Engineering Structures 33 (2011) 539–548
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Engineering Structures
journal homepage: www.elsevier.com/locate/engstruct
Analysis of the strain transfer in a new FBG sensor for Structural Health
Monitoring
Benjamin Torres, Ignacio Payá-Zaforteza
∗
, Pedro A. Calderón, Jose M. Adam
ICITECH, Departamento de Ingeniería de la Construcción, Universidad Politécnica de Valencia. Camino de Vera s/n, 46071 Valencia, Spain
a r t i c l e i n f o
Article history:
Received 19 May 2010
Received in revised form
2 September 2010
Accepted 2 November 2010
Available online 30 November 2010
Keywords:
Fiber Bragg grating sensors
Fiber optic sensors
Strain transfer
Structural Health Monitoring
a b s t r a c t
Structural Health Monitoring (SHM) is a topic of great interest in structural engineering due to the
ageing of the built infrastructures and the growing use of innovative structural systems and construction
materials. Although several sensing technologies have been developed for use in SHM systems, fiber optic
sensors, especially Fiber Bragg grating sensors (FBGs), are now attracting much attention due to their
advantages over other types of sensors.
This paper presents a new FBG strain sensor with an unsymmetrical packaging configuration designed
to be fixed to the surface of the monitored structure. A 3D Finite Element numerical analysis of the sensor,
packaging and adhesive used to fix the system to the host structure was conducted to study the influence
of (1) the thickness and mechanical properties of the adhesive, and (2) the configuration of the packaging
on the accuracy of the sensor. The results obtained from the numerical models show that the strains
measured by the proposed system and the actual strains in the host structure differ by less than 2.5%
due to the fact that the packaging contains only one layer of composite material in an unsymmetrical
configuration. The proposed design can thus be said to be an improvement compared to the typical
configurations of optical fiber surface strain sensors.
© 2010 Elsevier Ltd. All rights reserved.
1. Introduction
Structural Health Monitoring (SHM) has been defined by Li
et al. [1] as the use of in situ, continuous or regular (routine)
measurement and analyses of key structural and environmental
parameters under operating conditions, for the purpose of warning
of impending abnormal states or accidents at an early stage, as
well as giving maintenance and rehabilitation advice. Although
the formal establishment of this discipline is fairly recent
(the International Society for Structural Health Monitoring of
Intelligent Infrastructure was created in 2003 and the first journal
devoted exclusively to this field appeared in 1993) the history of
systems to monitor the behavior of bridges and buildings is much
older, as engineers such as Torroja and Nervi monitored some of
their innovative designs during the first half of the XXth century
(see [2,3] respectively).
A monitoring system is composed of three elements (see
[1,4–6]): a sensor system, a data processing system and a health
evaluation system. Sensors that detect the transmission of electri-
cal signals have traditionally been used in SHM, but in recent years
the use of fiber optic sensors (FOSs) has attracted a great deal of
attention in research and development, as reflected in both review
∗
Corresponding author. Tel.: +34 963877562; fax: +34 963877568.
E-mail addresses: bentorgo@upvnet.upv.es (B. Torres), igpaza@cst.upv.es
(I. Payá-Zaforteza), pcaldero@cst.upv.es (P.A. Calderón), joadmar@cst.upv.es
(J.M. Adam).
and application articles [1,7–12]. The main reasons for this inter-
est are the falling prices of FOSs added to their advantages over
conventional sensors. These advantages relate mainly to their flex-
ibility, embeddability, multiplexity, small size and immunity to
electrical or magnetic interference (see [4,13]). FOSs detect vari-
ations in the properties of light travelling through the optical fiber.
The fiber usually consists of three layers: the core, which is a thin
glass fiber, the cladding that confines the propagation of the light
within the fiber core, and an outer coating or jacket that pro-
vides the fiber mechanical strength and protects it from damage
and moisture absorption. Although there are three different FOS
technologies (point, long gauge and distributed sensors), the most
frequently used system at the present time is the Fiber Bragg
grating (FBG) point sensor [8].
FBG sensors cannot be installed bare in a structural element as
they are very fragile and the harsh environment of a construction
site and building structures would affect their durability and
behavior. Therefore several encapsulation or packaging techniques
have been developed to protect them (see [4,8,14]). It is common
practice to assume that the strain values recorded by FBG sensors
are the actual strains in the host structure, but this is not always
the case. In the common case of sensors that are first embedded
in a material or fixed to a plate and then glued to the surface of
the host structure, a discrepancy appears related to the thickness
of the embedding material or fixing plate and the thickness and
mechanical properties of the adhesive.
0141-0296/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.engstruct.2010.11.012