PSI - Issue 64

Ivan Markovic et al. / Procedia Structural Integrity 64 (2024) 1621–1627 Author name / Structural Integrity Procedia 00 (2019) 000–000

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To design the geometry, reinforcement and position of the DFOS-cables for the targeted failure mode as optimally as possible, non-linear calculations were first performed by means of the FEM-Software ATENA. Based on the ATENA results, the specimens were then fabricated in the laboratory and equipped with DFOS-cables. As all the experiments were performed in a closed lab space, with mostly constant temperature conditions, the temperature effects on the results were neglected. All eight specimens were loaded with a bending tensile machine and simultaneously the strains in DFOS-Cables were measured and recorded with an Optical Backscatter Reflectometer device (OBR). The obtained strains were subsequently compared and evaluated with the results of the FEM-modelling by ATENA-Software. Using the recorded load-deflection curve of the experiment, the ATENA models were further refined and better tuned. In this work, the crack detection and localization in both flexural and shear cracks will be investigated. 3. Test Set-Up The fibre optic strain sensing cables (DFOS) with a rugged sheathing (cable type BRUsens DSS, diameter 3.2 mm, V9-grip) were used for all measurements. They were bonded with a rapid hardening mortar in grooves in the concrete surface. The location and arrangement of grooves depend on the type of test (bending or shear test). For each of the beams designed for flexural failure (beam 1 to beam 4, dimensions width x height x length = 200 x 100 x 1200 mm, reinforced with bottom and top longitudinal reinforcement and with stirrups), two fibre optic cables were attached to the bottom reinforcement bars prior to concreting (see Table 1). Subsequently, after concreting and hardening, two longitudinal grooves were cut on the bottom side of the beam and after thorough cleaning of grooves, another fibre optic cable was placed into these grooves and fully bonded with a rapid hardening mortar (Figure 1, left). For the beams designed to fail in shear (beam 5 to beam 8, dimensions width x height x length = 150 x 180 x 800 mm, reinforced with bottom and top longitudinal reinforcement, beam Nr. 5 with stirrups and beams 6-8 without stirrups), the external optical cables were placed in the zones of critical shear crack in two different arrangements: perpendicular or diagonal (see Table 1 and Figure 1, right). This was done to observe advantages and disadvantages of both arrangements. For both arrangements, it is important not to deform optical fibre during placement too much, especially in the areas near loops, where the cable is bent.

Fig. 1. Arrangement of internal and external DFOS for: (left) Beam specimen targeted to flexural failure; (right) beams specimen targeted to shear failure

To measure the strains in the fibre optic cables during the flexural tests, the Optical Backscatter Reflectometer (OBR 4600) produced by the Company LUNA Innovations was used. In addition to OBR 4600 device, the FOS device (Fibre Optic Switch) was attached to it, so that multiple channels could be measured during the tests. The OBR instrument uses large wavelength coherent interferometry to measure Rayleigh backscatter as a function of position (local resolution) in the optical fiber. Spectral shifts in the measured backscatter are caused by external influences such as temperature or strain. These spectral shifts can be measured and subsequently scaled, allowing the acquisition