Research On Law Of The Relaxation Of Welding Residual Stress Under The Effect Of Alternate Loading

Welding is an important process in ship building industry.Residual stress will reduce when welding structure is subjected to the alternate cyclic loading.This phenomenon is called stress relaxation.It will cause influence to structure safety and fatigue strength.This dissertation uses thermal elastic plastic method and experiment to verify the accuracy of thin plate welding simulation.The welding progress and stress relaxation progress are simulated and different factors are studied.Then models of T-joint and grillage are built.Boundary condition and loading cases which are closer to reality are used to study the influence of different factors.The main contents are as follows:(1)The background and significance of the study are introduced,as well as the study of welding deformation,welding residual stress and the control of welding residual stress.The research status of stress relaxation under alternate loading is also introduced.(2)Basic theory of thermal elastic plastic method and main analysis flow chart are introduced.A finite element model is built to calculate the welding residual stress and the stress field after loading to verify the accuracy of simulation method.(3)The simulation method is used to study the relation between loading amplitude,loading sequence,stress ratio and stress relaxation.The results show that maximum loading amplitude has biggest influence on stress relaxation,and loading sequence has little influence on it when the specimen is drawn only.When there is reverse loading,if the loading amplitude is large and stress ratio is close to-1,some part will relax continuously and the first cycle has the largest influence on stress relaxation.(4)A T-joint model is used to study the influence of loading amplitude and stress ratio and two different direction of loading are used to simulate the stress relaxation.It shows that loading amplitude not stress ratio has great influence on the relaxation because of the low compression welding residual stress.It also shows that when two loads at different direction draw or compress the structure at the same time,stress relaxation is restrained.(5)CCS standards are used to calculate still water load and wave load to study the transverse plate residual stress relaxation.The results show that the stress relaxation is very conspicuous.Stress relaxation can reduce residual tensile stress.The reduction of residual tensile stress can reduce the speed of stress corrosion and fracture propagation and improve the fatigue strength.