to the mechanically orthotropic characteristics of the Graphite/Epoxy composite material, two FBG sensors

were orthogonally embedded along the two principle axes

in mid-plane of the specimen. When strain load was

applied along one of the axes, longitudinal and shear

responses of the structure were simultaneously monitored,hence its orthotropic properties were determined.

Further, any randomly oriented strain applied to the

specimen will be analytically quantified along the two sensors.

Recurring to surface mounted resistance strain gage

concept, the embedded FBG strain gage array wasrecalibrated, and its sensing alterability is quantified.

This work tends to provide a quantitative discussion

on FBG sensors’ residual erroneousness after an

optimized embedment, the conclusion may give designers

a reference to properly interpret FBG sensors’

performances, in case they are used as an embedded strain gage.

Keywords: smart structure, fiber optic sensor, compositematerial, embedded FBG strain gage.

1. INTRODUCTION

With rapid growth of aerospace industry, higher- performance and more sophisticated materials become

extremely demanded, which has initiated a new approach

to structural design and analysis, i.e. smart structure

concept, a structure being capable of sensing ambientstimuli and reflecting appropriate reaction by means of

actuation and control. The sensor network integrated in

the material monitors the structural variables (e.g. strain,

pressure, or temperature), subsequently convey the

abnormality to a servo mechanism, which generates a set

of instructions to produce the desired adapting behaviors,thus existing materials’ performance and adaptability are

enhanced.The essential issue to design a smart structure is to

find the most reliable and integratable sensors. Fiber optic

sensors offer the utmost potential to satisfy these requests.

Besides high sensitivity and advanced interrogation

techniques that have been commercially developed intelecommunications industry, fiber optic sensors possess

further distinctive benefits:

(1) Fiber optic sensors are always featured with smallsize and lightweight, therefore induce minutealteration to the host material’s mechanical properties.

(2) Most of optical phenomena are immune to

electromagnetic field, hence the sensors’ applicabilityis significantly enlarged.

(3) Most fiber optic sensors are susceptive to multiple

ambient factors (e.g. strain, pressure and temperature).

Thereby a multifunctional sensor system can be

manufactured.(4) Due low signal attenuation and broad transparent

spectrum, fiber optic sensors can be easily

multiplexed, thus large scaled sensor array is easily

realized. No matter the typology of mechanical load applied to

the structure, strain is always a regressive measurand todetermine the magnitude a material responds. Therefore

in most applications, fiber optic sensors are always

employed as fiber optic strain gauges. Since the instinct

photo-elastic characteristics, FBG (Fiber Bragg Grating)

sensor is regarded as the most promising candidate forstrain measurement.

For a preliminary understanding of Fiber Bragg

Grating, the approach shown in Figure 1 is adopted. [3]

Despite of the circular cross section of an optical fiber,consider only a medium stack, in which the multiple

layers are featured as interleaved refractive indices n1 andn2. The optical period is defined as the thickness of twoadjacent layers. Suppose a broadband light comprising

wavelength component B0 vertically launched into the

stack. Whenever the thickness of each layer happens to

equal B/4 (or B0/4 neff , B is the transformed wavelength

in the medium corresponding to B0 in free space, neff is

the average refractive index of the medium, thus

B= B0/neff ), reflections from the stack will interfereconstructively centered at B, performing as a

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transmission notch or reflection spike in wavelength

domain.

