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Abílio Silva1(*), Paulo Reis1, Piotr Miluski 2, Dominik Dorosz 2
1
C-MAST, Dept. of Electromechanical Engineering, University of Beira Interior, Covilhã, Portugal
2
Bialystok University of Technology, Faculty of Electrical Engineering, Bialystok, Poland
(*)
Email: [email protected]
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ABSTRACT
Optical non-destructive testing (NDT) has gained more and more attention in recent years, as
consequence of its non-destructive imaging characteristics with high precision and sensitivity.
Therefore, the main goal of this work is to study the viability of the single optical fibres
embedded to evaluate the mechanical properties of CFRP structures. For this purpose, flexural
tests were performed on specimens embed by a commercial single optical fibre (F-MLD with
coating diameter of 250 m). CFRP laminates with two different stacking sequences and
different thicknesses (2.7, 3.3 and 4.0 mm) were used. Finally, three different positions of the
optical fibre, along the laminate thickness, were studied. The results demonstrate that the
manufacturing technique is appropriate, because the mechanical properties of the laminates are
not affected by the sensors presence. It was also demonstrated that the sensor presents different
answer for different stacking sequences.
Keywords: Optical fibres, CFRP, Bending testing, Mechanical behaviour.
INTRODUCTION
The carbon fibre reinforced plastic (CFRP) are increasingly used in various fields of engineering
and this trend is likely to increase due to their excellent specific strength and stiffness [1-3].
Even though CFRP is being used in almost all modern aerospace structures as a primary
structural material, it is still difficult to precisely manufacture large scale CFRP structures and
ensure their structural integrity during operation [4]. Innovative techniques are required to
monitor the internal states of composite structures and utilize the obtained data to improve the
structural design, processing technologies and maintenance methods [1, 2, 4].
Smart materials are, by definition, materials capable to sense alterations occurring inside them,
to interpret them and to react to them by means of actuators. Applying those materials to the
conception of bridges, pressure vessels, aircrafts, etc., it is possible to know the general state of
degradation of the structure, before and during its use [5]. The development of smart composite
structures involves the integration of sensors (such as optical fibre sensors) into these advanced
laminated materials allowing in situ process monitoring and continuous structural health check
[1]. The evolution in the optical fibre sensors, in combination with advances in composite
material technology, has opened up the new field of fibre optic smart material structures, the
workability of composite materials ensuring a good embedding of fibre optic sensor into the
material [2, 5].
Optical fibre sensors have attracted considerable attention, since they are small, lightweight,
immune to electromagnetic interference, environmentally stable and they have very little signal
loss over extremely long distances [1, 4, 6-8]. Furthermore, they possess sufficient flexibility,
strength and heat resistance to be embedded relatively easily into composite laminates [4, 6].
Optic fibre sensors can be classified according to the light parameters that are modulated. There
are three types of sensors: the intensity, the phase and the wavelengths modulated [5, 9]. The
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most suitable sensor depends upon several factors such as the required sensor sensitivity, the
manufacture cost, the possibility to provide absolute measurements [5, 10].
RESULTS
Table 1 shows the number and sequence of the CFRP layers, where “Y” means laminate with
optical fibre and “N” the laminate without optical fibre (OF), “0i” the number of layers with
carbon fibres in the specimen length direction, and “90i” the number of layers with carbon fibres
in the width direction (i = 1, 2).
Table 1. Scheme of the CFRP laminates with and without optical fibres.
Type
Aver. thickness
[mm]
2.65
A
3.99
Opt. fibre
Packing sequence
N
0290202902029020290202
Y
02901(OF)9010290201
N
02902029020290202902
Y
02901(OF)901029020290201
Figure 1 shows a representative example of the nominal bending stress versus flexural
displacement curves, for laminates with different thicknesses and layup. As expected, increasing
the thickness increases the flexural strength for both configurations (A and B). On the other
hand, increasing the thickness decreases the displacement for both configurations.
Figure 1. Tensile test results
CONCLUSION
This study demonstrated that the manufacture method to embed the optical fibres on CFRP
laminates is adequate and the mechanical properties are not affected by the optical fibres.
A single optical fibre, without interferometric sensors, can be used to monitorize the health of the
composite laminates. Its sensory capacity is related with the optical signal intensity.
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The localization of the fibres is relevant on the optical signals obtained and their incorporation
between layers with perpendicular direction is favourable.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the funding by Ministério da Ciência, Tecnologia e Ensino
Superior, FCT, Portugal, under grants PTDC/SAU-BEB/71981/2026.
REFERENCES
[1] Abrate S. Impact on laminated composite materials. Appl. Mech. Rev 1991; 44: 155-190.
[2] Richardson MOW, Wisheart MJ. Review of low-velocity impact properties of composite
materials. Compos Part A-Appl S 1996; 27: 1123-1131.
[3] Reis PNB, Ferreira JAM, Antunes FV, Richardson MOW. Effect of interlayer delamination
on mechanical behavior of carbon/epoxy laminates. J Compos Mater 2009; 43: 2609-2621.
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