Integrated fiber Bragg grating incorporated textile

carbon reinforcement structures

L.S.M. Alwis1, K. Bremer2, Y. Zheng2, F. Weigand3, M. Kuhne4, R. Helbig3, B. Roth2

1
School of Engineering and the Built Environment
Edinburgh Napier University
Edinburgh, United Kingdom
L.Alwis@napier.ac.uk
2
Hannoversches Zentrum für Optische Technologien (HOT)
Leibniz Universität Hannover,
Hannover, Germany
Kort.Bremer@hot.uni-hannover.de
3
Sächsisches Textilforschungsinstitut e. V. (STFI),
Chemnitz, Germany
4
Materialforschungs und -prüfanstalt an der Bauhaus-Universität Weimar (MFPA Weimar)
Weimar, Germany

Abstract—An optical fibre Bragg grating (FBG) based strain data thus obtained are used for the prediction of potential
sensor network incorporated in a functionalized textile-based structural failure and to schedule follow-up maintenance to
carbon structure (FTCS) that can be used for structural health ensure safe operation. This is vital in addressing the economic
monitoring (SHM) is introduced. The aim of the design is not only benefits and ethical need for safe and sustainable infrastructures.
to monitor strain, but also to act as a structural strengthening In line with the safety aspects, it should also be noted the
mechanism in the target application. The integration of the optical application of appropriate strengthening mechanisms, i.e. such
fiber, i.e. incorporating the FBG sensors, on the FTCS is achieved as carbon fibre reinforcement polymer (CFRP) to strengthen
by “interweaving” the two elements, i.e. optical fiber and carbon concrete [2], are just as vital as monitoring the infrastructure
filaments, on a polymer textile substrate in a grid formation using
itself.
a specialized fabrication process. The thus obtained sensors was
then characterized and calibrated where a variation in the fibre FOS has a number of advantages over the conventional
length, i.e. resulting from strain, would induce a variation in the electrical sensors, most importantly being electrically passive
reflection wavelength of FBG sensors. To do so, the functionalized (hence safer to use) and therefore immune to electromagnetic
skein sample (incorporating FBG sensors) was subjected to interference. In addition, optical sensor systems are light weight,
varying elongation using a tensile testing machine by carefully small in size and have the capability of enabling a wider
incrementing the applied force. A good correlation between the bandwidth of data which is extremely useful to sensors systems
applied force and measured Bragg wavelength change was involving the interrogation of a collection of sensing elements
observed, showing the value of the dual-achievement of the
or a sensor grid, such as required by SHM applications [3-5].
proposed optical fiber-based mechanism in obtaining strain
measurement while being utilized as a strengthening agent. The most efficient SHM technique involves the monitoring
of strain in concrete structures [1][3]. The already established
Keywords—Carbon reinforcement, optical fiber sensor, FOS mechanism to do so is to somehow attach, using epoxy for
functionalized carbon structures, textile carbon structure, Structural example, a network of FOSs to the target structure and monitor
Health Monitoring, Fiber Bragg Grating the variations in applied strain using interferometric techniques
[2]. Fibre Bragg Grating (FBG) sensors have thus been utilized,
for instance, for the SHM of railway bridges [6]. However, since
I. INTRODUCTION this technique relies on glue to attach the sensors to the structure,
Current trend in engineering towards “smart systems” it does not fully transfer the strain to the sensing element. In
addressing multiple issues/purposes simultaneously has addition, it becomes a tedious installation having to manually
rendered the need for the implementation of sensor schemes that attach the sensors to cover the target region, not to mention the
are of multi-purpose nature. Recent advances in the uptake of likelihood of breaking the sensing elements while doing so.
fiber optic sensors (FOS) for Structural Health Monitoring Previous communication by the group [7] successfully
(SHM) has seen a wide range of research into the feasibility of established an effective force transfer between FOS (in a Mach-
“embedding” sensors within the target structure itself in order to Zehnder interferometric setup) and carbon structure, utilizing a
elicit higher sensitivity and for real-time data extraction [1]. The novel textile reinforcement structure design, which