Fig. 1 Understanding of FBG Theory

Therefore, central wavelength of the reflected spike ortransmitted notch can be expressed as

2 B eff n (1.1)

Similarly in FBG fiber, the core area is patterned with

above depicted refractive index alternation (so called“Bragg grating”) through certain UV inscribing technique

along fiber axis, so that specific wavelength components

of the input signal are reflected, while the rest remain

transmitted. The reflected wavelength is desirable bymeans of controlling FBG fiber’s optical period , as well

as the strength of the refractive index modulation.The most striking characteristics of FBG fiber are its

photo-elastic and thermal-optic behaviors, upon which

FBG fiber is always counted as the nominated strain andtemperature sensors. According to the literature, FBG

fiber’s effective refractive index and its periodic spacing between grating planes are affected by the changes in

strain and ambient temperature. [4] Thereby Bragg

grating’s central wavelength shifts while the strain load

and temperature changing. Relationship among these

variables can be expressed as

2 2 B

n nn l n

l l T T

T

(1.2)

Where the first term represents the strain-induced

portion of the central wavelength shift, which can befurther expressed as

(1 ) B B e z p S (1.3)

Where B is the wavelength of the light (1550nm in

this experiment), z is the strain in µ along fiber axis, Pe

is the effective strain-optic constant, which can be

obtained through following equation

2

12 11 12

2

eff

e

n p p p p

(1.4)

Where P11 and P12 are FBG fiber’s strain-optic tensors,

and is the poisson’s ratio. Although the physical

parameters of the optical fibers have certain deviations

among different manufacturers, averagely for a typicalgermano-silicate optical fiber, P11=0.113, P12=0.252,

=0.16, and neff =1.482. Using these parameters, one can

anticipate central wavelength shift of 1.2 pm as a result of applying 1 µ axial strain to the FBG fiber.

The inherent benefits of FBG sensors are particularly

enhanced when they are used in association withadvanced composite materials. Fiber reinforced

filamentary typology of the materials enables integrationof optical fibers within the structure, facilitates FBG

sensors to internally monitor physical parameters, such as

strain. Reciprocally, embedment effectively protects the

optical fibers from hostile environmental attacks, such asintensive radiation in space shuttle application.

2. EXPERIMENT

This work reports the entire optimization process of an

orthogonal FBG sensor array embedded inGraphite/Epoxy composite material. Moreover, the

performance of sensor system was observed and its

applicability is discussed. The experiment was dividedinto three successive steps:

(1) Determined the optimal layout for FBG sensors to be

embedded in Graphite/Epoxy composite laminate.

(2) Fabricated Graphite/Epoxy composite specimen,

simultaneously integrated FBG sensor array into the

structure.(3) After the fabrication process, strain load was applied

on the specimen, a set of central wavelength shifts

was obtained. Comparing the performances withthose of surface mounted resistance strain gages and

FBG fiber itself before embedment, the FBG strain

gage array was recalibrated, and erroneous deviationswere evaluated.

2.1 FBG Sensors Layout Optimization

In the experiment, FBG fibers were made of CorningSMF-28 photonic fibers, and gratings were inscribed at

Bragg Photonics Inc. NCT-301 graphite/epoxy prepreg

was used to fabricate the composite specimen. The


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material manufacturer is Newport Adhesives and

Composites, Inc.The specimen was designed with cross-ply lamination,

its lay-up sequence followed (08904)s, where “0”

represents ply’s graphite fiber orientation being alongwith specimen’s structural direction; while “90”

represents ply’s graphite fiber orientation being

perpendicular to specimen’s structural direction, i.e.

orthogonal to that of “0” plies; the subscript “s”

represents the lay-up is symmetric about the mid-plane of the entire stacking. The designed plate size was 200mm x

40mm along its structural orientation. Since specified

thickness of prepreg tape was 125µm [5] and totally 24 plies being stacked, final thickness of the specimen was

approximately 3mm. The detailed structural

understanding of the specimen is schemed in Figure 2.

Fig. 2 Schematic Structure of The Specimen

Due to the orthotropic characteristics of Graphite/

Epoxy composite, two FBG sensors were orthogonally

deployed in the specimen, along the two reinforcing fiber

directions (i.e. parallel and perpendicular to the structuralorientation) respectively. Central wavelengths of the two

gratings were apart from each other, so that when strainload was applied along the structure, specimen’slongitudinal and shear responses were simultaneously

monitored, consequently material’s orthotropic properties

are determined. Further, any randomly oriented strain

applied to the specimen will be analytically quantifiedalong the two sensors. In order to gain comparatively

uniform strain field along FBG sensor’s gauge length (in

approximately 10mm), the two orthogonally embedded

FBG sensors were located at the geometric center of the

specimen.Until now, there are still two concerns suspended in

FBG array layout:

(1) Should the FBG fibers be deployed parallel or perpendicular to the neighboring reinforcing fibers?

