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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results (see Figure 2 bottom), we decided to implement the HEI on static strength instead (Table 1) which is in the same range of fatigue strength reduction observed for other austenitic stainless steels from the literature (see Figure 2 top) in order to consider conservative HEI values. Additionally, considering the static HEI can be justified by the fact that in the FKM guideline, the SN curve parameters are derived from the tensile strength.

Table 1: Material parameter sources and HEI used for derivation of design SN curves ( HEI values refer to tensile strength)

Material parameter source

Material AISI 321 AISI 321 AISI 304L

HEI source

HEI 0.92 0.89 0.92

3.1 certificate of demonstrator part

this project

literature Jürgensen et al.

Jürgensen et al. 2024

this project

this project

D a = 10 mm d i = 7 mm t = 1.5 mm

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150,5

Fig. 4. Testing of a sample component of bended tube sections from AISI 321 stainless steel under pulsating internal pressure loading which was conducted for as received and hydrogen pre-charged samples. Part geometry (top), test setup (bottom left) and failure position (bottom right) As the experimental points are located above the SN curve, the prediction is conservative and the FKM SN curve predicts lower stress amplitudes at the same number of cycles. Both the experiments at specimen and component level were pre-charged up to the saturation content over the complete wall thickness and values of hydrogen content are thus comparable for specimen and component level. Also, the test frequency was similarly chosen at specimen level (f = 0.5 - 1 Hz) and component level (f = 1 Hz). For the uncharged / as received components, the fatigue assessment leads to a utilization factor of 1.27 using the material properties from the certificate of the manufacturer (Fig. 5 left). For the hydrogen pre-charged samples (Fig. 5 right), the SN curve under hydrogen is significantly flatter, thus increasing the value of inverse slope k and finally leading to a utilization factor of 1.17. Our demonstration use-cases show that the modified FKM fatigue strength assessment can be applied in a conservative manner in hydrogen conditions for components of austenitic stainless steel which were pre-charged up to their saturation content. Further