978-1-5090-1012-7/17/$31.00 ©2017 IEEE

simultaneously acts as both the reinforcement of concrete and its B. Textile carbon net structures
SHM mechanism. The work presented here is a follow-up An embroidery technology was developed at the Saxon
design where FBG sensors are integrated into a textile-based Textile Research Institute (STFI), Chemnitz, Germany to
carbon reinforcement structure for the first time for SHM. manufacture the functionalized carbon structure. The developed
Unlike traditional FBG-based strain sensing mechanisms, the embroidering technique allows the simultaneous processing of
new design will be inherently incorporated within the carbon fibers and optical fiber, i.e. optical fiber incorporating the
strengthening mechanism of the structure, thereby achieving the FBG sensors. Depending on the application, tailored carbon
dual purposes of SHM and strengthening. structures can be manufactured, i.e. several layers of carbon
In light of the discussion above, the paper presents the filaments with up to 3.200 tex, as well as very different grid
development of a tailored functionalized textile-based carbon structures. A FTCS structure with integrated optical fiber
structure (FTCS) with integrated FBG sensors using a “weaving sensors is shown in Fig. 2.
and embroidery” technique of optical fibers and textile carbon
filaments in a net-based construction. The proposed mechanism
achieves simultaneous purposes of acting as the reinforcement
and the SHM mechanism, thereby saving a considerable amount
of time and money. Furthermore, unlike the case where glue is
involved, any applied strain would be directly transferred to the
sensing element itself, ensuring improved accuracy of the strain
profile obtained of the target structure.

Fig. 2. Fabricated textile-based carbon reinforcement structure with

II. FUNCTIONALIZED TEXTILE-BASED CARBON integrated optical fiber sensors.
STRUCTURES
In order to fabricate the FTCS the FBG sensors as well as
A schematic of the FTCS design with integrated FBG the carbon fibers were embroidered on a polyvinyl alcohol
sensors is shown in Fig. 1. In this design, the proposed FTCS (PVA) nonwoven substrate. To obtain the final grid-like carbon
acts as both the reinforcement as well as the SHM mechanism. structures, several layers of carbon fibers were embroidered
In order to fabricate the FTCS first the FBG sensors were subsequently on the PVA nonwoven substrate. After the
manufactured, which were integrated afterwards, during the embroidering fabrication process is complete, the nonwoven
embroidery of the FTCS. substrate was removed by dissolving the PVA in approximately
50 °C hot water.
III. SENSOR PERFORMANCE
The performance of the fabricated structures with integrated
FBG sensors was evaluated using a tensile testing machine
(MFC T3000), as shown in Fig. 3a. In order to measure the force
transfer between the textile carbon structure and optical fiber
sensors, only the carbon structure was mounted to the tensile
testing machine. Furthermore, during the evaluation the
spectrum of the FBG sensors was obtained using the BBS and
the OSA again. In Fig. 3b a measured FBG sensor spectrum is
illustrated.
(a) (b)

Fig. 1. Schematic of functionalized textile-based carbon reinforcement

structure with integrated FBG sensors.

A. Fiber Bragg grating sensors

The FBG sensors have been fabricated in house using a KrF
excimer laser (ATLEX 500 FBG) and the phase mask technique
(IBSEN 1070 nm) on photosensitive single-mode fiber
(Thorlabs GF1 or Fibercore SM1500). The fabrication of the
FBG sensors has been monitored in real time using a broadband
light source (BBS Opto Link C band) and an optical spectrum
analyzer (ANDO AQ6317B). The resulting Bragg wavelength Fig. 3. Evaluating the sensor performance using a tensile testing
of the fabricated FBG sensors are λB = 1558.7 nm with a machine (a) and recorded FBG sensor spectrum (b).
reflectivity of approximately 40%. The measured sensor performance of the functionalized
textile-based carbon reinforcement structure sample is shown in
Fig. 4. The sensor performance shows a good correlation
between the applied force and the measured Bragg wavelength V. CONCLUSION
change ΔλB. In total, three subsequent measurements on the A textile-based carbon structure with integrated FBG
same sample were performed. As illustrated in Fig. 4, a linear sensors that can be used for SHM has been introduced and
response (0.00159 nm/N) with relatively small drift (4.16×10-5) evaluated. The functionalized structures have been fabricated
were obtained for the three measurements of the same sample. using a modified embroidery machine that has been developed
at the STFI in Chemnitz, Germany. The developed embroidery
machine allows the cost-efficient fabrication of tailored
functionalized textile-based carbon structures. The evaluation of
the sensor performance indicates a linear correlation between
applied forces and measured Bragg wavelength changes
(0.00159 nm/N) with a small drift (4.16×10-5). Based on the
successful evaluation presented here, currently the performance
of the structures is evaluated for when the structure is embedded
into concrete elements.

ACKNOWLEDGMENT
The authors acknowledge support of the Bundesministerium
fuer Bildung und Forschung (BMBF) within Grant Number
Fig. 4. Bragg wavelength shift of the functionalized textile-based 03ZZ0345.
carbon reinforcement structure due to applied load.

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