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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The position of the strain peaks from the DFOS were compared with the position of the cracks from the experiments. The cracks could already be localized in very early cracking stages, much before they became visible by human eye. After a certain load (approximately 40% of max load), a typical bending crack pattern becomes clearly visible. If the specimen is subsequently loaded further, individual “dominant” crack or cracks increase disproportionately in contrast to the other cracks, which still exist but whose width remains almost constant, so that only one or a few very large strain peaks (depending on the number of dominant cracks) can be detected. Thus, for crack detection at large deflections, it must be considered, that next to dominant cracks, many finer cracks still exist in the whole bending specimen. The calibration of cracks widths was done by method by (Rodriguez et al. 2015, 2019) and resulted in a very good agreement between measured and calculated crack widths in different loading stages, see some selected comparisons in Table 2. This fact can be used as another motivation to use DFOS for real structures in the engineering practice.

Table 2. Comparison of measured cracks widths by DFOS and by manual measurement for beam specimen Nr. 3, targeted for bending failure (DFOS calculation of crack width was done according to Rodriguez et al. 2019. Load (kN) Crack Width by DFOS (mm) Crack width from manual measurement (mm) 6.2 0.012 Not visible 16.9 0.36 0.25 36.4 0.81 0.75

By selecting and slightly calibrating the material parameters, we achieved for all four bending tests, that the complete load-deflection curve in ATENA was approximately the same as the experimental curve. Only at loading above 80% of the ultimate bending strength, the differences of the experimental and measured curves are slightly larger. The reason for this could be the underestimation of stirrup reinforcement in the bearing resistance of the FEM model. The different states of the bending behavior (uncracked, cracking, cracked, reinforcement yielding) are also clearly visible in FE-modeling. To make the model more plausible, the load-deflection diagram is also calculated by a simplified analytical model, where only tensile reinforcement considered. The difference in the plastic range can be explained by the stirrups and the compression reinforcement, which were not considered in the simplified analytical calculation. 5. Shear tests: comments and selected results of the measurements For the specimens Nr. 5 to 8, the main goal was to achieve a shear fracture, which is why the DFOS were placed in grooves externally, at the critical shear zone on the lateral beam surface. Since shear cracks always occur with a certain inclination to the horizontal, for the specimens 5 and 6 the perpendicular DFOS-grid was used to map the crack patterns, and for specimens 7 and 8 we applied an optimized diagonal DFOS-grid. In addition to the external DFOS placed in grooves, in the case of the specimen 5 (reinforced with longitudinal reinforcement bars and stirrups), two DFOS cables were also fixed internally to the four stirrups with cable ties to compare the internal strains with the externally bonded DFOS cables. The crack detection and crack localization of shear cracks on specimens 5 to 8 by means of DFOS was successful. Generally, the position of the strain peaks from the DFOS corresponds very well with the position of the cracks from the experiments, as given in Fig. 4. In the next paragraphs, the DFOS-measurements on beam specimen Nr. 5 will be shown and discussed. This is the only one of the specimens designed for shear failure, which was reinforced with longitudinal reinforcement bars and with stirrups. The specimens 6, 7 and 8 were reinforced only by longitudinal bars. From the early-loading stages, the DFOS at specimen Nr. 5 indicated a clear increase of strain, especially in the bottom part of the beam (Fig. 4, points C1 and C2), which corresponds to the formation of small cracks which are not visible by human eye. It is very important to be able to register shear cracks in early cracking stages, as small shear