PSI - Issue 75

16

Fabrice Deleau et al. / Procedia Structural Integrity 75 (2025) 392–418 Deleau Fabrice, Emmanuel Persent, Guillaume Coudouel, Guillaume Perrin/ Structural Integrity Procedia (2025)

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Fig. 11: S-N graphic of the results for each methodology with SN curves

The proposed methodology provides a result between (API 16F, 2022) and the (DNV GL RP C203, 2020) . It can be explained by the physical quantities involved. The (API 16F, 2022) criterion considers a stress amplitude based on the highest tension stress (the critical plane fully consists in the highest tension range plane), which can be conservative for ductile materials, and the (DNV GL RP C203, 2020) approach considers the combination of the octahedral shear stress for the amplitude, with the octahedral normal stress for the correction, which is less conservative (the critical plane fully consists in the octahedral plane). The proposed methodology uses the same amplitude stress as (DNV GL RP C203, 2020), but with the mean highest normal stress which is closed to the one considered as range by the (API 16F, 2022). Then, the correction used to apply the mean stress effect is different: the coefficient according to (DNV GL RP C203, 2020) considers a linear ratio of the stress range and the mean stress, while the one used for the new methodology, is based on Gerber’s parabola, so the influence of the mean stress is quadratic. 4. Experimental investigations 4.1. Set-up configuration Tests were conducted using an Instron industrial universal testing machine. This hydraulic system enables fatigue testing under severe mechanical tensile loading conditions. Ball joints were incorporated into the mechanical setup to ensure purely axial loading on the small-sized i-Clip prototype (Fig. 12).