PSI - Issue 75

Hannes Schwarz et al. / Procedia Structural Integrity 75 (2025) 625–632 Schwarz, Fliegener, Rennert / Structural Integrity Procedia (2025)

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safety factors to cover uncertainties regarding the local hydrogen concentration and possible interaction between hydrogen diffusion and mechanical loads are not yet considered in our example.

Fig. 5. Results from component testing: Results for uncharged specimens (left) and hydrogen pre-charged specimens (right) of stainless steel AISI 321 tube sections under internal pressure load (x-axis = number of load cycles to failure from test, y-axis = maximum nodal principal stress from finite-element simulation)

3. Summary and Conclusions In this work, we demonstrate how component dimensioning in hydrogen environment can be performed based on the well-established FKM guideline (Rennert et al. , 2020). We modified the fatigue strength assessment scheme by implementing fatigue ratios as an indicator for the hydrogen influence, considering the ratio of fatigue strength at a reference number of load cycles as well as the ratio of inverse slope, similar to hydrogen embrittlement indices ( HEI ) which are known from the literature to characterize the hydrogen influence on static properties. We analyzed existing literature data in order to quantify the hydrogen impact on fatigue properties in a systematic manner. As a practical use case, we apply the fatigue strength assessment on bended tube sections from stainless steel loaded with pulsating internal pressure in reference and hydrogen pre-charged condition. Analyzing our use case, we can conclude: • Most results on fatigue properties (i.e. SN curves) of austenitic stainless steels under hydrogen conditions presented in the literature show a significant reduction of fatigue strength by factor 0.7 to 0.8. The fatigue strength is primarily reduced at lower number of cycles (< approx. 10 5 ), whereas in some cases for higher number of cycles (> approx. 10 5 ) the fatigue strength can even be slightly increased. The slope of the SN curve under hydrogen conditions therefore decreases and the SN curve becomes flatter. The ratio of inverse slope k H2 / k ref typically reaches values between 1.5 and 3. In general, there is few data available in the literature which may be related to the fact that fatigue testing in hydrogen conditions is very time consuming due to the required low frequency in the range of approx. 1 Hz in order to fully enable the maximum degradation of material properties caused by the hydrogen atmosphere. A lot more future testing effort is needed in order to generate a reliable basis of material properties which can be used for strength assessment in general for all material classes. • The experiments which were conducted in our project on hydrogen pre-charged AISI 304L samples at specimen level, as well as bended tube sections of AISI 321 at demonstrator part level in general agree to the behavior described in the literature and summarized in the previous passage. • The fatigue strength assessment acc. to FKM guideline, considering HEI values which were calibrated on