PSI - Issue 38

Alexander Erbe et al. / Procedia Structural Integrity 38 (2022) 192–201 Author name / Structural Integrity Procedia 00 (2021) 000 – 000 represents fairly well the axis ratios of = 0 and the = -0.5 and the axis ratios of 1 and -1 in a conservative manner. To investigate whether the position of crack-initiation coincides with the position of the hot-spot strain, two results are exemplary given in Figure 7 left, for an experiment with an axis ratio of = -1 and right, for = -0.5. In both cases, macroscopic cracks grew under an orientation of about 45° relative to both loading axis. This emphasizes again the idea of evaluating the individual resulting hot-spot strain when running biaxial experiments and cruciform like specimens. Figure 7: left figure: specimen with macroscopic cracks and merged contour-plot from FEM for = -1 and T=400°C, right figure: specimen with macroscopic cracks and merged contour-plot from FEM for = -0.5 and T=400°C Strain-controlled = -1 strain controlled = -0.5 199 8

3.2. Influence of temperature and hold times Based on the performed test series at 400 °C it could be concluded that the influence of the applied axis ratio on the crack initiation cycles can be described by simply shifting the uniaxial crack initiation line by a specific factor. This key result is now used and applied to experiments at 500 °C with and without hold time where this shift factor is applied as well. For this, as outlined in section 2.2, several experiments at 500 °C were carried out as well to investigate the temperature influence. In a further step the strain controlled experiments were performed with the same test parameters and axis ratios but with an additional hold time of 3 min in tension and compression in order to reveal the combined influence of creep and various axis ratios on the lifetime. Again, at first the suitability of the evaluated hot-spot position for crack-initiation is cross-checked by means of the crack position and confirmed, Figure 8. Figure 8: left figure: specimen with macroscopic cracks and merged contour-plot from FEM for = -1 and T=500°C right figure: specimen with macroscopic cracks and merged contour-plot from FEM for = -0.5 and T=500°C without dwell time