PSI - Issue 64

Henrik Becks et al. / Procedia Structural Integrity 64 (2024) 1279–1286 Henrik Becks / Structural Integrity Procedia 00 (2019) 000 – 000

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peak, followed by fiber section 2. Despite the noticeable peak, the strain in fiber section 4 remains significantly lower than the strain in fiber sections 1 and 2. Examining the strain profiles in the transverse direction (derived from the mean value of all points in the respective fiber sections along the cropped 26 cm specimen length), it is evident that the crack initiates on one side of the specimen due to a slightly eccentric loading of the test – a typical phenomenon in uniaxial tensile testing (Figure 3d-f). 4.2. Monotonic behavior with reinforcement In Figure 4, the FOS results of a reinforced monotonic test are depicted for a section of 34 cm. Figure 4a illustrates the strain behavior immediately after the first crack. The strain level is very low, and there is no strain maximum but rather a strain minimum. The reason for this is once again the eccentricity of the specimen. However, unlike in the monotonic test without reinforcement, this eccentricity lies in the weak axis of the specimen, causing one side to compress (where the measuring fiber is applied) while the other side tears apart. Subsequently, the specimen cracks at five additional locations within the measurement field (Figure 4b-e), which is evident from the pronounced strain peaks that correlate well with the respective crack positions. When comparing the strain profiles of the two fiber segments, it is noticeable that the measurements mostly exhibit very similar results. However, there are also instances where the peak strains are slightly different (cf. Figure 4c), potentially due to a disrupted bond between measuring fiber and concrete surface.

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Figure 4. FOS results of a monotonic tensile test with reinforcement: a-e) strain profiles after the emergence of distinct cracks.

In Figure 5a, the loading of the experiment is depicted with respect to the time, showing the crack initiation and various points of interest. In addition to the five cracks mentioned above, further cracks occurred outside the measurement field. Figure 5b displays the temporal evolution of the mean strain. The capability of the measuring fiber to monitor the strain of the concrete surface in high resolution both before (<100  m/m) and after crack formation (>1000  m/m) is demonstrated. While this allows for predicting crack initiation, it also enables the calculation of crack opening long after crack formation. Even at 95% of the maximum load – when reinforcement is already yielding – FOS can still represent the strain remarkably well without significant data loss (Figure 5c). Only large strain gradients are not properly recorded. Shortly before the reinforcement ruptures, the measuring fiber fails (Figure 5a, b), meaning that ultimately around 98% of the relevant test duration was recorded in high resolution.