In this paper, the building blocks for a smart glass connector platform are presented. The connector is meant to connect optical fibers, which cannot be connected using conventional connector or splicing technology, e.g. optical sensing fibers embedded in composite structures for structural health monitoring purposes. We are setting up a technology platform using aluminosilicate glass substrates as interposer between 2 optical fibers that need to be connected. Using femtosecond laser technology, this interposer can be equipped with basic building blocks such as low-loss intermediate single mode waveguides, couplers and Bragg grating sensors. To this end, femtosecond laser-written waveguides are inscribed in aluminosilicate glass (Xensation® Cover float glass) and the laser parameters are optimized. Waveguides with estimated propagation losses of 0.65 dB/cm are obtained and are subsequently used to make couplers in glass. These couplers can be tuned in coupling ratio between 87:13 and 15:85 by varying the coupling length from 1 to 4.5 mm at a pitch of 17 μm. In addition, fs-laser written Bragg gratings were manufactured, with the goal to add monitoring capabilities to the interposer. These were realized by modulating the fs-laser pulse train and as such a reflectivity up to 5.9% was achieved in the C-band for 23 mm long Bragg gratings. When properly designed, these can be made compatible with standard read out equipment for fiber Bragg gratings (FBGs).
The possibility of embedding optical fiber sensors inside carbon fiber reinforced polymer (CFRP) for structural health monitoring purposes has already been demonstrated previously. So far however, these sensors only allowed axial strain measurements because of their low sensitivity for strain in the direction perpendicular to the optical fiber’s axis. The design flexibility provided by novel photonic crystal fiber (PCF) technology now allows developing dedicated fibers with substantially enhanced sensitivity to such transverse loads. We exploited that flexibility and we developed a PCF that, when equipped with a fiber Bragg grating (FBG), leads to a sensor that allows measuring transverse strains in reinforced composite materials, with an order of magnitude increase of the sensitivity over the state-of-the-art. In addition it allows shear strain sensing in adhesive bonds, which are used in composite repair patches. This is confirmed both with experiments and finite element simulations on such fibers embedded in CFRP coupons and adhesive bonds. Our sensor brings the achievable transverse strain measurement resolution close to a target value of 1 μstrain and could therefore play an important role for multi-dimensional strain sensing, not only in the domain of structural health monitoring, but also in the field of composite material production monitoring. Our results thereby illustrate the added value that PCFs have to offer for internal strain measurements inside composite materials and structures.
Embedded optical fibre sensors are considered in numerous applications for structural health monitoring purposes. Since
the optical fibre and the host material in which it is embedded, have different material properties, strain in both materials
will not be equal when external load is applied. In this paper, an experimental evaluation of the response of uni-axial
fibre Bragg grating sensors embedded in small cross-ply composite laminates subjected to out-of-plane transverse
loading is discussed.
Satin woven composites can be seen as a series of connected unit cells with a definite length and width. Along the length
and width of one unit cell, local strain fields can vary significantly due to the yarn interlacing pattern. Embedded Type I
FBGs with a 80 μm- and 125 μm cladding diameter are used to study the internal transversal strain variations in unloaded
and tensile loaded thermo-plastic 5-harness satin weave composite test specimens. The residual strains are
examined at different (axial) load levels up to 300MPa and 1.2million cycles. Differential transversal strains up to 0.07%
are presented. Results show that the embedded FBGs are capable of measuring long term the transversal strain
distributions. It is found that the yarn interlacing pattern of the satin weave composite causes complex and random local
strain fields during loading and that, even though axial strain measurements are matching very well with external strain
measurements, a large scatter exists in local transversal strain states between the different composite test-coupons.
Fiber Bragg gratings written in Bow-tie fiber and in highly birefringent micro-structured optical fiber are embedded in a
carbon fiber reinforced epoxy. The Bragg peak wavelength shifts are measured under controlled bending, transversal
load and thermal cycling of the composite sample. The results evidence the feasibility of using micro-structured fibers in
structural integrity monitoring. We obtain similar axial as well as transversal strain sensitivities for the two embedded
fiber types. We also highlight an important advantage of the micro-structured fibers for this application which is the low
temperature dependence of the birefringence.
This paper presents the transverse response of fibre Bragg grating sensors embedded in small cross-ply composite
laminates. Since the optical fibre (isotropic) and the host material (anisotropic) have different material properties, strain
in both materials will not be equal when load is applied. Therefore, the strain transfer from host material to the embedded
sensor (optical fibre) has been determined using FE-simulations. Composite laminates with embedded optical fibre
sensors were submitted to transversal stress. This load case induces high birefringence effects in the core of the optical
fibre by which, using the calculated strain transfer coefficients together with multi-axial strain formulations, the authors
were able to measure the total strain field of a carbon fibre reinforced plastic specimen.
A new method is proposed for mounting a Fiber Bragg Grating to a substrate material and in this way forming an optical
strain gauge. The strain gauges were optimized for usage in aeronautical applications and hence with stringent
requirements on their performance. The installation procedure is easy, fast and reliable and happens by means of a
specially designed mounting tool called a sensor pad. It is used in combination with a UV-curable adhesive. The
performance of these sensors was investigated in an extended research program and the results of this program will be
presented. It will be shown that the proposed optical strain gauges exhibit superior performance compared to their
electrical counterparts and hence that this is a very promising method for strain sensing.
Combining the functionalities of fiber Bragg gratings (FBGs) and microstructured optical fibers (MOFs) offers
promising technological perspectives in the field of optical fiber sensors. Indeed, MOFs could overcome some of the
limitations of FBGs in conventional fibers for sensor applications. The added value of MOFs stems from the ability to
design an optical fiber in which an FBG acts as a sensor with a selective sensitivity, e.g. a sensor that is sensitive to
directional strain but not to temperature. For this purpose we use a MOF with a phase modal birefringence on the order
of 8×10-3. A FBG in this MOF yields two Bragg peak wavelengths, with a wavelength separation that depends on the
phase modal birefringence of the MOF. We characterize these FBGs for transversal loads on a bare fiber and compare
the results with simulated sensitivities. Then, we embed the sensor in a composite coupon and measure the response of
the Bragg peak wavelengths as a function of the applied transversal pressure on the composite material. This allows
drawing conclusions on the advantages of FBGs in MOFs for sensing applications.
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