Fatigue Crack Propagation Behavior Of Marine High-Strength Steel Considering Welding Residual Stress Redistribution

Ship and offshore engineering structures,which use welding as main joined way,are prone to fatigue failure problem in welded areas during service.Due to artificial factors and complexity in the welding process,welded joints contain crack-like defects inevitably.Those defects tend to evolve into long cracks,thus lead to fracture failure.Fracture mechanics is a more suitable method to estimate residual fatigue strength for welded structures,and contribute to great economic significance and engineering value.However,interaction between crack propagation behavior and residual stress redistribution behavior in residual stress field,as well as residual stress relaxation due to plasticity of crack tip,cause the discrepancy between fatigue assessment and practical cases of welded structure.Therefore,this paper studied Ni-Cr-MoV marine high strength steel,and carried out experimental research and numerical simulation research on long crack propagation behavior and residual stress redistribution behavior of butt-welded specimens.The main works of this paper sketch as following:(1)Mechanical and crack propagation tests were carried out to obtain parameters of material mechanical properties and fatigue crack propagation model.Firstly,elastoplastic behavior under monotone tensile load and cyclic tension-compression load was studied.Based on Chaboche back stress theory,the cyclic hardening elastoplastic model of the material was established.Meanwhile,constant amplitude crack propagation test for compact tensile specimens were carried out in order to study stress ratios influence on crack propagation behavior.Intrinsic crack propagation models were established to predict crack growth rate under residual stress field in the following research.(2)Crack propagation and residual stress redistribution tests of butt-welded specimens were carried out.At first,this paper designed and prepared butt-welded middle tesion specimens.X-ray residual stress measuring equipment was deployed to measure residual stress distribution of spcimens and hence discuss difference.During crack propagation test,the residual stress redistribution test on the specimen surface was carried out simultaneously.In these tests,crack propagation behavior and residual stress redistribution behavior,including data of crack length,cyclic loading times,stress and strain status of crack extension line,were monitored.Those data were used to support numerical simulation in the following research.(3)A numerical simulation research of fatigue crack propagation considering plastic residual stress redistribution was conducted.Based on ABAQUS secondary development technique and extended finite element method,this paper established a continuous simulation method of crack propagation and residual stress redistribution.This method considers influence of continuous variation of plastic residual stress redistribution during crack propagation behavior and can describe time sequence characteristics of the crack propagation behavior accurately.Residual stress redistribution behavior as well as residual stress intensity factor during crack propagation were calculated by the proposed method.And simulation results were compared with experimental data and theoretical solutions,and so the accuracy of the method were verified.Meantime,simulation results show that stress ratio effect of residual stress intensity factor on crack growth does exist under elastoplastic analysis.Based on the simulation results,this paper also proposed a residual stress intensity factor model which concerned stress ratio effect and crack scale.Through this model and material intrinsic crack propagation models,fatigue crack growth assessment of butt-welded specimen was conducted.The prediction was comperared with experimental data.