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Pacific Science Review, vol.16, A, no. 1, 2014, pp. 1~8
Advances of FRP-Based Smart Components and Structures
Yung William SASY CHAN * and Zhi Zhou**
Abstract: Fiber-Reinforced Polymer (FRP) composites are widely used in civil industry since a couple of
decades. This paper is an object of understanding smart components and structures based on FRP. Basic
principles of the intelligent structures made of FRP and Optical Fiber sensors are introduced. Some
significant up-to-date smart elements used as reinforcing and health monitoring of structures are also
mentioned in details. Moreover, certain applications of smart FRP systems in civil engineering are
enunciated briefly. The results show that smart bars based on FRP are very useful and could replace
conventional steel. Additionally, FRP-OF-OFBG is one of the most advanced techniques for local and
global monitoring. However, interface strain for externally reinforcing systems needs specific
characterization to overcome debonding effects. Finally, problems analysis of existing applications based
on carbon fiber composite are point out, following by some possibilities of new design smart FRP.
Keywords: Fiber-reinforced polymer (FRP), Optical fiber, Smart sensor, Smart structure, Structural
health monitoring.
(electrical strain gage, accelerometer, etc.) have been
installed for integrity evaluation of Bridges, dams,
buildings, and so on [4-5]. However, these techniques
present some limitations in application like durability
and high cost. Moreover, due to new trends of smart
materials, standard transducers have been replaced by
intelligent sensors. Nowadays, most detectors are
based on smart materials like Piezoelectric, Shape
Memory Alloys (SMA), electro-magneto-rheological
fluid, and fiber optic [6-8].Hence, all smart structures
up-to-date are based on the properties of the smart
materials employed for the design. In other words,
sensors performances and quality depend on the
advantages and disadvantages of the materials
utilized.
The concept of intelligent structure is based on
safety and economic improvement concerns, weight
and time saving, sensing, and auto-control. For these
reasons, many attempts of actuating and/or sensing
systems have been introduced by researches. For
example, a smart aggregate-based active-sensing
system was introduced to evaluate damages severity
of concrete shear wall. As a result, fragility of PZT
patch when directly embedded into concrete could be
overcome [9]. Smart self-healing Reinforced
Concrete (RC) beams with super-elastic SMA and
fiber optic sensors for temporary repair cracks have
been demonstrated in [10]. Self-repair of fissures by
INTRODUCTION
Since recent decades, Smart structures and
components have been becoming an interesting area
to monitor civil engineering infrastructures [1-2].
Indeed, all structures or/and parts with an integration
of sensors and/or actuators systems are classified as
“Intelligent structures”. They are able to provide a
self-structural health monitoring and/or an actuating
response without human intervention. Moreover, new
and existing large infrastructures are built
everywhere, which are suffering of different
deterioration. Thus, smart systems facilitate
Structural Health Monitoring (SHM) tasks from
construction to service phase [3].
Traditional methods using conventional sensors
have been applied to detect the keys parameters of
structures in the past decades. Many sensors
* PhD Candidate
School of Civil Engineering,
Dalian University of Technology, China
E-mail : [email protected]
** Professor
School of Civil Engineering,
Dalian University of Technology, China
E-mail : [email protected]
1
Yung William SASY CHA
AN * and Zhi Zhou**: Advances of FRP--based Smart Components and Structurres
mechannical performannces. Fig.1 illusstrates the basic
principples of smart sysstem based on FRP
F
and Opticaal
Fiber sensors (OF
FS). Consequeently, a brieef
introduuction of comm
mon used fiber optic
o
sensors andd
FRP m
materials are giveen.
embeedding SMA wires into con
ncrete were also
a
developed, but theermal or electrrical actuation is
needded as activationn [11]. Long-teerm monitoringg of
concrete bridge by using many typ
pes of Fiber Opptic
(FO)) sensors has beeen proposed in
n some literaturres.
Fiberr Bragg Gratingg (FBG) sensorss are bonded at the
surfaace or integratedd into concrete with
w stainless stteel
or eppoxy resin encaapsulation techn
niques were fouund
in [12]. Moreover, proof
p
ABS encclosures were ussed
for Tsing
T
Ma Bridge, in order to protect FBG sennsor
from
m moisture andd dust [13]. Nevertheless,
N
these
methhods seem to bee inconvenient for durable heaalth
moniitoring due to fragility
fr
of Glasss fiber, difficultties
of sensors
s
installaation, and corrrosion effects of
certaain materials.
