Study On The Fatigue Performance Of Welded Joints In Steel Bridges Under Prior Corrosion And Complex Stress Fields

Environmental corrosion and fatigue are important factors affecting the durability of steel bridge. Welded joints of steel bridge are mostly in complex stress states in practical engineering. Thus a study on the fatigue perfmance of welded joints in steel bridges under prior corrosion and complex stress fields is carried out.(1) The research on fatigue life assessment methods, multiaxial fatigue failure criteria and mutiaxial fatigue tests is introduced in this dissertation. Corrosion types and influencing factors of steel bridges, corrosion prediction models and corrosion test methods are summaried. Mechanisms of corrosion fatigue crack initiation and propagation are described. The research progress on corrosion fatigue life assessment is summaried by domestic and foreign scholars.(2) According to domestic and foreign literatures, full penetration load-carrying fillet cruciform welded joints with inclined angles (0°,15°,30°,45°) are constructed to simulate the complex stress fields by the interaction of normal and shear stresses. The linear extrapolation method, the quadratic extrapolation method and the 1 mm stress method are used to calculate hot spot stress of welded joints. The proper mesh density is deterimined by different mesh sizes. Hot spot normal and shear stress concentration factors varying with inclined angles are analyzed. The equivalent stress method, the interaction equation method and the bi-parametrical critical plane method are used to assess fatigue life, and calculated results of fatigue life compares with expertimental results. It is recommended to use the linear extrapolation method to calculate hot spot stress and the equivalent stress method to predict fatigue life of welded joints under complex stress fields.(3) A lot of corrosion depth models are summaried. Corrosion pit notch and corrosion pit crack quantitative technology are introduced. Finally the simplest power function model is determined to represent the relationship between corrosion depth and time. To study the effect of individual pit size on fatigue life, one pit is taken as a spherical notch, and two different pit depths are considered.(4) Fatigue tests of full penetration load-carrying fillet cruciform welded joints with 45° inclined angle are carried out under three different corrosion degrees, and S-N curves are obtained. The effects of different corrosion damages on hot spot stress concentration factors and fatigue notch factors are studied. A corrosion fatigue life impact factor defined by corrosion pit depth and stress range is proposed, and its expression is fitted by test results. The results show that corrosion pits have an effect on fatigue performance of weld joints, but it is not very sensitive. Different reductions of fatigue strength of weld joints are produced by different corrosion depths and stress ranges.(5) Taking full penetration load-carrying fillet cruciform welded joints with 45° inclined angle under different corrosion degrees as the research object, the stress intensity factor of surface crack of welded joints is calculated based on numerical methods. Effects of different corrosion pit size, crack aspect ratio and crack depth on stress intensity factor are analyzed. The results show that pit depth plays a major role on stress intensity factor, and the effect of pit radius on it is relatively small. The surface point is mode I cracking, and the deepest point is mixed modes I and II cracking. Effects of pit depths, crack aspect ratios and crack depths on stress intensity factor at the surface point are greater than the deepest point.(6) Fatigue crack growth rate parameters of cruciform welded joints are obtained from S-N curves under different corrosion degrees. A numerical method is taken to simulate fatigue crack growth of cruciform welded joints under different mixed mode fatigue crack growth criteria. Crack shape correction factors under different mixed mode fatigue crack growth criteria are fitted by polynomial regression. The maximum circumferential stress criterion is used to modify fatigue crack growth rate. The relationships of effective stress intensity factors and crack depths, crack growth angles and crack depths, and crack depths and cycle times are given under different stress ranges and corrosion degrees. The simulated crack growth path compares with the actual path. The results show that the simulated crack growth path agrees well with the actual path. The fatigue lives predicted by modified crack growth rates agree better with fatigue test results.The crack shape correction factor based on the maximum circumferential stress criterion can be used to calculate fatigue life.(7) A fatigue damage model of weld joints is obtained in view of damage mechanics. Crack size is used to define a damage variable, and two common fatigue damage evolution models, the Chaboche model and the Lemaitre model, are given. Material parameters of the Chaboche damage evolution equation of cruciform weld joints under different corrosion degrees are determined by fatigue test results. Damage curves and fatigue life prediction equations of cruciform weld joints are obtained. The results show that the crack size can be used as the damage variable of welded joints, and the Chaboche model also can be used to assess the damages of welded joints.