We demonstrate the application of distributed fiber optic strain sensing based on optical frequency-domain reflectometry for the early identification and location of fatigue cracks in welds in steel tubular test specimens. This involved subjecting two welded tubular specimens, equipped with surface-mounted optical fiber sensors, to a resonant bending load. We continuously measured the strain distributions in the test specimens without any interruption throughout the entire duration of the test. The measurements were carried out with a spatial resolution of 2.6 mm and a strain resolution of 1 με. The test samples were designed so that the fatigue cracks initiated from the inner surface of the pipe at the weld root and propagated through the wall thickness towards the outer surface. The fatigue tests were performed under two stress ranges. In Sample 1 (higher stress), we successfully detected and located the strain concentration region at approximately 5% of the specimen’s lifetime before the breakthrough. For Sample 2 (lower stress) the initiation of the hotspot was detected around 27% of the specimen’s lifetime before the breakthrough. We demonstrate that our method allows for on-the-fly detection and location of fatigue cracks originating from the inner surface of the specimens' wall.
A fibre optic sensor design is proposed for simultaneously measuring the 3D stress (or strain) components and temperature inside thermo hardened composite materials. The sensor is based on two fibre Bragg gratings written in polarisation maintaining fibre. Based on calculations of the condition number, it will be shown that reasonable accuracies are to be expected. First tests on the bare sensors and on the sensors embedded in composite material, which confirm the expected behaviour, will be presented.
Composite materials are increasingly used in aerospace applications, owing to their high strength-to-mass ratio. Such materials are nevertheless vulnerable to impact damage. It is therefore important to investigate the effects of impacts on composites. Here we embed specialty microstructured optical fiber Bragg grating based sensors inside a carbon fiber reinforced polymer, providing access to the 3D strain evolution within the composite during impact. We measured a maximum strain of -655 με along the direction of impact, and substantially lower values in the two in-plane directions. Such in-situ characterization can trigger insight in the development of impact damage in composites.
This paper demonstrates that epoxy-based single mode polymer waveguides with Bragg gratings can be realized in very thin (down to 50 micron) polymer foils which are suitable for strain sensing when integrated inside glass fiber reinforced polymer composite materials. The single mode waveguides were fabricated using laser direct-write lithography and the gratings were realized using nanoimprint lithography. These steps were performed on a temporary rigid carrier substrate and afterwards the functional layers were released yielding the thin, flexible sensor foils which can be laser-cut to the required dimensions. The Bragg grating-based polymer waveguide sensor foils were characterized before and after embedding into the composite. As expected, there was a blue shift in the reflection spectrum because of residual strain due to the embedding process. However, the quality of the signal did not degrade after embedding, both for 50 and 100 micron thick sensor foils. Finally, the sensitivity to strain of the embedded sensors was determined using a tensile test and found to be about 1 pm / microstrain.
Composite materials are extensively used in a wide array of application markets by virtue of their strength, stiffness and lightness. Many composite structures are replaced today not only after failure but also before, for precautionary reasons. Adding optical sensing intelligence to these structures not only prolongs their lifetime but also significantly reduces the use of raw materials and energy. The use of optical based sensors offer numerous advantages i.e. integrability, high sensitivity, compactness and electromagnetic immunity. Most sensors integrated in composites are based on silica fibers with Bragg gratings. However, polymers are an interesting alternative because they present several advantages. They have high values in the opticalconstants involved in sensing, are cost-effective and allow larger elongations than silica. Moreover, planar optical waveguides represent an interesting approach to be further integrated e.g. in circuits. We present a comparison between Ormocer®-based and epoxy-based polymer waveguide Bragg grating sensors. Both polymers were screened for their compatibility with composite production processes and for their sensitivity to measure temperature and stress. Ormocer®-based sensors were found to exhibit a very high sensitivity (-250 pm/°C) for temperature sensing, while the epoxy-based sensors, although less sensitive (-90 pm/°C) were more compatible with the epoxy-based composite production process. In terms of sensitivity to measure stress, both materials were found to be analogous with measured values of (2.98 pm/μepsilon) for the epoxy-based and (3.00 pm/μepsilon) for Ormocer®-based sensors.
