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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Fig. 7. Example of atmospheric conditions under which long-term DFOS measurements can be made.

a)

b)

Fig. 8. The passage of different trains along the monitored section: a) passenger train crossing the bridge; b) freight train.

Figures 9a and 9b show examples of the measurement results in the form of strain profiles as a function of length (for the upper and lower sensors, respectively) over four subsequent measurement sessions (from January to April 2023). For the sake of legibility, a 50 m section has been chosen for presentation, in which the analysed bridge structure is located. In subsequent measurements (in monthly cycles), due to the increase in temperature with respect to the zero measurement made in January, the average strain values move towards negative values, corresponding to the formation of compressive stresses. This phenomenon is explained by the thermal expansion of the steel, which is constrained by the connection of the rails to the substrate. Thermal compensation of the DFOS strain results is necessary to determine the strains corresponding to the actual change in structural length, as well as the part directly related to the mechanical stresses (i.e. excluding any free elongation or shortening). As rails are not a free element, thermal changes play a significant role in generating mechanical stresses.

a)

b)

Fig. 9. Example strain profiles measured by the a) upper and b) lower sensor in the first four sessions (on a selected measuring section of 50 m).

However, the restraint of the rail by the bond to the substrate is not perfectly uniform along its length. It is therefore worth noting that as the temperature gradient progressed from one session to the next, the local effect of