Study On Fatigue Cracking Behavior Of Welded Joint In Steel Box Girder Of Highway Bridges And Its Repairing Effect

Steel box girder is widely used in highway bridges because of its excellent mechanical performance.However,the fatigue cracking problem of its complex structures is serious under vehicle loads,which threatens the safety of bridges.It is an important way to improve the durability of steel box girder structure to find out the spreading law of fatigue cracking and put forward reasonable maintenance and reinforcement methods.In this paper,combined with the theory of material mechanics and fracture mechanics,fatigue cracking behavior and maintenance and reinforcement methods were studied through fatigue test and numerical simulation method.This paper focuses on the fatigue details of two typical steel box girders: the root-to-rib weld and the arc notch of diaphragm.By analyzing the fatigue life,crack development path and section characteristics of the typical parts,the fatigue performance and cracking characteristics were revealed.The influence of factors on fatigue cracking and crack propagation was discussed.Based on the above analysis,corresponding steel plate reinforcement methods were proposed for the two structural parts respectively.The strengthening parameters and technology were suggested,which were verified in real bridges.The main research work and conclusions are as follows:(1)Fatigue tests were carried out on two typical structural details of the root-torib weld and the arc notch of diaphragm.The fatigue strength of the typical structural details was analyzed by nominal stress method and hot spot stress method.The cracking behavior and propagation characteristics at different structural details were studied.The variation law of crack propagation path and rate was revealed.The results show that there are multiple crack sources when the fatigue crack of the root-to-rib weld is initiated,which will produce multiple short cracks and fuse with each other to form a long crack.The propagation path of the crack at the arc notch of diaphragm changes with the crack length.It shows that the stress of the crack is complicated in the process of crack propagation,which proves that it is a typical mixed-mode crack.Furthermore,the influence of pavement on fatigue performance was studied by comparison.It is found that pavement can reduce the fatigue stress amplitude and improve the fatigue life to a certain extent,but has no significant effect on the fatigue strength of the structure.(2)The distribution law of local residual stress along the plate thickness direction at the root-to-rib weld was analyzed by microhardness test and numerical simulation.The key factor of fatigue cracking was explained.Through Co-simulation using ABAQUS and FRANC-3D,the initiation position and propagation behavior of fatigue cracks of the root-to-rib weld are studied and revealed under different stress modes.On this basis,the influences of the geometry of unfused parts,penetration rate of weld and plate thickness on fatigue crack cracking were analyzed.It is found that the influence of factors on cracking is different before and after cracking.Geometrical shape and penetration rate of weld have little influence on crack propagation stage.Increasing roof thickness is beneficial to delay fatigue crack propagation.(3)In order to analyze the influence of fatigue cracking of typical structural details on local stress,finite element model was established.Three kinds of cracks were studied,included horizontal cracks grown at roof-to-rib weld,through cracks at roof,cracks at arc notch of diaphragm.The deformation law of crack profile and stress field at crack tip under vehicle load were discussed.Moreover,the influence of different crack characteristics on nearby structure details and pavement were analyzed.Based on this,the influence of multiple cracks on local stress was analyzed,and further compared with the single crack case,which provided a basis for the subsequent reinforcement design.(4)Based on the stress analysis results of fatigue cracking at the root-to-rib weld and the arc notch of diaphragm,steel plate and angle steel were used to reinforce the fatigue cracked part.The strengthening design of the typical fatigue part was further studied.For the crack length of different structural details,the reinforcement methods are studied respectively.The influence of the size of strengthened plate,adhesive layer material and bonding position on the strengthening effect is explored.The value range of relevant reinforcement parameters of each part is suggested,which provides reference for the subsequent reinforcement test of real bridges.(5)Fatigue cracks at roof-to-rib weld and arc notch of diaphragm in a real bridge in China were repaired by bonded steel plate and bolting steel plate respectively.The change rule of the crack-tip stress field before and after reinforcement were compared,and the maintenance effect were investigated,based on the field monitoring data.The results of more than 2 years tracking observations verify the initial effectiveness of steel plate reinforcement method in the fatigue crack repair.