PSI - Issue 80

M. Bennebach et al. / Procedia Structural Integrity 80 (2026) 136–145 Author name / Structural Integrity Procedia 00 (2019) 000–000

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a portion of the pressure cycles applied.

Fig. 8. Sample pressure cycles applied.

4. Fatigue life calculations For fatigue life calculations, several approaches either derived from design standards or from state-of-the-art methods were investigated. The first methodology is based on CODAP, which provides specific fatigue criteria and is widely used in the French pressure vessels industry. This standard gives detailed guidelines for the design and verification of pressure equipment, considering the cyclic loading in service and appropriate safety factors. Application of this approach involves the use of analytical methods to estimate fatigue life; with results that are often highly conservative. The second methodology is based on several state-of-the-art criteria using advanced modeling and simulation techniques to predict fatigue life. This allows for more accurate analysis, considering the local stress/strain state and major influencing factors on durability. It also gives us the ability to run real time analysis from measured strain gauges data, facilitating then the hybridization between physics-based models and data driven ones. As critical areas are welded zones, the equivalent stress used in fatigue calculations is the hot spot stress derived from a quadratic extrapolation. The hot spot stress approach was initially used for welded tubular joints in the offshore sector and pressure vessels analysis since the 1960’s; then extended to the case of plated welded structures. In the IIW recommendations, the hot spot stress corresponds to the maximum principal stress in the base metal at the weld toe, considering the effects of stress concentration due to the overall geometry of the considered detail, but excluding the local stress concentration effects due to weld local geometry and discontinuities. It is applicable to fatigue failure at weld toes. Traditional approach to derive this stress is based on linear or quadratic extrapolation of strains or stresses from two or three reference points at certain distances from the weld toe. The non-linear notch stress effect, not included in the structural hot spot stress definition, is considered to vanish within a distance 0.3 to 0.4 times the plate thickness from the weld toe. That’s why IIW recommends making extrapolation from points which distance from the weld toe is over 0.4 times the plate thickness. The extrapolation procedure and hot spot calculation either from measurements or FE calculations are illustrated in figures 9 and 10.