PSI - Issue 79

Martin Sladký et al. / Procedia Structural Integrity 79 (2026) 421–432

422

Nomenclature Symbols FAT

fatigue class

m N R

slope of the S–N curve fatigue life in cycles load asymmetry ratio multiaxiality coe ffi cient

s

T σ

scatter range index

t

wall thickness

2 α ∆ λ

notch opening angle

stress range normalized to the corresponding FAT class

∆ λ P 50%

normalized stress range at 10 6 cycles, corresponding to a survival probability of 50 %

stress range

∆ σ

actual notch radius fictitious notch radius

ρ

ρ f ρ ∗

material-dependent stress-averaging distance

Subscripts hs

used to denote values related to the hot-spot stress used to denote values related to the notch stress used to denote values related to the nominal stress

n

nom

ref

used to denote reference values corresponding to a wall thickness of 25 mm

life assessment of these joints is essential to ensure the structural integrity and long-term reliability of such high performance components. Within established engineering standards, the fatigue life assessment of welded joints is typically addressed in general fatigue-oriented design guidelines, such as British Standard BS 7608:2014 + A1:2015, Eurocode EN 1993-1 9:2005 / AC:2009, and the FKM Guideline by Rennert et al. (2020), as well as in welding-oriented codes, including the American Welding Society Code AWS D1.1 / D1.1M:2020. It may also be incorporated into standards intended for particular applications where welded joint fatigue represents a key design consideration, such as the ASME Boiler and Pressure Vessel Code ASME BPVC.VIII.2-2023. This study adopted as its primary theoretical reference one of the most comprehensive sets of recommendations for the fatigue design of welded joints, issued by the International Institute of Welding (IIW). Its principal guideline, Recommendations for Fatigue Design of Welded Joints and Components, authored by Hobbacher and Baumgartner (2024), covers all three major stress-based prediction approaches employed in this study. In addition, the IIW has published several complementary documents that elaborate on specific aspects introduced in the main guideline, in cluding detailed recommendations for the fatigue design of welded hollow section joints by Zhao and Packer (2000), the hot-spot stress-based approach by Niemi et al. (2017), the notch stress-based approach by Fricke (2012), and the statistical evaluation of fatigue data by Parmentier et al. (2023). Among the employed fatigue life estimation methods, the nominal stress-based approach represents the most tradi tional and straightforward technique. As described by Macdonald (2011), nominal stress is defined as the stress value una ff ected by local stress concentrators and can generally be determined using only basic analytical relationships, without requiring complex numerical analysis. However, this approach has only limited ability to transfer results from one geometry to another, making it suitable primarily for well-established and standardized welded configurations. Moreover, as noted by Maddox (2002), determining nominal stress in complex geometries, particularly those involv ing hollow sections, can be ambiguous. The e ff ect of plate thickness deviations from the reference value of 25 mm is directly incorporated into the S–N curve by adjusting the FAT class. For components with wall thicknesses below the reference value, a beneficial thick-