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.
We present the use of microstructured optical fiber Bragg grating-based sensors for strain and temperature monitoring inside the concrete buffer of the Belgian supercontainer concept, demonstrated in a half-scale test in 2013. This test incorporated several optical fiber sensors inside the concrete buffer for production and condition monitoring. The optical fiber sensors presented here consist of small carbon-reinforced composite plates in which highly birefringent Butterfly microstructured optical fibers, equipped with fiber Bragg gratings, were embedded. The double reflection spectrum of these MOFGBs allows to simultaneously monitor strain and temperature, as confirmed by comparison with data obtained from thermocouples and vibrating-wire sensors installed near the MOFBGs.
We present shear stress sensing with a Bragg grating sensor fabricated in a highly birefringent microstructured optical fiber. This sensor has a shear strain sensing resolution of 0.04 pm/με when embedded in a shear loaded adhesive bond. We achieve discrete shear stress mapping in an adhesive bond by embedding a multitude of these sensors at different locations in the bond line. Experiments and numerical modeling show the limited influence of angular misalignment of the sensor on its shear stress response. Finally, we discuss the cross-sensitivity of this sensor to shear strain and temperature.
The application of shear stress sensors in structural health monitoring remains limited because current sensors are either difficult to implement, they feature a low measurement resolution or the interrogation of the output signal is complex. We propose to use fiber Bragg grating-based sensors fabricated in dedicated highly birefringent microstructured optical fibers. When embedded in a host material, the orientation angle of the fiber should be chosen such that their polarization axes are aligned parallel with the direction of maximum shear stress when the host is mechanically loaded. We present experimental results of sensors embedded in the adhesive layer of single lap and double lap structural joints. These tests demonstrate that when the joints are tension loaded, the embedded sensors have a shear stress sensitivity of around 60 pm/MPa. We study the influence of the adhesive material on the sensor response, as well as the influence of sensor orientation and location in the bond line. Finally, we demonstrate the minimal thermal cross-sensitivity of the shear stress sensitivity of this sensor.
We report on the use of a fiber Bragg grating (FBG) based sensor written in a photonic crystal fiber (PCF) to monitor the cure cycle of composite materials. The PCF under study has been specifically designed to feature a high phase modal birefringence sensitivity to transverse strain and a very low sensitivity to temperature. We exploit these particular properties to measure strain inside a composite material in the out-of-plane direction. The embedded FBG sensor has been calibrated for transverse and axial strain as well as for temperature changes. These FBGs have then been used as embedded sensors during the manufacturing of a composite material in order to monitor how strain develops inside the composite during the cure cycle. We show that our sensors allow gaining insight in the composite cure cycle in a way that would be very difficult to achieve with any other sensor technology.
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.
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.
This paper discusses a new sensor design based on optical fibre Bragg gratings which is being developed in the
framework of the MASSFOS-project ('Multi-Axial Stress and Strain sensing of thermo hardened composite elements
using Fibre Optic Sensors'-project). The objective of this ESA
co-funded project is to develop a monitoring system
which measures dynamically the multi-axial stress and strain plus temperature in thermo hardened composite elements.
The sensor consists of a 'High Birefringence'-fibre in which two gratings have been inscribed; each grating yields two
distinct Bragg peaks. The first grating is sensitive to the total stress field in the material, while the second one is isolated
from transverse stress components. By measuring the four Bragg peaks of the sensor, it is theoretically possible to
determine the total strain field, plus the temperature inside a composite material. Static experiments (a tensile test and a
compressive test) have already been carried out to prove the feasibility of the sensor embedded in a composite laminate.
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