C
Composites
fibeer reinforced po
olymer (FRP) like
l
carboon fiber have beeen used in aero
ospace since maany
decaddes ago for airccraft frame prod
duction. They own
o
goodd mechanical prooperties such ass high tensile strress,
high young moduluus in fiber direcction, lightweigght,
and so
s on [14]. Its applications in Civil Engineerring
begaan just in the early
e
1980sfor rehabilitation and
a
repaiir of damaged structures.
s
They
y could also usee as
reinfforcing elementts for new prrojects to replaace
convventional materiaal like steel bar [15]. Furtherm
more,
they have been usedd to package Fiber Optic senssors
with a good strainn sensitivity sim
milar to the bare
b
OFB
BG. Therefore, many
m
researcheers have been doone
in order to obtain sm
mart component based on FRP and
a
Opticcal fiber sensors. In spite of effforts done, furtther
progrress are still needed for su
uccessful damaage
detecction, long-term
m monitoring, best
b
accuracy, and
a
so onn.
Inn this paper, the basic priinciples of sm
mart
compponents are presented
p
with
h further detaails,
following by a brieff description of some optical fibber
sensoors. In additiion, some sig
gnificant existting
advaances smart FRP
P systems and th
heir applicationss in
civil engineering arre individually described.
d
Finally,
brieff discussion annd proposals are
a established in
orderr to face challennges of current systems.
s
Fig 1. Schematic proccess of smart co
omponent basedd
on FRP andd optical fiber seensors.
Fiber Optic Sensingg
Opttical fiber sensoors are very pro
omising tools foor
sensingg due to theeir potential advantages
a
likke
immunnity to electrom
magnetic, high sensitivity, highh
corrosiion resistance, small in sizee, non-electricaal
devicess and so on [16]].
BASIC
C PRINCIPL
LES
F
Fig 2. Some fibeer optic sensors principles
The principles of the smart com
T
mponents are rellied
to: (1)
( the sensingg principles of the optical fibber
sensoors used, and (2) the propertiees of the FRP for
In aaddition, they reespond to a chaange in intensityy,
phase, frequency, polaarization, waveelength or modee,
2
Pacific Science Review, vol.16, A, no. 1, 2014, pp. 1~8
when exposed to environmental effects [17]. Many
kinds of sensors have been deployed in literatures,
including Bragg Grating, distributed sensing
(BOTDA, OTDR), Fabry-Perot Interferometry, Long
gage Grating, SOFO, etc. Among them, Fiber Bragg
grating (FBG), distributed optical fiber and FabryPerot (FP) show great interest in civil engineering
applications. Sensing principle of each sensors
mentioned above, following by a brief comparison
are given in Fig.2 and Table 1 respectively.
EXISTING SMART FRP
COMPONENTS AND STRUCTURES
Smart FRP Anchorage Systems
Anchorage is one of key components playing
important role for structural integrity. In such system,
axial stress and interfacial stress developed along the
component need accurate and real-time measurement.
However, it is quite difficult to evaluate these stress
level due to sensors installation difficulties.
Consequently, two (02) kinds of smart systems could
overcome these problems. One is distributed smart
FRP anchor rod and the second is smart FRP
anchorage, based on FBG and OF.
Table 1. Comparison between some fiber optic
sensors
Type of
sensors
Typ
e
FBG
QD
FP
Poi
nt
BOTDA
D
Measure
ment
type
ε
t
d
ε
t
p
ε
t
Resolu
tion
High
High
Interrogati
on
technique
Wavelengt
h
Phase
Low
Frequency
(0.5
m)
Note: D: Distributed; QD: Quasi-distributed; ε: Strain
t: Temperature; d: Displacement; p; Pressure
BOTDA: Brillouin Optical Time Domain Analysis.
Fiber Reinforced Polymer Composites
Fig 3. Schematic of configuration and fabrication
process of the distributed FRP anchor rod [20]
FRP materials are composed of matrix (epoxy
resin) and fiber (Aramid, Carbon, Glass, and Basalt).
They have high corrosion resistance, good
mechanical properties almost similar to those of
conventional steel along the fiber direction. Various
successful applications were found to replace
conventional methods for reinforcing, retrofitting,
and repairing [18-19].
Therefore, it is obvious to say that mechanical
properties of FRP are strong enough for optical fiber
protection, and obtaining smart reliable sensing
component.
Fig 4. Strain sensitivity comparison between bare OF
and built-in OF.
