Fatigue Assessment Of Rib-to-Deck Weld In Orthotropic Steel Decks Based On Averaged Strain Energy Density

Orthotropic steel decks have been widely employed in various types of steel bridges due to their advantages,such as the high strength-to-weight ratio,the wide ranges of application,the accelerated construction and erection,and the convenient for intelligent manufacturing.However,as the environmental effect affected and the traffic load and volume increased,the large number of welded joints resulted from the manufacturing process and the considerably high stress concentration induced by the structural system and stress characteristics have made the fatigue problem been prominent,which has become the key issue that restricts the application and development of orthotropic steel deck bridges.Since the rib-to-deck welded joint is one of the most important structural details in orthotropic steel decks,it is of particular significance to clarify its fatigue failure mechanism and to accurately evaluate its fatigue performance.At present,the commonly used traditional fatigue assessment methods have many limitations when evaluating the fatigue performance of rib-to-deck welded joints.Such as,the nominal stress method cannot take into account the multiple fatigue failure modes and the determination of nominal stress is quite difficult,the mesh-sensitive of hot-spot stress method,and the significantly change of local stiffness induced by the introduction of fictious round for equivalent notch stress method.In this dissertation,from the viewpoint of local characteristics of fatigue failure of welded structures,the fatigue failure mechanism and fatigue assessment method of rib-to-deck welded joints are comprehensitively investigated on the basis of the notch stress theoy.The following main studies are included in this dissertation:(1)In order to consider the effect of local nonlinear stress concentration of weld geometries on the fatigue performance of rib-to-deck welded joints,the local stress distributions around the weld toe and weld root notches are analyzed according to the notch stress field theory.The coupled effects of weld geometries and loading conditions on the dominate fatigue failure modes are also investigated and validated against the experimental results.The critical influencing factors on the fatigue performance of rib-to-deck welded joints are revealed.(2)The mesh-insensitive averaged strain energy density is employed as fatigue strength index to consider the effects of weld geometrical parameters and loading conditions on the fatigue performance of different fatigue failure modes.Reanalysis on a large number of fatigue test data available in literature are performed to propose the fatigue strength W-N curves in terms of averaged strain energy density.The averaged strain energy density based method that is capable of uniformly evaluating the multiple fatigue failure modes at rib-to-deck welded joint is established.Moreover,the fatigue failure mode transition relationship of rib-to-deck welded joint is revealed by analyzing the correlation between the averaged strain energy density and the weld geometrical parameters.(3)Several full-scale segment models are fabricated and tested under constant or variable amplitude loads to investigate the fatigue behaviors of rib-to-deck welded joints.The fatigue crack initiation and propagation processes are monitored through the staged static loading tests,based on which the fatigue failure modes and fatigue performance of rib-to-deck welded joints are investigated by taking advantages of three-dimensional finite element simulation.The feasibility,applicability and reliability of the previously proposed averaged strain energy density method and the corresponding W-N curves are validated.(4)To overcome the modeling complexity and the high cost of computation when using the finite element method to obtain the local parameters at rib-to-deck welded joints,the internal loadings resulted from the vehicle wheel loads acting on the orthotropic steel decks are decomposed into several elementary loading modes according to the principle of superposition and equivalent equilibrium conditions.Then,the parameter sensitivity analysis and multiple parameters regression analysis are carried out to derive the evaluation equations for notch stress intensity factors and T-stress for the elementary loading modes.A rapid evaluation method that is capable of calculating the average strain energy density of rib-todeck welded joints within arbitrary weld geometrical parameters and load modes is proposed.(5)Under the assumption of linear cumulative damage criterion,the fatigue limit state equation that can take into account both the weld geometrical parameters and the vehicle loads is derived based on the engineering structural reliability theory and the previously proposed averaged strain energy density fatigue assessment method.Considering the probability distribution function of the weld geometrical parameters,the fatigue life reliability of rib-todeck welded joints in orthotropic steel decks of a long-span cable-stayed bridge is evaluated,and the influences of weld geometrical parameters on the reliability of various fatigue failure modes are also investigated.