Fatigue Performance Research On U-rib Structure

Orthotropic steel structure with U-rib is widely used in large and medium span bridges due to the advantages of light weight,high ultimate strength,rapid erection and easy assembly.However,the fatigue problem of orthotropic steel structure is more prominent.Rib-to-deck weld joint(RD joint)is one of the typical fatigue cracking positions,and the inspection and repair of the cracks initiated at RD joint is difficult,which seriously affects the structure safety.Improving the fatigue performance of RD joint is one of the effective ways to promote the development of U-rib structure.In this thesis,the fatigue performance of different types RD joints was studied from the aspects of local stress concentration and fatigue crack propagation behavior by fatigue test combing with numerical analysis.The main research contents of this thesis are as follows:(1)According to the investigation on the development of RD weld joint of the Urib structure,the fatigue test was conducted on traditional RD weld joint using singlesided welding process with two new RD welded joints welding by U-rib inner welding technology and single side weld and double side forming process.The RD weld joint using single side weld and double side forming process shows excellent fatigue performance.(2)A three-dimensional finite element model of RD weld joint was generated.By introducing the virtual notch circle at the crack initiation point of fatigue specimens,the difference in effective notch stress with single side weld? double side weld and different weld penetration rate was studied.The results indicated that improving the penetration rate on the double side weld has little effect on improving the fatigue performance of the weld joint.It can change the weld transition form to reduce the local stress concentration and further improve the fatigue performance.(3)A method based on extended finite element method(X-FEM)and secondary development of open source finite element software is proposed to simulate threedimensional fatigue crack propagation.The method is implemented by using the command stream similar to Python language in Code-Aster.The extended finite element method uses level set method to characterize the crack so that there is no need to mesh the crack interface.The crack propagation is realized by the level set update method and the accuracy of fracture parameter results can be ensured by refinement of crack font grid using adaptive division algorithm.(4)The initial defect was assumed to be a single semi-elliptical crack to study the fatigue performance of RD joint from the perspective of fracture mechanics.The method described above was applied to the three-dimensional crack propagation simulation of the segment models of U-rib structure.The location of the initial defect along the longitudinal direction of the bridge and the initial crack shape were changed.The numerical simulation shows that the negative impact of the initial defect location within a certain distance is almost same,and the cracks in different shapes will eventually stabilize in a certain form and continue to propagate.(5)The cracks were placed on the inside and outside of welded joint to study the effects of deck plate thicknesses,single-sided and double-sided weld on the fatigue life.Increasing the plate thickness and the using double-sided weld can reduce the amplitude of the stress intensity factor at the deepest point and the surface point of the crack,and slow down the crack growth rate so that the fatigue performance of RD joint can be effectively improved.