Study On The Influence Of Residual Stress On Fatigue Crack Propagation Of Typical Welded Joints

A large number of welded structures which made of high strength steel are used in modern ships and marine engineering structures.Different parts of ship are connected as a whole by the welded structures.These welded structures include common flat welded joints and typical ship welded joints from floating production storage and offloading structures.It is unavoidably for these welded joints to generate welding residual stress during the process of welding.The welding residual stress will have bad effects on the fatigue strength of the whole structure while high strength steel is very sensitive to it.They have been subjected to the alternate loads caused by waves for a long time during their service and easily generate cracks in the area of welding seam.Eventually the crack propagation will cause the fatigue fracture.At present,there are few studies on the influence of structural strength caused by the changes of welding residual stress.Therefore,the research on the influence of residual stress on the fatigue strength of typical welded joints has important reference value for reasonable evaluation and prediction of fatigue strength of welded structures.The effects of welding residual stress and fatigue strength of typical welded joints of high strength steel were studied based on the heat-elastic-plastic theory and fatigue crack propagation related theory.The main research works and conclusions are as follows:(1)The numerical simulations of residual stress of butt welded flat model 1,butt welded flat model 2 of high strength steel and typical ship welded joint are carried out.The experimental investigation of residual stress of model 2 is carried out by neutron diffraction method.It is found that the numerical simulation results of welding residual stress match with the experimental measurement results of residual stress.The maximum value of transverse welding residual tensile stress of model 1 and model 2 both appear near the area of weld toe,and the stress far away from welding seam is relatively small.(2)The study on the rule of residual stress release of typical ship welded joint under external load is carried out.It is found that the tensile stress of the structure considering welding residual stress is larger than that of the structure without residual stress.After the external load is loaded and unloaded,the residual tensile stress would be released,and the amount of released stress is proportional to the magnitude of the load.(3)The study on the rule of residual stress redistribution of flat model 1 with long welding seam and typical ship welded joint after the initial surface crack appeared is carried out.It is found that the transverse residual tensile stress near the area of crack front is larger,and the residual tensile stress at the deepest location of the crack front rise significantly compared to that in the intact structure.(4)The study on the fatigue crack propagation of flat model 1 with long welding seam and typical ship welded joint under the influence of residual stress redistribution is carried out.It is found that the crack size continues to increase as the crack propagates.The transverse residual tensile stress near the area of crack front is always larger,and the changes of the tensile stress at the deepest location of the crack front are significant.Meanwhile,the stress intensity factor of the crack front increases as the crack propagates.