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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specimens under monotonic loading; (3) Reinforced specimens under stepwise increasing fatigue loading. The testing procedure varied depending on the test type, but at the beginning of each test, the steel clamps (Figure 2a) were always loaded with a normal force of 75 kN. To achieve this, the test setup is equipped with two hydraulic cylinders (top and bottom of the specimen) located in the middle of the clamping area (Figure 2c). Subsequently, the monotonically tested specimens were loaded until crack formation with a constant rate of 0.1 mm/min. After crack formation, plain specimens were stopped, while reinforced specimens were tested further until failure of the reinforcement with an increased rate of 1 mm/min. In fatigue loading experiments, first, the mean load level was applied with a rate of 0.1 mm/min. Afterward, the specimens were cyclically tested with a frequency of 0.1 Hz. While the lower load level remained constant at 1 kN, the upper load level started at 5 kN and was increased by 0.5 kN every ten cycles until failure. In the following, one experiment of each test type is presented and discussed. 4. Experimental results and discussion 4.1. Monotonic behavior without reinforcement To present the FOS results of the monotonically loaded tests in a comprehensible manner, Figure 3 displays just a section of the specimen measuring 26 cm. As described in Section 3.2, four sections of measuring fibers run longitudinally along the specimen. To differentiate these sections, they are depicted in various shades of green and numbered 1 to 4. Three different loading stages are depicted – namely 1%, 50%, and 99% of the maximum load – to track the progression of crack growth.

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0 50 100 150 strain [ ] FOS,mean  µm/m

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Figure 3. FOS results of a monotonic tensile test without reinforcement at various load levels: a-c) strain profiles in longitudinal direction and d-f) strain profiles in transverse direction.

The influence of the notch is evident right from the outset of the experiment (Figure 3a), attributed primarily to the reduced stiffness in the notch region. At 50% of the maximum load, it becomes apparent how the average strain increases in all fiber sections, yet also how a strain maximum slowly develops in the outer fiber sections (1 and 4) in the area of the notches (Figure 3b). This phenomenon is attributed to the fracture process zone, which progresses from the notches toward the interior of the specimen with increasing loading. Just before the crack is fully formed (Figure 3c), the gradual crack growth is clearly discernible. Fiber section 1 exhibits the most pronounced strain