Fatigue Cracking Mechanism About Diaphragm Of Steel Bridge Deck Considering Welding Residual Stress

With the popularization and the increase in service time of steel box girder bridges,fatigue cracking problems of the arc incision of diaphragms have occurred frequently,and have attracted attention in the field of bridge engineering.The stress condition of this part is complicated,and there are many factors that affect its fatigue cracking.While being affected by the wheel load of the vehicle,it is very likely that there will be a large welding residual stress,which brings many difficulties to the research of fatigue cracking.In order to scientifically understand the fatigue cracking mechanism,this paper takes the steel bridge deck diaphragm as the research object,establishes the finite element model of steel box girder bridge under the load of the vehicle,and analyzes the stress response characteristics of the arc incision under the combination of 72 load cases.A thermo-elastic-plastic finite element model for the welding connection between the U rib and the diaphragm is established,and the welding temperature field and stress field are clarified.The stress test and experimental study of the diaphragm is carried out,and the distribution of residual stress at the edge of the arc incision is analyzed.And on this basis,a simplified model for the superposition of the welding residual stress and the wheel load stress of the diaphragm is established,and a fatigue assessment method of the steel bridge deck diaphragm based on strain energy is proposed.The research content and conclusions are as follows:(1)The general situation of the application of orthotropic steel decks and the failure modes of fatigue cracking are introduced,especially the cracking phenomenon of the arc incision is particularly prominent,affecting the safe operation of the bridge.The research status at home and abroad is summarized and summarized from the three aspects of steel bridge deck fatigue,arc incision fatigue and welding residual stress.This leads to the problems that the stress condition of the arc incision and the mechanism of fatigue cracking are not clear.(2)In order to clarify the stress on the wheel load of the diaphragm vehicle,the ABAQUS software was used to establish a finite element model of the steel bridge deck under the load of the moving vehicle.The model was loaded with a standard fatigue III model,with a total of 72 load cases.And focus on the stress analysis of the arc incision.According to the calculated stress amplitude,the fatigue life of the diaphragm was predicted by the traditional S-N fatigue performance evaluation method.The fatigue life is only 0.1 years,which is far from the actual projected cracking life of 8-9 years.(3)In order to obtain the distribution of the residual stress field of the diaphragm and to clarify the fatigue cracking mechanism of the diaphragm,a finite element model of the diaphragm-U rib welding connection is established by ANSYS software.Dynamic monitoring was carried out to obtain the welding temperature field and stress field of the component,and the residual stress distribution of the diaphragm and U rib welding was further analyzed.The residual stress test was carried out by X-ray diffraction method,and the test results werecompared with the simulation calculation data.The results show that there is a large residual tensile stress near the arc incision of the diaphragm,the peak value is close to 230 MPa,the high level of welding residual stress is the key factor causing fatigue cracking of the arc incision.(4)Aiming at the problem that the traditional S-N fatigue assessment method with stress amplitude as a parameter cannot quantify the contribution of residual stress to fatigue damage,this paper obtains the distribution of the integrated superimposed stress at the edge of the arc incision by establishing a superposition model of the welding residual stress and the wheel load stress.Situation,and the strain energy method was used to calculate the cumulative fatigue damage and life prediction of the diaphragm.The results show that,considering the welding residual stress,the fatigue life of the diaphragm is about 8 years,which is consistent with the actual situation of the project.