International Journal of Fatigue

Mooring chains are critical components in floating offshore energy systems, where fatigue failure may compromise structural integrity and operational reliability. Conventional fatigue assessment is generally based on standard S-N or T-N curves, which do not explicitly capture the local multiaxial stress–strain conditions governing crack initiation. This work develops and validates a finite element (FE) multiaxial fatigue methodology for chain links under constant-amplitude tension-tension loading. Elasto-plastic FE simulations of three-link chain assemblies are combined with the Smith-Watson-Topper (SWT), Walker, Fatemi-Socie and Dang Van criteria to evaluate fatigue indicator parameter distributions and predicted fatigue lives. The numerical predictions are validated against fatigue tests performed on commercial DIN 764 studless chains and supported by fracture surface observations. The results show that fatigue failure is governed by competing critical regions at the outer crown and kt point, while contact-zone damage is mainly associated with local fretting. The SWT and Walker criteria reproduce the experimentally observed mode I failure regions, while Walker provides the best fatigue life predictions, with all but one estimate within a factor of two of the experimental results. The improved performance is attributed to its calibrated mean-stress sensitivity. The Walker criterion provides the most suitable balance between physical consistency and quantitative prediction capability for the tested chain links.

In this study, the initial and fatigue-evolved yield surfaces of Laser Powder Bed Fusion (LPBF)-fabricated stainless steel (SS) grade 316 L were experimentally characterised for three build orientations: horizontal (XY), inclined at 45° (ZX45), and vertical (Z). Uniaxial tensile tests revealed that XY printed specimen exhibit higher mechanical properties than those printed in other orientations. Initial yield surfaces were determined in axial-shear stress space using sixteen proportional loading paths and were fitted using isotropic (Tresca and Huber-von Mises-Hencky) and anisotropic (Hill48 and Szczepiński) yield criteria. The results demonstrated clear deviations from isotropic yielding, with the Szczepiński criterion providing the best agreement with the experimental yield points. Axial stress-controlled fatigue tests were subsequently performed with periodic interruptions at intervals of 20,000 cycles. At each interruption, the yield point was measured to capture the progressive evolution of the yield surface. The results showed orientation-dependent distortional hardening, characterised primarily by asymmetric evolution of axial tensile and compressive yield strengths, while shear yielding remained largely unchanged. Microstructural analysis confirmed that this evolution correlates with orientation-dependent changes in lattice misorientation and accumulated plastic strain.