Research On The Fatigue Crack Propagation Of The Typical FPSO Welded Joints Considering The Residual Stress Redistribution

Many ships and large marine structures including FPSO are mostly welded by high strength steel,and the residual stress induced by welding has an effect on the fatigue strength of the structures,and high strength steel is more sensitive to the welding residual stress.FPSO bears constantly alternating load caused by waves and other load for a long time during service,and crack is produced easily at weld toe of the large steel structures.At present,there are few studies on the mutual influence of fatigue crack propagation and residual stress field.The study of fatigue crack propagation influenced by residual stress is very important to evaluate the fatigue life of the welding joint of FPSO and marine structures.Welding residual stress of butt welding joint of plate and T-shape joint of FPSO is studied in this paper.The related theory of heat-elastic-plastic theory and fatigue crack propagation is applied.The propagation of the crack of the welding joint with initial crack is studied,and the redistribution of residual stress is considered.The main research work and conclusions are as follows:(1)The initial welding residual stress of the intergrated structure of typical butt welded joint of plate and T-shape joint is studied.The position of the maximum transverse residual tensile stress is near the weld toe.So the fatigue crack is produced easily at weld toe,and the simulated values and the measured values are of agreement.The effects of the external load on the release of the residual stress are studied for both welded joints.It shows that the residual stress will release under external load,and the value of the released residual stress becomes larger with the increase of the external load.(2)The surface crack is added near the weld toe of the two kinds of intergrated welded joints,and the redistribution of the residual stress of the welded joint with initial crack was calculated with the method of numerical simulation.It can be seen: tensile residual stress at the crack tip becomes larger with the surface crack.The tensile residual stress releases near the location where the initial crack opens.(3)The residual stress is redistributed as the crack propagates.The position of the maximum residual tensile stress is always on the crack front and the value is becoming larger due to the propagation of the surface crack.The opening of crack causes the release of transverse residual stress on the surface.(4)The stress intensity factor K on the crack front increases with the propagation of the crack,and it means an accelerated trend of crack propagation.And the node at the crack tip on the surface is the first node to reach the fracture toughness values.