PSI - Issue 75

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

626

2

components and many others. So called hydrogen embrittlement results in a severe reduction of fracture strain and reduction of area which can be observed in tensile tests. In some cases, extreme embrittlement leads to sudden failure within the globally elastic regime and all features of ductile failure behavior will vanish. This was extensively studied since the 60s by NASA and others. While the underlying theories of hydrogen embrittlement are still controversially discussed, there exists at least a basic understanding about the susceptibility of various engineering steels to hydrogen embrittlement based on their range of strength properties, chemical composition, heat treatments and other influencing factors. The current challenge lies in appropriate codes and standards which allow particularly smaller companies without own testing capabilities to realize a safe design of their components. Few existing codes are either limited to very specific components (such as ASME B31.12, 2019 for hydrogen pipelines) or consider the hydrogen influence in a very conservative way (AD2000, 2012). Further analysis of the state of the art of hydrogen readiness of most relevant codes and standards see Fischer et al. , 2023. The German FKM guidelines (see Rennert et al. , 2020; Fiedler et al. , 2019; Berger et al. , 2018) which are applicable to arbitrary engineering applications are considered as an ideal basis for component dimensioning also in hydrogen atmosphere. Nevertheless, the existent guidelines are not validated to be used in hydrogen atmosphere. The aim of the research project related to this work is to investigate the applicability of the guidelines if material properties in hydrogen atmosphere are considered e.g. in the form of so-called hydrogen embrittlement indices ( HEI = property in hydrogen / property in reference atmosphere, e.g. fracture elongation or reduction of area) which are widely used in the literature to characterize the effect of hydrogen on the static material properties. In contrast, the focus of our paper is the application of the fatigue strength assessment according to the FKM guidelines in hydrogen atmosphere by implementing fatigue ratios of material properties which are defined in analogy to the HEIs . Following this methodology, we evaluate literature data in a systematic way to determine the fatigue ratios, validate the procedure by using experiments conducted within the project and finally apply the modified fatigue strength assessment to demonstrator components in the form of bended tube sections from austenitic stainless steel which have been tested in reference condition and in a hydrogen pre-charged condition. Based on the results of the validation scenario, we discuss in how far the existent FKM guidelines can already be used in hydrogen environment as well as open questions and future research which is needed for a general extension of the guidelines to hydrogen applications.

Nomenclature c H2

Hydrogen concentration Saturation concentration

[wppm] [wppm]

c S

Inner diameter Outer diameter

[mm] [mm]

d

D

Strain

[-]

ε

Frequency

[Hz]

f

Research association mechanical engineering

FKM

Hydrogen

H 2

Hydrogen embrittlement index Inverse slope of SN curves

[-] [-] [-]

HEI

k

Stress concentration factor / notch factor

K t

Pressure Radius

[bar] [mm] [MPa] [mm]

p r σ

Stress / Stress amplitude

Thickness

t