(2) Where should the FBG fibers be placed better in

thickness among the plies?

Many of the concerns about embedded optical fibers’obtrusiveness are focused on the interlaminar lenticular

resin-rich pocket, which forms when optical fibers are

buried in laminated composite host, since this resin

pocket acts as an interlaminar discontinuity and poses a

potential reliability hazard to both sensor and host underapplied loads. [6] Then the first concern is interpreted as:

What layout pattern could best reduce the resin-rich

pocket, hence minimize the harmful effects to both thehost structure and the FBG sensing.

According to the literature, it is reported that resin

pocket becomes smaller as the angle between optical fiber

and adjacent reinforcing fibers decreases. Upon this

deduction, structural discontinuity (resin pocket) in thematerial will be minimized when FBG fiber is buried

parallel to neighboring reinforcing fibers, and maximized

when FBG fiber is perpendicular to neighboringreinforcing fibers.

In the experiment, an unidirectional square-shaped

pre-specimen was fabricated with ordinary optical fibers

embedded parallel and perpendicular to the neighboringreinforcing fibers, then the pre-specimen was cut in

stripes along the structural edges, hence the resin-rich

pocket cross sections were exposed in the side face of the

stripes. Each stripe was observed under SEM (ScanningElectronic Microscope), the geometric status of the resin-rich pockets were recorded

(a) (b)

Fig. 3 Resin-pocket geometries when(a) Optical fiber is perpendicular to neighboring

reinforcing fibers(b) Optical fiber is parallel to neighboring

reinforcing fibers

Upon observations, it is approved that when optical

fiber is embedded perpendicular to the neighboringreinforcing fibers, a big resin-rich pocket is generated;

while there is almost no resin-rich pocket observed when

the optical fiber is embedded along with the neighboringreinforcing fibers, therefore the mechanical geometry is

optimized, hence induces the least effect to both the hoststructure and the FB sensor’s performances.

According the literature, it has been observed that the

closer the embedded optical fiber to the outer layer or the

interface between cross-plied layers, the moredeformation induced to host structure, thus results in more

stress field concentration in vicinity of optical fibers, and

adds more errors to the FBG sensor. [7] It has been


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modeled that in case the embedded optical fiber is

deployed parallel to the neighboring reinforcing fibers,when the optical fiber is four layers away from laminate’s

surface or the interface between cross-plied layers, the

induced structural deformation is sufficiently suppressed.Therefore, the second concern is effectively answered

regarding where the optimal place is for embedded FBG

fibers among the laminate layers.

With subject to above discussions, it is determined

that the FBG array embedded laminate specimen followsthis lay-up criteria: (04F004904F9090408), i.e. the FBG

sensor along with the structural orientation was embedded

between the fourth and fifth layers where the neighboringreinforcing fibers were also oriented along with the

structural direction; the FBG sensor perpendicular to the

structural orientation was deployed in the mid-plane of the laminate (i.e. between the twelfth and thirteenth

layers) where the neighboring reinforcing fibers were

parallel to the FBG fiber. The schematic lay-up structure

of the specimen is shown in the diagram below

Fig. 4 Optimized Layout of FBG Sensor Array

Moreover, the FBG fibers were recoated with

polyimide before embedment, due to polyimide’s superb

tolerance under high temperature (which will be

experienced during curing process), as well as its thermaland mechanical properties better match the host structure

comparing to other commercial coating materials.

2.2 Composite Specimen Fabrication

In the experiment, major steps to fabricate the

composite specimen are depicted as below:

(1) PREPREG LAY-UP. During this process, prepreg

was cut into desired patterns. After surface wascleaned, each tape was laid up in designated sequence.

