Fracture Mechanics-based Fatigue Evaluation Of The Rib-to-deck Welded Joints Of The Orthotropic Steel Bridges

Orthotropic steel decks have been widely used in long span bridges over theworld because of high load-carrying capacity, low density, short installation time andeasy maintenance. However, since the stress concentration caused by the suddenchange of the geometry and the initial defects in the U-rib-to-deck welded joints, thesmall cracks at the welded joints can easily grow and lead to the collapse of the bridgestructures.The traditional fatigue analysis methods for the steel brid’ge welded joints arebased on S-N curves. However this method cannot describe the characteristic of thecyclic stress in U-rib-to-deck welded joints, where the compressive stress is muchlarger than the tensile stress. Furthermore, the S-N curve can only predict the primaryfatigue life of the crack. A new method for the fatigue analyses of orthotropic steelbird’ges welded joints is based on fracture mechanics. When calculating the stressintensity factors K through finite element analysis, the simplified2D strain planecrack mode is always used by some researchers, which implies that the crack atrib-to-deck welded joints is very long, so the resulting fatigue analysis is not accurate,Moreover, Paris Law and Forman Law in the fracture mechanics are only suitable forthe case of cyclic stresses with R〉-l.In this thesis, the ANSYS is used to analyze the stress field in the welded jointsof the orthotropic steel bridges under different loading conditions. Then the3D localcomputational model of a welded joint is created by PATRAN, and the fatigueanalysis module AGILE, which is based on finite element method and symmetricGalerkin boundary element method, is used to perform the fatigue analysis. Thisthesis presents for the first attempt to employ3D fracture fatigue model for therib-to-deck welded joints of orthotropic steel bridges. The change of stress intensityfactors in the crack tip as the variation of the crack depth is analyzed under3D stressconditi’on which cant be achieved in2D crack models. The study presented in thisthesis provides some useful guideline for the3D fracture mechanics-based fatigue lifeevaluation of the rib-to-deck welded joints of orthotropic steel bridges.