Research On Fatigue Performance Of Welded Joints Of Steel Bridges Under Complex Environment And Stress Field

Steel structure is widely used in bridge construction because of its many advantages.However,fatigue damage is likely to occur in steel structures due to long-term service under alternating loads,and the safety of bridge structures is seriously affected.In a complex environment with corrosive media and high temperature of fire,it is theoretically more difficult to accurately predict the remaining fatigue life of steel bridges.As one of the most important connection methods of steel bridges,welded joints are mostly affected by complex environments and alternating loads in actual projects.Therefore,it is necessary to carry out research on the fatigue performance of welded joints of steel bridges under the action of complex environments and stress fields.In this paper,two different types of welded joints with Q420 q D high strength steel as the base material were used as the research object.Based on the combination of finite element and test methods,the effects of corrosive media and high temperature of fire on the fatigue performance of welded joints were analyzed.With reference to the design dimensions,the finite element models of the two welded joints were established.For the three working conditions of no treatment,corrosion-only treatment and corrosion and fire treatment,the fatigue life of the two welded joints was estimated based on the S-N curve method.The fatigue tests of 6 sets of 10 specimens under three working conditions of two kinds of welded joints were carried out,and the fatigue performance change laws of two kinds of welded joints under three working conditions were studied.It provides a theoretical basis and reference basis for the assessment of the remaining fatigue life of the existing steel bridge and the fatigue and fire-resistant design of the steel bridge to be built.The main research contents and results are as follows:(1)The research status of the fatigue problem of welded joints of steel bridges was introduced.The fatigue life assessment methods of welded structures of steel bridges were summarized.The corrosion types,influencing factors,prediction models and corrosion tests of steel bridges were introduced.The corrosion fatigue failure mechanism and influencing factors of steel bridges are summarized.The effect of high fire temperature on the residual stress distribution and size of welded joints was explained.The finite element analysis method and theory of the existing welding process were briefly introduced.(2)With reference to domestic and foreign documents and actual test conditions,two types of welded joints,cross joints and lap joints,were designed.Based on the ANSYS finite element software platform,the three-dimensional geometric models of the two welded joints were established.The thermal-stress coupled field analysis sequence method was adopted,the combined heat source was applied,and the life and death unit technology was combined.After welding,the fire temperature load was applied to the welded joint according to the fire standard heating curve.The size and distribution of the transient temperature field and spatial residual stress of the welded joint during the welding process and the fire high-temperature treatment process were obtained.The results showed that during the welding process,the maximum temperature of the weld pool of the cross joint was close to 2500 ℃,and the maximum temperature of the weld pool of the lap joint exceeded 2700 ℃,which both exceeded the melting point of Q420 q D steel at room temperature.As the distance from the heat source became shorter,the temperature of the node becomes higher.After being affected by the high temperature of fire,the residual stress of the two welded joints showed different degrees of decline,and the residual stress was generally symmetrically distributed according to the position.(3)Two static analysis models of welded joints with unequal numbers of corrosion pits in a single fillet weld were established.The relationship between the stress concentration factor and the fatigue life and the number of corrosion pits in a single fillet weld was obtained,respectively.Based on the FE-SAFE software,the fatigue life of the three working condition models was predicted.The results showed that the stress concentration factor of lap joints was greater than that of cross joints.Under the same stress load of the two welded joints,the fatigue life of the untreated model,the corrosion and fire model and the corrosion-only model decreased in turn,and the fatigue life of the cross joint was always higher than that of the lap joint.(4)Fatigue tests of two kinds of welded joints under three working conditions were carried out,and the corresponding S-N curves were obtained.The macrostructure and microstructure of the fatigue fracture and fatigue damage were analyzed.Based on the results of numerical simulations and the design curves of national codes,the fatigue characteristics and fatigue life of the two types of welded joints were evaluated.The results showed that corrosive media and high temperature of fire will indeed affect the fatigue performance of welded joints.Under the three conditions,the fatigue strength of the cross joint was greater than the fatigue strength of the lap joint,and the actual fatigue life of the two welded joints was in good agreement with the numerical simulation results.The two types of welded joints with corrosion-only treatment were affected by the corrosion time and degree of corrosion,which were quite different from the calculated values of the design curve formulas of national classification societies.The fatigue life of these two welded joints without treatment and with corrosion and fire treatment can be better evaluated by the design curve of GB code.