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.
Photonic Crystal Fibres (PCFs) are well known for allowing the implementation of specific waveguiding features that
cannot be achieved with conventional optical fibres. This results from the design flexibility of the holey structure in the
PCF cladding and/or core regions. Today PCFs have found applications for example in supercontinuum generation,
optical sensing and fibre lasers. They are now also being combined with fibre Bragg gratings, more specifically in the
fields of optical fibre sensing and all-fibre laser applications. In this contribution we discuss how we applied micro-optical
design methods based on commercially available software such as MODE Solutions and FDTD Solutions from
Lumerical Solutions, Inc. and COMSOL Multiphysics® combined with MATLAB® scripting and additional
optimization methods to develop microstructured fibres for three different purposes, i.e. PCF structures that facilitate
Bragg grating inscription, PCF structures that enable temperature insensitive pressure measurements and bendable PCFs
with a very large mode area for high power short pulse fibre lasers. For the three cases we describe the fibre design
methods and property simulations as well as the tolerance studies that take into account manufacturing imperfections as
well as possible variations in material parameters.
Microstructured optical fibers are increasingly used in optical fiber sensing applications such as for example optical fiber
based structural health monitoring. In such an application the fiber may experience substantial mechanical loads and has
to remain functional during the entire lifetime of the structure to be monitored. The resistance to different types of
mechanical loads has therefore to be characterized in order to assess the maximum stress and strain that a fiber can
sustain. In this paper we therefore report on the extensive set of tensile tests and bending experiments that we have
conducted both on microstructured optical fibers with an hexagonal air hole lattice and on standard optical fibers. We use
Weibull statistics to model the strength distribution of the fibers and we follow a fracture mechanics approach in
conjunction with microscopic observations of the fractured end faces to study crack initiation and propagation in both
types of fibers. We show that the failure strain of microstructured fibers is about 4.3% as obtained with tensile tests,
compared to 6.7% for reference fibers. Although the mechanical strength of microstructured optical fibers is lower than
that of the standard fibers it is still adequate for these fibers to be used in many applications.
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