(2) OPTICAL FIBER INGRESS/EGRESS.

Simultaneously during plies laid up, FBG fibers were

deployed between corresponding layers along the

desired orientations. The rest of the optical fibersexposed out of specimens were protected with 0.9mm

Teflon buffers, which are able to withstand high

temperature up to 500°C(3) MOLDING. After being laid up, raw ply stack was

placed in a fabrication mold, and then the mold was

sealed.(4) CURING. The sealed mold was placed in an

autoclave for high temperature and pressure curing.

The mold was vacuumed in autoclave to remove

additional air and volatile gases generated during

matrix polymerization.(5) SPECIMEN TRIMMING. At the end of curing

process, specimen was removed from the mold,

whose edges were trimmed and polished by adiamond saw. Special care must be taken when

trimming the edges where the optical fiber was

ingress/egress.

Fig. 5 Final State of The Specimen

2.3 Performances of Embedded FBG Strain Gage

Although the FBG sensor array was embedded in

accordance to the optimized layout, one can still expect

from previous observations that natural host structure is

somewhat perturbed due to the foreign entity, thuscomplex transverse stress field is interactively induced in

the vicinity between the host material and FBG fiber.Therefore, FBG sensors’ optical responses will be alteredfrom that of itself before embedment, hence calibration

becomes essential once the FBG sensor is embedded.

However, two suspicions need to be proposed whenever

FBG is used as an embedded strain gauge:

(1) May embedded FBG sensor fail before reachingdesired measurement range, so that render the sensor

useless for further transduction tasks?

(2) To what extent the interpretation of the embedded

FBG sensor’s signal is altered due to residual stressas well as the additional transverse stresses when the

specimen is pulled?

To answer these issues, before embedment, bare FBGfibers were pulled and their central wavelength shifts

were quantitatively characterized. Further, after FBG

embedded composite specimen was fabricated, an

orthogonal resistance strain gage array was mounted on

specimen surface, which geometrically coincided withembedded FBG strain gages. Thereby during the tension

progress, both readings from the two gauge systems were


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logged, in which the resistance gage is used to calibrate

the FBG gage; while FBG gage performance will becompared with that of itself before embedment, so that the

erroneous deviation is quantified. The setup of the

experiment is shown as below. During experiment session,temperature remained constant.

Fig. 6 Experiment Setup

According to the orthogonal resistance gage array,

specimen’s longitudinal and shear responses were profiledduring a strain load incremental progress.

(a) (b)

Fig. 7 Specimen’s mechanical Response(a) Along (b) Transverse to Strain Load

Figure 7 (a) indicates a linear relationship betweenstrain load and specimen’s elongation along with the load.

With this diagram, one can figure out specimen’s strength

modulus along loading direction. However, Figure 7 (b)

illustrates an insignificant and irregular displacement

along specimen’s shear direction. Due to the tinymagnitude of the fluctuation, it is believed specimen’s

shear strain is so small that its poisson’s ratio approaches

zero. Further, one may make the first assumption thatthere would be no apparent central wavelength shift forthe corresponding FBG gage perpendicular to the loading

direction.

During entire tension progress, both FBG sensorssurvived, and no failure behavior observed. Meanwhile,

based on strain readings from corresponding resistance

gage, embedded FBG gage’s photo-elastic behavior is

plot and sensitivity is recalibrated in figure below. Not

only, performance difference before and after embedmentare compared. Apparently FBG sensor’s behavior

deviates from its bare fiber status. It is believed this

deviation is induced by embedded fiber’s vicinaltransverse stress in addition to axial strain load. Even

though the plot gives reader a first sense that FBG gage’s

sensitivity is enhanced after embedment, this

enhancement is erroneous.

