PSI - Issue 64

Bertram Richter et al. / Procedia Structural Integrity 64 (2024) 1208–1215 Richter et al. / Structural Integrity Procedia 00 (2019) 000–000

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monolithic DFOS with 1 mm diameter and smooth surface, which was glued with an epoxy resin to the longitudinal rebar. Special type of DFOS– fiber in metal tube (FIMT)– were integrated directly into two strands of the post tensioned tendon (hereafter called “smart_tendon”) and into one of the prestressing strands (hereafter called “smart_strand”). The FIMT are integrated between the wires through a sophisticated winding process and held in place by friction alone without use of adhesives (Gläser, 2023). The concrete DFOS and the reinforcement DFOS were measured by a Rayleigh backscattering based interrogator unit ODiSI 6100 (Luna Innovations Inc., 2020) with a spatial resolution of 1.3 mm and a measurement frequency of 1.56Hz. The smart_tendon and smart_strand were interrogated by a Brillouin backscattering based fTB5020 (fibrisTerre Systems GmbH, 2019). The spatial resolution was set to 0.2m, resulting in a spatial sampling of approximately 5cm. The frequency scan step was set to 5MHz, giving a refresh rate of approximately 0.008Hz across the three FIMTs. The tare of all fiber optic measurements was set just before first loading. To evaluate force–displacement behavior, the midspan deflection was measured by a draw-wire sensor. Bearing displacements were measured by linear variable displacement transducers and the applied forces were measured by load cells at each load application point. 2.4. Data evaluation The assessment of huge amounts of DFOS data is still a challenge (Janiak et al., 2023; Richter et al., 2023). The free open-source software framework fosanalysis (available at https://github.com/TUD-IMB/fosanalysis/) streamlines the data analysis process from parsing measurement data files, selecting parts of the data, preprocessing the data, up to crack detection and crack width calculation (Richter et al., 2023). The following plots show median strain profiles over a time span of 30 s during plateaus where the load was held constant. Apart from the temporal median, no further preprocessing was carried out. 3. Results The strain profiles for three selected load levels are presented below. Fig. 4 shows the strain profiles at t 3 with F = 80 kN for all investigated DFOS. While the concrete DFOS provides a strain profile with pronounced strain peaks at the crack locations, the reinforcement DFOS shows several disturbances. Firstly, even after applying the temporal median, several isolated spikes of large positive or negative strain values remain. These spikes are referred to as harmful strain reading anomalies (Bado et al., 2021). Secondly, some strain peaks are clipped and the trunk of the strain peak is widened (e.g., at x = −0.1 m), indicating a detachment of the DFOS from the reinforcement, indicated with arrows in Fig. 4. The detachments can cause a melding of closely neighbored peaks. Those effects impose challenges on automatic crack detection. In general, strain profiles from DFOS attached to the reinforcement or embedded in the concrete matrix differ strongly (Richter et al., 2023). The installation goal for the reinforcement DFOS, measuring “pure” steel strains, could not be achieved. The direct contact with the surrounding concrete led to the strain profile being influenced by the concrete. Other installation methods (Bado et al., 2020) are recommended if pure steel strains are to be measured. However, these are more laborious and susceptible to damage to the DFOS.

Fig. 4. Comparison of strain profiles of different sensors at t 3 ( F = 80 kN)