Study On Numerical Simulation And Reinforcement Method Of Fatigue Crack Refinement In Rib-to-Diaphragm Of Steel Box Girder

With light weight,high intensity and wide applicability,steel box girder bridges and orthotropic steel bridge decks(OSD)are widely used in the construction of long-span bridges.However,due to the coupling of internal and external factors,such as geometric defects,numerous welds,welding residual stress,wheel loading and so on,the fatigue problem of OSD structure is prominent,and the most serious issue is the fatigue cracking at the rib-to-diaphragm of steel box girder.For the fatigue performance evaluation and fatigue crack reinforcement need to be further studied.Based on an existing steel box girder bridge,this thesis takes two cracks at the rib-to-diaphragm of steel box girder as research objects and adopts refined numerical simulation modeling.What the thesis focuses on is as follows:(1)Look at and summarize the development of fatigue research,simulation of welding residual stress,and research status of fatigue crack reinforcement.(2)The mechanical characteristics of rib-to-diaphragm are obtained through multi-scale modeling method.The thesis is to evaluate the fatigue performance of rib-to-diaphragm welding joints by nominal stress method,hot spot stress method and effective notch stress method,and analyze the characteristics of different fatigue assessment methods.(3)The temperature field and residual stress field of dynamic welding process are obtained by using thermoplastic finite element method.And the results show that there will be a large residual stress on the weld.The longitudinal residual stress even exceeds the yield strength of the material.And the peak residual stress at the arc-shape notch reaches 207 MPa.The varing characteristics of welding residual stress under cyclic loading are obtained by simplifying and simulating dissipation mechanism of welding residual stress under cyclic loading.The relaxation of welding residual stress mainly occurs in the first cycle,and only when the welding residual stress and external load are superimposed into the yield state,can the redistribution of welding residual stress occur.(4)The thesis takes the arc-shape notch of diaphragm as the research object,and combined the vehicle load characteristics with the welding residual stress to obtain the fatigue performance evaluation method based on S-N curve of base metal.The calculation results show that the fatigue evaluation method can evaluate fatigue performance of arc-shape notch very well,which provides a new idea for fatigue evaluation.(5)The thesis clarifies basic concept of extended finite element method and calculates stress intensity factor of the focus position by starting from extended finite element method and introducing 3-D initial crack then chooses the propagation mode of the crack.Therefore,analyzing the equivalent stress intensity factor provides the basis for fatigue crack growth.The amplitudes of stress intensity factors under different cracklengths are obtained to calculate the fatigue life.(6)Based on the previous research results,the thesis points out two kinds of reinforcement plan for fatigue cracks at rib-to-diaphragm,which are bonded angle steel and STC pavement reinforcement.The thesis calculates reinfored stress and equivalent stress intensity factor amplitude,judges the crack propagation state,then evaluates the reinforcement scheme.And the calculation shows that the bonding angle steel can improve the stress state of U-rib weld toe very well.Finally,the thesis recommends sticking angle steel plus STC pavement strengthened for this kind of fatigue crack.