Fig. 8 Performance of Longitude FBG Gage

Unexpectedly, unlike the irregular behavior of thecorresponding resistance gage, perpendicularly deployed

FBG gage performed a downward central wavelength

shift with longitudinal strain load increment, even thoughthe magnitude was not big. Despite the fact that

specimen’s shear strain approached zero, embedded FBG

gage still monitored structural reactions. However, this

structural variable is not along shear direction, instead, it

is transverse stress variation in embedded FBG fiber,which is induced by longitudinal strain. This phenomenon

from another point of view approves the erroneousness

embedded FBG gage in its axial strain interpretation.

Fig. 9 Performance of Transverse FBG Gage


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3. DISCUSSION

Besides thermal effects, embedded FBG strain gage’s

interpretation issues are manifested in vicinal transverse

stress sensitivity. In the most revealing form, FBG gage’s

central wavelength shift can be expressed as: [8]

1 f f

B B z iS n S ds B (3.1)

Where is the strain tensor depicting the all-

dimensional strain states in FBG fiber, S is the normal

strain component everywhere tangent to fiber axis, and

f

iS f

z

B is FBG’s instinct grating wavelength when vicinal

stress field is zero. Consider if the embedded FBG straingage is subject to radial compression from host structure

only, but is prevented from axial expansion, should we

expect the FBG gage to experience a central wavelength

shift? The answer is yes, because although the embeddedFBG strain gage dose not change in length, its central

wavelength still changes since its refractive index,

according to photo-elastic theory, is a function of thestress field in the fiber. Apparently if, for example, the

sensor is considered to contain axial strain information

only, there will be errors additively arising. [9] Acting as

“interferential” sensitive components, these errors deviate

FBG gage’s performance, hence degrade its accuracy.Beyond pivotal calibration procedure, sensor

designers still need to know: to what extent FBG gage’s

response has been deviated, or in the other word, whetherthe desired axial measurand is possibly overwhelmed in

the deleterious noise? However, it is possible to followresistance strain gage concept to devise meaningful

quantifications of errors due to transverse strainsensitivity, in which synthetic response of an embedded

FBG strain gage can be generalized as

0

f f B f

z z tx x ty F S F S F S y

(3.2)

Where is the normalized central wavelength

shift, is gage’s sensitivity to axis strain; F is its

sensitivity to transverse strain in

z F tx

direction; is its

sensitivity to transverse strain in direction. For

convenience, all sensitivities are normalized by , so

that final sensor response becomes

ty F

F

y

z

f f f

z z tx x ty y F S K S K S (3.3)

Where and are two normalized transverse

strain sensitive factors. Scheme of FBG fiber’s coordinatesystem is shown as below.

tx K ty K

Fig. 10 Scheme of FBG Fiber’s Coordinate

Recurring to resistance strain gage precept, embeddedFBG sensor is assumed (or more precisely, “desired”) to

respond to axial strain only. Using this approach, it is

possible to define normalized central wavelength shift of

an “ideal” FBG strain gage as

' f

z FS (3.4)

Where the superscript prime indicates the ideal sensor

and is the ideal axial sensitivity. One can notice thatthis expression is consistent to the situation of a bare FBG

fiber. Errors due to transverse stress can then be

quantified simply by finding the relativeness betweenactual and ideal response

F

'

100% 100%'

f f y x

z tx ty f f

z z

S S E F F K K

S S

(3.5)

This expression reveals that sensor’s error is a function of:

(1) The difference between actual and ideal axialsensitivity, which is believed to be induced by axial

mismatch (or the residual stress) between host

material and embedded FBG fiber;

(2) The transverse strain sensitive factors, which are

resulted from the instinct photoelastic nature of theFBG gratings;

(3) The ratios

f

x

f

z

S

S and

f

y

f

z

S

S , which indicate the extent of

the undesired transverse stress during measurement.

Recurring the scenario into experimental analysis,

where test data from embedded FBG gage represent anerror-deteriorated performance, while data from bare FBG

fiber represents an “ideal” case. Hence upon Figure 8,

transverse stress induced


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3

3

1.56 10 1.23 10

1.23 10

26.8%

E

3

(3.6)

26.8% erroneous deviation from its ideal state.

