PSI - Issue 38

S. Häusler et al. / Procedia Structural Integrity 38 (2022) 230–237

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S. Häusler et al. / Structural Integrity Procedia 00 (2021) 000 – 000

Both graphs show that some curves have a plateau at the beginning, while degradation starts almost immediately for others. Correlating the course of the lines of graph (a) with the damage evolution in the images in Figure 4, it can be seen that this behaviour depends on the emergence of cracks. In the specimen corresponding to the turquoise line (50k cycles) only hairline cracks developed until 25% of the lifetime. Thus, no loss in stiffness appeared, while the green line ’s specimen (57k cycles) showed two major cracks spreading from the left and thus a steep stiffness degradation emerged. Both lines cross at around 60% of lifetime, where the cracks of the green line ’ s specimen (57k cycle) have grown further through the width of the specimen. During the same life fraction, in the turquoise line ’ s specimen (50k cycles) three dominant cracks grew through the width following a sudden stiffness drop of about 40%. Even though the damage onset differs more than 10000 load cycles between these specimens, final failure is only 7000 cycles apart. Undergoing a rapid stiffness degradation, the specimen with delayed damage onset fails first. Comparing the first three lines of graph (a) (8k, 8.7k and 12k cycles) with the representative results of Cadavid et al. (2017), it stands out that despite the varying progresses of the degradation, the magnitude short before final failure is similar. However, only the black line has a comparable course, since the other two (lilac and blue) show a delayed degradation onset. The following table displays the initial stiffnesses used for the normalisation in Figure 3: Table 2a: Initial stiffness used to normalise the curves of the experiments shown in Figure 3 (a) and the respective stress amplitude R = -1 Curve 8k 8.7k 12k 50k 57k 139k 225k 330k 1,04M 2,2M E init GPa 15.3 16.6 15.6 16.3 16.9 16.9 16.1 16.6 16.5 17.1 MPa 26.8 27.1 25.9 21.2 19.9 19.1 17.8 15.6 14.1 12.9 Table 2b: Initial stiffness used to normalise the curves of the experiments shown in Figure 3 (b) and the respective stress amplitude R = 0.1 Curve 6k 19k 29k 48k 92k 146k 215k 687k E init GPa 16.1 15.8 16.5 16.3 16.0 15.3 16.0 15.8 MPa 12.8 11.7 12.2 10.6 10.0 9.9 9.6 8.5 Figure 3 (b) shows the stiffness degradation of the tests with R = 0.1 for specimens with Θ = 90°, where basically three courses appeared. The orange line (215k cycles) shows hardly any degradation, while the black and dark blue ones (6k and 19k cycles) degrade to 20-30% of the initial cyclic modulus. The remaining lines gather between those extreme cases. The backlight images demonstrate that the comparably rapid degradations of the black and dark blue Figure 4: Backlight images for a) 0%, 25% and 60% of lifetime of the specimen with Θ = 90° (visually dominant cracks are highlighted with green arrows) and b) of Θ = ±45° specimen before testing and the representative damage state close to final failure.