PSI - Issue 64

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

1682

2

1. Introduction Distributed fibre optic sensing (DFOS) is increasingly used in civil engineering and geotechnics [Barrias et al. (2016), Bado and Casas (2021)]. The main feature, and at the same time an advantage and superiority over conventional point-based measurement methods, is the possibility of carrying out measurements of selected physical quantities over the entire length ranging from a few centimetres to even several hundred kilometres [ Güemes et al. (2010)]. Therefore, the advantages of this technique become particularly apparent when monitoring long structures such as roads and embankments [ Sieńko et al. (2022) ], bridges [ Sieńko et al. (2023)], tunnels or pipelines [Popielski et al. (2021)]. The measurement results, instead of a single value obtained in a single point in time , are profiles of strain [ Piątek et al. (2023) , Sieńko et al. (2019) ], temperature [Lu et al. (2023)], displacement ∆ [Bednarski et al. (2021), Bednarski et al. (2021), Siwowski et al. (2021)] or vibrations ∆ [Shang et al. (2022), Kishida et al. (2022)], both as a function of time and as a function of length (equation 1). This allows direct detection of local events such as cracks in the concrete [Howiacki (2022)], stress concentrations, buckling, local settlement, ground cracking, leakage and others. ( ), ( ), ∆( ), ∆ ( ) → ( , ), ( , ), ∆( , ), ∆ ( , ) (1) To perform distributed (geometrically continuous) measurements, progressive interrogators (optical data loggers) use three fundamental physical phenomena, such as Rayleigh scattering [Kishida et al. (2013)], Brillouin scattering [Bastianini et al. (2019)] and Raman scattering [Lu et al. (2023)]. The first two are used to measure strains caused by both mechanical and thermal actions. The third allows only the measurement of temperature profiles along the length. Typically, Rayleigh scattering is used for strain measurements with very high spatial resolution (up to 1000 points per metre of sensor) over distances of the order of a few hundred metres, or for vibration (strain rate) measurements at very high frequency (up to 40 kHz) over distances of up to 150 km. Therefore, this technique is often used to measure linear objects exposed to dynamic influences, such as railway lines [Vidovic and Marschnig (2020)]. Brillouin scattering, on the other hand, with limited spatial resolution (typically between 1 and 5 points per metre), is used to measure strain over distances of tens of kilometres. Similar ranges are achieved by Raman scattering, which can be used as part of the thermal compensation of the other techniques, or as a stand-alone tool to detect temperature change events (e.g. leaks, fires). It should be emphasised that monitoring such long distances using traditional point methods (Figure 1) is very often inefficient or economically unviable, and in many cases technically impossible.

Fig. 1. Conventional spot gauges vs. distributed fibre optic sensors (DFOS) on a railway line - scheme.

The optical phenomena discussed above are used in many types of instrumentation, each with its own advantages and limitations. However, in order to build an effective DFOS system, it is necessary to select not only the appropriate parameters of the interrogators, but also the sensors, their installation methods, or the way of acquiring, processing and interpreting the recorded data. Each project should be treated on an individual basis.