When FBG grating is used as embedded strain gage,

only the variables tangent to fiber axis are desired, whiletransverse vectors act as noise additive to the meaningful

signal. Recurring to the concept of normal detectors, only

if signal is always obtrusive to noise, FBG strain gage issaid to be practical in a certain application, otherwise

signal will be overwhelmed in noise, so that strain gage

becomes useless. Or in the other word, the embedded

FBG fiber would be rather used as pressure sensor thanstrain sensor, in which transverse field becomes the

dominating measrand.

Upon the discussions, precept of Signal-to-Noise-Ratio (SNR) in engineering applications could be also a

means to quantify embedded FBG strain gage’s

applicability. In this experiment, one can calculate SNR

of the embedded FBG strain gage along loading direction:

'

'

3

3 3

10log

1.23 1010log

1.56 10 1.23 10

SNR

(3.7)

5.71dB

Hence interferential signals were suppressed with 5.71dB

below desired axial strain measrand.

4. CONCLUSION

Through the experiment, an optimal layout criterion is

developed in case FBG fiber array is integrated in

composite lamination as embedded strain gauges, so that

perturbations are minimized for both structural health andsensor interpretation. The criterion is: FBG fibers have to

be embedded parallel to the neighboring reinforcing fibers,

and as far as possible from laminate’s outer surface and/orthe interface between cross-plied layers.

After optimal patterned FBG fiber array embedded

Graphite/Epoxy composite specimen was fabricated,strain load was applied to the specimen, performances of

embedded FBG strain gages were observed andrecalibrated with geometrically coincided resistance strain

gages mounted on specimen surface. Comparing this

photo-elastic plot with that of bare FBG fiber beforeembedment, erroneous deviation is obtained, and

embedded FBG strain gages’ applicability is quantified by

means of SNR (Signal-to-Noise-Ratio) for the optimizedlayout.

5. ACKNOWLEDGEMENT

The authors would like to thank CanadianMicroelectronics Corporation (CMC) for their technical

support, and Natural Sciences and Engineering ResearchCouncil of Canada (NSER) for their financial assistance,

as well as Dr. V. S. Hoa and his PhD student Mr. Xiao

Bao for their assistance in composite specimen fabrication

and measurement.

Reference

[1] López-Higuera, JM et al, “Strain and TemperatureTransducer on One FBG”, Smart Structures & Materials,

Proc. SPIE, Vol. 4328, 2001

[2] Andreas Othonos, Kyriacos Kalli, “Fiber BraggGratings, Fundamentals and Applications in

Telecommunications and Sensing”, Artech House, 1999

[3] S. O. Kasap, “Optoelectronics and photonics

principles and practices”, Prentice Hall, 2001

[4] Andreas Othonos, “Fiber Bragg Gratings”, Review of Scientific Instruments, 68(12), 4309 – 4341, December

1997

[5] Newport Adhesive s and Composites, Inc., “NCT-301Product Data Sheet”, 2003

[6] A. Dasgupta, et al, “ Prediction of Resin-PocketGeometry for Stress Analysis of Optical Fibers Embedded

in Laminated Composites”, Smart Materials and

Structures, Vol. 1, No. 3, pp. 101 – 107, 1992

[7] James S. Sirkis, et al, “Fiber Optic Smart Structures:Chapter 4, Optical Fiber/Composite Interaction

Mechanics”, Edited by Eric Udd, John Wiley & Sons,

Inc., 1994

[8] J. S. Sirkis, “A Unified Approach to Phase-Strain-

Temperature Models For Smart Structure Interferometric

Optical Fiber Sensors”, Optical Engineering, 32(4), 752 –773, 1993

[9] Eric Udd, “Fiber Optic Smart Structures”, John Wiley

& Sons, Inc., 1995


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