PSI - Issue 64

Łukasz Bednarski et al. / Procedia Structural Integrity 64 (2024) 1681 – 1688 Author name / Structural Integrity Procedia 00 (2019) 000-000

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the sleepers on the strain distribution of the rail - particularly in its lower part - became apparent in the DFOS data. A close-up of a graph showing this phenomenon is shown in Figure 13. It should be noted that this effect does not occur on the upper sensor. With DFOS measurements it is therefore possible to analyse the strain distributions of the rail not only as a function of its length but also as a function of its height. With this type of measurement, the problem of selecting the optimum location for spot sensors is completely eliminated. This is because the distributed analysis in the length domain does not allow extreme values of the measured physical quantities to be overlooked.

Fig. 10. Example strain profiles measured by the lower sensor in session S04 (close-up on a selected section of 20 m).

Based on the strain profiles recorded along two lines at a known distance, it is possible to directly determine the curvatures in the analysed plane [Piątek et al. (2023), Bednarski et al. (2023), Monsberger et al. (2021)] (in this case in the vertical plane).Furthermore, by assuming appropriate boundary conditions at the beginning and end of the analysed section, it is possible to determine vertical displacements (settlements, deflections) expressed directly in millimetres. In practice, it is very common to define the boundary conditions on the basis of geodetic measurements or by locating the start and end of the sensor in zones considered stable (i.e. where it is reasonable to assume zero displacements). 3. Summary and conclusions The results obtained using the DFOS-based designed for rail deformation monitoring, briefly discussed within this article, is one of the largest of its kind in the world, both in terms of the total length of fibre-optic monolithic sensors installed directly on the rail surface and in terms of the number of optical interrogators to measure different physical quantities. It should be emphasised that, in addition to axial strain (which is usually the main measurement parameter in DFOS systems), the designed system additionally enabled geometrically continuous measurements of temperature and vibration (strain rate), as well as the calculation of curvature and vertical displacement. The technique of geometrically continuous fibre optic measurements opens up new possibilities in the analysis of the technical condition of structures, especially linear structures such as railway lines, road embankments or pipelines. Despite its many advantages and undoubted benefits, it should be stressed that it is not a "plug and play" solution. In other words, each project should be preceded by a detailed analysis in order to select the individual parameters of the interrogators and sensors, but also the methods of installation, data acquisition and its post processing, presentation and physical interpretation. Today's challenges in the field of long-distance DFOS measurement focus on the development of effective thermal compensation methods and post-processing algorithms, as well as the possibility of measurement automation. Interdisciplinary collaboration between engineers, mechanics, electronics, programmers and IT specialists is required to stream data from the interrogators to remote servers equipped with appropriate software to process and visualise the large amounts of data. The benefits of automating DFOS measurements are well worth the effort. It is notable that the first implementations of such an approach were carried out in the UK in 2022.