3
Yung William SASY CHAN * and Zhi Zhou**: Advances of FRP-based Smart Components and Structures
Distributed Based Smart FRP Anchor Rod
The smart FRP anchor rod is made of distributed
optical fiber sensor (BOTDA) and FRP. The
manufacturing process is shown in Fig.3. In order to
monitor full-scale axial strain, built-in OF sensors
was adopted as sensing part. Calibration tests on bare
OF and smart FRP sensors are done for strain
sensitivity comparison. By using the data from [20]
and Origin software, strain sensitivity comparison
curves were depicted in Fig.4.According to the graph
and linear fitting results, a very small difference
value (0.002 MHz/µε) is notified between the strain
sensitivity of bare OF and FRP anchor rod. Therefore,
we can conclude that the FRP do not change too
much the sensitivity of the optical fiber sensor.
Smart FRP Bars
FRP bars were introduced to replace conventional
steel bars for reinforced concrete (RC) structures. For
structural safety improvement, advances on smart
FRP bars were produced in Harbin Institute
Technology (HIT). Many sorts of intelligent bars
could be found in literature reviews. However, the
most significant are cited with further details here.
Smart Basalt-FRP Bars (BFRP bars)
The system consists of built-in OFS, embedded
into B-FRP during fabrication process by pultrusion,
like depicted in Fig.3. Moreover, based on data from
[22], strain sensitivity of bare OF and BFRP are
shown in Fig.7 and Fig.8. According to the results, a
Fig 5. Smart FRP anchorage in reference [21].
Smart FRP Anchorage Based on FBG
Anchorage based on FRP-FBG was developed in
order to measure accurately its axial strain states.
Height (08) FBG sensors were embedded along the
FRP rod [21]. The design is shown in the Fig.5.
Using data from reference [21], stress-strain
relationship of FRP rod (Fig.6) is established. Results
show a correlation coefficient of 0.999 and young’s
modulus of 51.85 GPa.
Fig 7. Strain sensitivity of bare OF
Fig 8. Strain sensitivity of Smart BFRP bar
Fig 6. Stress-strain relationship of FRP rod.
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Pacific Science Review, vol.16, A, no. 1, 2014, pp. 1~8
good coefficient correlation and slightly difference
exists between bare OF (R2 = 0.99988) and smart
FRP system (R2 = 0.99986). Hence, smart bar has
the strain sensitivity similar to the bare OF. Zhou et
al [23] have also conducted similar studies of smart
bars based on CFRP-OFBG and GFRP-OFBG. The
strain sensing coefficient of each smart bar is 1.21
pm/µε and 1.19 pm/µε respectively. The coefficient
of correlation are 0.9999 and 0.99982 respectively
too. From these results, we can confirm that a good
linearity is obtained for every type of FRP
composites used.
OFBG has been used to monitor losses in prestressed
RC beams [25]. However, it could only detect losses
at local point of structure. As an advance sensing
technique for force losses, smart FRP-OF-FBG bars
allow full-scale measurement along its length [26].
By using the method described by Zhou et al [24], a
smart steel strand was applied to replace conventional
steel strand. The sensing principles of the smart
strand are based on the Brillouin techniques
(frequency shift) and Fiber Bragg grating
(wavelength shift). Therefore, we could obtain local
and global measurement of force losses (see Fig.9 for
sensing configuration). Moreover, a simultaneous
measurement of strain and temperature have been
demonstrated in [27], by using one multi-signal
optical fiber sensor. The smart steel strands consist of
one FRP bar surrounded by six (06) steel wires. A
series of prestressed RC beam tests were carried out
to verify the performance of the smart steel. Load cell
was applied to compare the values acquired from
intelligent system. The results show that the
monitored values from smart steel strand agree well
with those from load cell with a relative variance less
than 8%. Moreover, a slight relative error less than
0.25% between FBG and BOTDA sensors is noticed,
which indicates a great cooperating capability.
Smart FRP-OF-FBG Bars
Full-scale measurement can be realized by
combining “point sensors” and “distributed sensors”.
The smart FRP bars presented here is a result of a
hybrid distributed OF sensors and FBGs aligned in a
single optical fiber, embedded into FRP [24]. The
sensing configuration of the smart bar is described in
Fig.9. Table 2 shows the linearity of sensor data and
measurement accuracy. We can notify from results
that a slight decrease of linearity between bare OFFBG and FRP-OF-FBG exists.
Smart Externally FRP Systems
Nowadays, infrastructures, especially bridges are
suffering of deterioration due to material ageing in all
over the World. New constructions require high
investment and time-consuming. Therefore, repairing
techniques by using FRP composites are suitable in
order to minimize financial and time-consuming.
Many systems for strengthening and repairing based
on FRP have been adopted in literatures. However,
some interesting issues are presented here.
Fig 9. Schematic sensing configuration of
BOTDA/R-FBG system (a) light switch method and
(b) coupler method [24]
Smart Embroidery FRP Sheet
FRP sheets are strongly used for column, columnbeam joints, and masonry wall strengthening.
