PSI - Issue 75

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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. 8: Stress triaxiality factor at maximum load.

One can analyse the variation of the orientation of the principal stresses directions to determine the proportionality of the load. It is presented in Fig. 9, and this angle rotates up to 90° in certain locations of the lug grooves of the connector between the minimum and the maximum tension instant. This allows us to understand that standard fatigue criteria are not completely applicable. The effects of non-proportional loading, known to intensify damage (Lemaitre et al., 2020), are acknowledged. This work begins to improve the standard methodology by considering the recommendations of (API 17G, 2019). Annex D specifies that the stress range needs to be evaluated in the direction where the 1st principal stress has the highest tensile value."

Fig. 9: Variation of the principal stress axes orientation during a loading cycle in deg.

2.2. Observations and assumptions from simulations on small-size model for fatigue calculations The analysis of the numerical results inside the lug grooves of the connector provides the following observations and assumptions for every spatial position of the investigated areas: - The load is not proportional during the cycle, i.e. the ratios between the principal stresses change, and the principal basis strongly rotates during the cycle. - The maximum stress state, called max  , is obtained at the instant with the maximum tension max F , because the Von Mises’s stress reaches its maximum at this moment. - Reciprocally, the minimum stress state, called min  , is obtained at the time increment with the minimum tension min F , because the Von Mises’s stress is minimum at this moment.