PSI - Issue 64

5

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

1685

a)

b)

Fig. 5. Location of DFOS sensors on the rail: a) cross-section; b) view after installation.

The sensors were delivered to the site in coiled sections (Fig. 6a) and connected together to form a long loop (it is possible to connect DFOS sensors in series to provide the loop configuration required by some interrogators or to reduce the number of measurement channels). Due to the necessity of bonding more than 2 km of sensors, procedures have been developed and a special machine (Figure 6b) has been developed to automate this process. Measuring axial strain requires a good bond between the sensor and the structure being monitored. For this reason, the rail surface was thoroughly cleaned beforehand with a jet of pressurised water. The fibre optic pigtails (signal wires) were protected in wells within the embankment (Figure 6c).

a)

b)

c)

d)

Fig. 6. a) Sensors delivered on site in coils; b) special automated machine for installation; c) view of the pigtail protection well; d) different optical interrogators inside the measurement car.

During the sessions, optical interrogators were located in the measurement car (Fig. 6d), where the appropriate temperature and humidity conditions for their correct operation were ensured. The pigtails from the protection wells were lead into the cabin and connected directly to the interrogators. The measurements were carried out over a long period (one year), taking into account the extremely variable weather conditions (Fig. 7) and, in particular, the significant changes in temperature. In the January measurements, the recorded rail temperature (measured with a pyrometer - Fig. 8a) was - 15°C, while in July it was already +45°C. The introduction of appropriate thermal compensation methods for such large temperature gradients is crucial for the correct interpretation of the measurement results. In addition, the lower and upper sensors can operate simultaneously in different solar conditions (see Figure 5b), depending on the date and time of the measurement and the current weather conditions. It should be noted that within the monitored section of the railway embankment there is a two-span concrete bridge (Fig. 8a), which provides local ground stiffening. The influence of the transition zone (between the relatively soft earth embankment and the relatively stiff structure) on the rail deformation is one of the subjects of a detailed analysis based on the recorded measurement data. This analysis is carried out both with regard to long-term measurement results (twelve measurements over one year) and with regard to short-term train runs at different speeds (Fig. 8a and 8b).