W. De Waele

Transversal Load Sensing With Fiber Bragg Gratings in Microstructured Optical Fibers

We present fiber Bragg grating based transversal load sensing with a highly birefringent microstructured optical fiber. For the bare fiber, the change of the Bragg peak separation under a transverse line load was simulated with a finite-element model and experimentally verified. We also show that microstructured optical fibers with fiber Bragg gratings can be successfully embedded in a carbon fiber reinforced composite material. The linear dependence of the Bragg peak separation to a transversal stress in the composite sample was measured to be 15.3 pm/MPa.

Monitoring of fibre reinforced composites with embedded optical fibre Bragg sensors, with application to filament wound pressure vessels

Tests carried out on bare optical fibres with a Bragg sensor show the feasibility of using these sensors for strain sensing. They have been embedded into simple composite laminates and have been subjected to static loading in bending tests. The measured strain from the Bragg sensor is perfectly linear with the applied force. Optical fibres with a Bragg sensor have also been embedded into filament wound pressure vessels. Tests carried out on such a pressure vessel include both static and slowly varying load schemes. The Bragg signal is nearly perfectly linear with the applied pressure. The results demonstrate the applicability of Bragg sensors for continuous monitoring of composite materials.

Multi-axial strain transfer from laminated CFRP composites to embedded Bragg sensor: I. Parametric study

Embedded optical fibre sensors are considered in numerous applications for structural health monitoring purposes. However, since the optical fibre and the host material in which it is embedded, will have different material properties, strain in both materials will not be equal when load is applied. Therefore, the multi-axial strain transfer from the host material to the embedded sensor (optical fibre) has to be considered in detail. In the first part of this paper the strain transfer will be determined using finite element modelling of a circular isotropic glass fibre embedded first in an isotropic host and second in an anisotropic composite material. The strain transfer or relation depends on the mechanical properties of the host material and the sensor (Youngs modulus and Poissons ratio), on the lay-up of the composite material (uni-directional lay-up/cross-ply lay-up) and the position of the sensor in a certain layer. In the second part of the paper the developed strain transfer model will be evaluated for one specific lay-up and sensor type.

Response of FBGs in Microstructured and Bow Tie Fibers Embedded in Laminated Composite

Fiber Bragg gratings in bow tie fiber and highly birefringent microstructured optical fiber are embedded in a carbon fiber reinforced epoxy. The Bragg peak wavelength shifts of the embedded gratings are measured under controlled bending, transversal loading, and thermal cycling of the composite sample. We obtain similar axial and transversal strain sensitivities for the two embedded fiber types. We also highlight the low temperature dependence of the Bragg peak separation of the microstructured fibers, which is an important advantage for this application. The results show the feasibility of using microstructured fibers in structural integrity monitoring.

Multi-axial strain transfer from laminated CFRP composites to embedded Bragg sensor: II. Experimental validation

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, the strain in both materials will not be equal when external load is applied. Therefore, the strain transfer from the host material to the embedded sensor (optical fibre) was studied in more detail in the first part of the paper. This second part presents 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. This loading case induces high birefringence effects in the core of the optical fibre. Using the numerically determined strain transfer coefficients (Luyckx et al 2010 Smart. Mater. Struct. 19 105017) together with multi-axial strain formulations, the authors were able to measure with reasonable accuracy the total strain field inside a carbon fibre reinforced plastic specimen.

Numerical and experimental study of the fatigue of threaded pipe couplings

Threaded pipe couplings are used in the oil and gas industry as an alternative for welding and in applications where pipes should be frequently coupled and uncoupled as is the case for drill pipes. To maintain a sealed and secure connection while being subjected to external variable loads, they are commonly preloaded by using threaded connections with a conical shape. In this study a series of finite element analyses of conical threaded API Line Pipe couplings is carried out to quantify the influence of its contact parameters (coefficient of friction, preload and taper angle). In the assembled pipes, small sliding of the thread contact surfaces is still possible due to elastic deformation under external loads. This means, the contact zones of the threads can change under load, which complicates the analysis of the couplings. It was found that the coefficient of friction between the threads had a significant influence on this sliding. The shape of the threads inherently causes local stress concentrations, which can in turn initiate fatigue cracks. To validate the results of the finite element simulations, both static and dynamic tests were carried out on an API Line Pipe coupling. The preloading of the connection was carried out on a torque machine and an experimental fatigue test was carried out on a four-point bending test setup. Strains measured by strain gauges on the connections are in good agreement with the strains predicted by the finite element simulations. Appearing fatigue cracks emanated from the root of the last engaged thread of the male part of the connection, which is the region with the highest stress concentration factor in the numerical model.

Feasibility study on measuring axial and transverse stress/strain components in composite materials using Bragg sensors

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.

Efficient Fatigue Testing of Tubular Joints

Tubular joints are intensively used in off-shore structures for shallow waters. Depending on the sea conditions and the type of structure, the design can be fatigue driven. This is particularly the case for off-shore wind turbines, where turbulences are generating a fatigue loading. Any improvement of the fatigue performance of the tubular joint would be beneficial to reduce the weight and the cost of the structure. To assess efficiently the fatigue resistance of the tubular joint, a testing method has been developed based on the resonance principle. The complete circumference of the welded joint can be loaded, successively in the in-plane and out-of-plane modes at a frequency close to 20hz. Finite element computations were used to investigate the feasibility of the concept. Then, an X-node was made and successfully tested to investigate the stress distribution along the weld. The experimental results were compared with finite element computations, giving a good agreement.

Benchmarking the response of Bragg gratings written in micro-structured and bow tie fiber embedded in composites

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.