PSI - Issue 75

Nina Grözinger et al. / Procedia Structural Integrity 75 (2025) 642–649 Author name / Structural Integrity Procedia (2025)

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In Fig.3b and 3d the LVDT signals for AISI 321 and its analytical functions are plotted. For 0.4 % strain amplitude, the signal in HTW follows the analytical function very well, while at 0.3 % strain amplitude the analytical function is not met in the first tenth of cycles. At the beginning of the test, the machine had reached a LVDT value that was too high and then had to be readjusted. Therefore, the recorded force signal is disturbed at the beginning and the increase in the stress curve does not reflect the true behavior. In all specimens out of AISI 321, there is a distinctive softening. Hardening occurs only very slightly at 0.4 % strain amplitude. a b

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Fig. 3. (a), (c) Progression of the maximum stress of the AISI 321 specimens in air and HTW; (b), (d) LVDT signals from AISI 321 air and HTW experiments and analytical function. The quantitative values of the lifetime are shown in Table 3 for each case, and also the ratio of them is calculated, which expresses the reduction effect of fatigue life in HTW, the so-called factor (Gao et al. 2021). For a direct comparison between all air specimens respectively all water specimens, all curves of the maximum stress are plotted in Fig. 4. The results show a tendency for softening to be less pronounced with ER 308L than with AISI 321, especially in air. On the other hand, the hardening of ER 308L in air is more pronounced. Looking at specimens tested at the same strain amplitude in the same medium, it can be seen, that the maximum stress in ER 308L is lower than in AISI 321. The number of cycles until load drop for those specimens tested with the same parameters is quite similar in each case, except for the specimens with 0.3 % strain amplitude in air.