Fatigue Life Analysis Of Invar Sheet Welded Joint Of LNG Carrier Liquefied Tank Containment System

Nowadays, the environmental protection draws more and more attentions all over the world. It can be predicted that in the future, the needs and development of natural gas will be growing increasingly. The demand of LNG carrier as the conveyance of LNG is also growing increasingly.Welding is one of the key technologies of shipbuilding, the weld quality have direct influence on the tensile strength and fatigue life of weld seam. Invar steel is the special material used as the bulkhead of LNG liquefied tank and there are a lot of Invar steel welded structure exist in the liquefied tank, and there exists high welding residual tensile stress in the weld seam and heat affected zone of welded structures. Therefore, it’s important to study the influence of Invar steel welding residual stress on the fatigue life of the LNG carrier liquefied tank bulkhead.Firstly, the welding finite element model was established based on the MSC. Marc software. The welding temperature field during the welding process and post-welded stress field were simulated and analyzed; the numerical simulation model was verified by the measurement results measured by Xstress3000instrument. Calculation results and experimental results show that there exists high longitudinal residual tensile stress in the weld seam and heat affected zone, the maximum stress is higher than the yield strength of Invar steel; the transversal welding residual stress is relatively small; the peak of the longitudinal residual stress tend to decrease and the high stress region tent to increase with the increase of welding current.Secondly, compared with ignoring the influence of welding residual stress, considering the welding residual stress and after the sloshing load is applied, the weld and heat affected zone still exist higher longitudinal welding residual stress, and the node fatigue life of the weld and heat affected zone was decreased drastically, welding residual stress has a greater impact on fatigue life of Invar steel sheet. Optimal design for the nodes which have the lower fatigue life have been performed and the results show that reducing the longitudinal residual stress of the node can significantly improve the fatigue life.Finally, the influence of the release of residual stress on the fatigue life have been studied, the results show that the longitudinal welding residual stress have a substantial release and the fatigue life increased significantly under certain stress ratio and stress amplitude.