However, sudden failures could occur during lifespan
of the system, and lead to serious damage. Thus, a
need of adequate monitoring is required. For this
purpose, embroidery method could solve the problem
of FBG sensors fixation at designated position [28].
The embroidery smart sheet is got from an
Table 2. Linearity (R2)of sensor [24]
Sensor
Bare-OF-FBG
FRP-OF-FBG
FBG
1.0000
0.9995
BOTDA
0.9992
0.9995
BOTDA-FBG
0.9999
0.9996
The practical application of smart FRP bars was
mainly for prestress losses monitoring. Smart FRP5
Yung William SASY CHAN * and Zhi Zhou**: Advances of FRP-based Smart Components and Structures
embroidery machine, by fixing the fiber optical
sensors accurately at the carbon fiber as show in the
Fig.10.
DISCUSSION & PROPOSALS
Based on the existing smart components cited
above, their advantages and disadvantages are
discussed in this part. Hence, the following remarks
and proposals are addressed:
(a) Up to date, Smart FRP-OF-FBG sensing
principle is the most convenient technique for global
and local monitoring. Smart bars based on this
sensing technique are promising tools for health
monitoring of prestressed reinforced concrete
structures. However, further studies are need in order
to improve the existing methods, as well as
ameliorate the measurement range of the smart bar to
detect damages until total rupture.
(b) The embedded techniques of OFS into FRP do
not affect the mechanical properties of systems.
Additionally, fabrication process (pultrusion, wetlay-up, hand lay-up, etc.) causes easy integration of
optical sensors.
(c) Two (02) similar smart anchorage systems
were described. For that one, which using 8 FBG
sensors, a combined OF-FBG in a single optical fiber
might be good solution in order to reduce the cost of
the sensors, as well as improve its durability.
(d) Surface bonded OFBG technique for smart
NSM strips is not reliable for long survivability of
sensors. New issues overcoming these disadvantages
should be adopted.
(e) The embroidery techniques seem good issues
to fix FBG at designed position into FRP sheet.
However, for large sheet, the smart system cost
expensively due to the number of FBG embedded in.
According to these remarks mentioned, FRP
based smart components and structures are still in
progress and need further development. Instead to
only detect damages on structures (prestress loss,
strain, etc.), a careful care on the survivability of the
components is also better to realize. For example,
Takeda, S-I. [30] used FBGs to measure the swelling
strain and coefficient of moisture expansion of CFRP
laminates in order to determine their suitability for
practical use. In addition, FRP materials as protective
layers of OFS are very sensitive in harsh environment.
Above the Transition temperature (Tg), mechanical
performance of the epoxy matrix will change, which
in turn, change the whole properties of the composite.
Smart NSM CFRP Strip
Near Surface Mounted (NSM) is one of externally
strengthening systems. Prestressing method makes
the possibility to use the whole tensile strength of the
CFRP strips. However, brittle properties of the
material could lead to global structural failure.
Consequently, accurate and real-time monitoring of
the prestress forces methods are needed. Here, the
authors developed surface bonded OFBG sensors on
CFRP strips for stress monitoring objective [29]. The
configuration of the smart system is shown in Fig.11.
For comparison, strain gauge is also bonded at the
opposite face of the strip. According to the results, it
was concluded that the OFBG sensors integrated into
the NSM CFRP strips could measure the prestress
loss, as well as detect damages during loading
process. Furthermore, the strain values gathered from
FBG sensors agree well with those from Strain gages.
Fig 10. Production of embroidery smart FRP sheet
based on FBG [28].
Fig 11. OFBG bonded on the surface of the NSM
CFRP strip (Smart NSM CFRP strip) [29].
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Pacific Science Review, vol.16, A, no. 1, 2014, pp. 1~8
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Moreover, we can design some new components
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CONCLUSION
Advances smart components and structures based
on FRP were introduced with further details in this
paper. A brief understanding of system principles
was given. Among all smart sensors, Optical fiber
sensors are very interesting to be embedded into FRP.
Test results show that the bare OF do not affect the
properties of composite materials, nor change its
physical shapes. Some smart FRP components for
internally and externally reinforcement was reviewed.
It is noted that the OF-FBG sensing principle along a
single fiber is very attractive for simultaneous
measurement of temperature and strain, as well as for
prestress loss monitoring.
According to the remarks gave for each system,
most researches did not take account the inconvenient
utilization of the materials when components are used
in very harsh environment.
In spite of these progresses, many discoveries and
improvements are still ongoing in this research area,
by designing new sensors based on FRP and optical
fiber more robust and flexible to resist in bad
conditions.
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