PSI - Issue 75

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

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2.2. Testing conditions For both materials, strain-controlled fatigue tests were carried out in air and HTW, each at 280 °C. The oxygen in the HTW was not limited. The strain amplitudes were 0.3 %, 0.4 % and 0.5 % across the various tests, see also Table 2, the strain rate always 0.4 %/s. To keep the water in a liquid state, the pressure in HTW was set to 80 bar. The specimens in HTW are pre-autoclaved for 4 to 5 days to make sure that the oxide layer is developed to a state comparable to literature data (Veile and Weihe 2024).

Table 2. Testing matrix

Strain amplitudes tested

Air

HTW

AISI 321 ER 308L

0.3 %, 0.4 %

0.3 %, 0.4 %

0.3 %, 0.4 %, 0.5 %

0.3 %, 0.4 %, 0.5 %

All test were finished, when the load peaks had dropped to 75 % of the value it showed at stable material behavior.

2.3. Methodology The testing principle used in this work, was established at MPA Stuttgart (Veile and Weihe 2024), (Veile et al. 2024a), (Veile et al. in review), (Kammerer 2020). It has the advantage, that no extensometer is needed for the HTW experiments. So, there is no problem with contact corrosion which is known from the literature (Kammerer 2020), (Vankeerberghen et al. 2020). Basically, the following strategy is used: in air, the strain is controlled locally by an extensometer as it is usually done. Simultaneously, a LVDT measures the global displacement (see Fig. 2a). Then, a mathematical function is fitted to this LVDT signal. By combining linear, logarithmic and exponential terms, it is possible to approximate the signal very precisely. For more details, see (Veile and Weihe 2024). In HTW, the experiment is controlled globally by the LVDT which follows the mathematical function (see Fig. 2b). a b

Fig. 2. (a) Setup for testing in air; (b) Setup for testing in HTW.

3. Results and discussion Fig. 3a and 3c shows the comparison of the air and HTW experiments for AISI 321. In all the results presented, the crack occurred in the gauge section. It can be seen, that the lifetime of the HTW specimens is lower than for the specimens tested in air environment. This is also the case for the ER 308L specimens. The detailed results of the ER 308L testing can be found in (Grözinger et al. 2025).