Experimental and analytical investigation on cryogenic fatigue and fracture behavior of 5% Ni steel welded joints for LEG type B cargo tank

Type B cargo tanks in Very Large Ethane Carriers (VLECs), constructed with 5 % Ni steel, exhibit non-negligible probability of cryogenic liquid leakage due to fatigue failure in welded joints under cyclic loads and the presence of pre-existing surface cracks. This study systematically investigates the fatigue and fracture behavior of 5 % Ni steel submerged arc welded (SAW) joints through tensile, fracture toughness, and fatigue crack propagation (FCP) tests at 23 °C and -110 °C. Results demonstrate the SAW joints exhibited superior cryogenic mechanical properties compared to those under ambient condition. A novel methodology integrating critical fatigue fracture location identification and FCP life analysis was developed, utilizing a Weibull-distributed random wave-induced load spectrum generated via Python to simulate operational stresses. FCP analysis using failure assessment diagram (FAD) and the Paris law model revealed critical crack sizes and remaining life at tank vertical support toes. Additionally, the influence of material properties and temperatures on FCP life was quantitatively evaluated. This study addresses the lack of fracture toughness and Paris parameters for 5 % Ni steel SAW joints, providing essential reference for damage tolerance design and safe operation of VLEC type B tanks under cryogenic conditions.