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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Sensors are a key component of the overall system as they are fully integrated into the structure being monitored. Whilst it is technically straightforward to replace an interrogator, it is challenging to replace or repair a sensor that is installed within the operating structure. Therefore, once sensors are installed, their service life should be estimated in the same way as the service life of the structure itself. Special care should be taken when selecting sensors, and it should be noted that there are at least several solutions with the same declared capabilities (e.g. axial strain measurement), but which in practice give significant differences in the quality of the obtained results [Howiacki et al. (2023)]. Among the solutions proposed on the market are monolithic composite sensors [Bednarski et al. (2022)], whose spatial visualisation is shown in Fig. 2a. The main features of these sensors include: • high accuracy (sensing fibre fully integrated into the monolithic core with no intermediate layers), • high durability and resistance to harsh environmental conditions (use of suitable corrosion resistant composites), • high deformability (up to ±4 %) to ensure correct sensor operation even under extreme loading conditions (e.g. within local cracks, fractures, buckling, as well as measurements outside the linear-elastic range of the structure; e.g. plastic deformation of steel starts at around 0.2%), • can be embedded in the medium (e.g. in concrete or soil) or bonded directly to the surface (e.g. on steel or composite elements - Fig. 2b), • possibility to adapt the sensor parameters to the requirements of a specific project (e.g. size and shape of the cross-section, type of material, axial and bending stiffness, strength, external braiding, number of internal fibres, minimum bending radius, etc.).

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Fig. 2. (a) Spatial visualisation of selected DFOS monolithic sensors; (b) example of application to measurements on a composite surface.

It should be noted that Poland is currently a world leader in the development and production of monolithic sensors and their implementation for monitoring engineering and geotechnical structures as part of DFOS systems. Typical projects include both short-term laboratory measurements (e.g. as part of doctoral research) or field measurements (e.g. test loads on bridges or piles), as well as long-term measurements under changing operational conditions. Monolithic sensors have been successfully installed in countries such as Germany, the Netherlands, the UK, Japan, Australia, the USA, Mexico and others. Selected field projects carried out in Poland are shown in Figures 3a - 3i. The remainder of this article discusses one of the largest systems in Poland, which used more than 2 km of monolithic DFOS sensors to monitor rail deformation.

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