Finite element analysis on fatigue crack propagation behavior at weld root of steel bridge deck and longitudinal rib

ObjectiveThis study investigated the fatigue crack characteristics and failure mechanism at weld root. It conducted analysis on fatigue crack propagation behavior at inner-side of weld root that have not been fully considered in previous studies.MethodA finite element model of cracks, at weld root of steel bridge deck and longitudinal rib, was established based on linear elastic fracture mechanics and extended finite element method. The stress intensity factors and influencing lines of two crack tips in different loading conditions were obtained. The fatigue propagation behavior of cracks at different initial crack positions and with different geometric configurations were studied.ResultThe initial crack at inner-side of weld root had greater influence on weld root stress curve variation rate than it did on weld toe. Stress at weld toe was gradually less than that at weld root. Two crack tips at inner-side of weld root were both mixed propagation with pattern Ⅰ and pattern Ⅱ. Crack development led to the structural stiffness attenuation and the bending moment increasing, which deflected the crack propagation path and caused the sudden variation in crack intensity factor Ⅰ and factor Ⅱ. The propagation process of crack upper tip presented three distinct stages, and the stable propagation stage accounted for about 55.3%. Different initial crack depths had significant effects on the propagation rate of upper tip and the propagation path of lower tip towards weld joint. The cracks with different initial positions at weld root indicated no significant difference in the fatigue characteristics. Compared with inner-side cracks, the outer-side cracks not only affected the fatigue behavior at welded joint, but also reduced the bearing capacity by 5.8%.ConclusionInitial cracks and geometric characteristics at inner-side weld root significantly affect the stress evolution, crack propagation pattern, propagation path, fatigue behavior and bearing capacity at welded joint. The differences of initial crack depth and inner-outer position would lead to stiffness degradation, bending moment increasing, and bearing capacity reduction. It indicates that the fatigue damage at inner-side weld root should be repaired in a timely manner.