| Due to its advantages of light weight,long span capacity and quick construction,steel bridge has been widely used in long-span highway and railway bridges.At present,the phenomenon of the vehicle overloads has become increasingly serious during the service life of steel bridges.With the lack of management and maintenance measures,the fatigue problem of steel bridges has become increasingly prominent.In the design and construction of steel bridges,there are some welded details with complicated local geometric structure and stress field.The fatigue performance of these welded details need to be further studied.Therefore,the girder's new structural details of steel truss bridge were taken as the research object to carry out the research on the fatigue performance experiments and fatigue life assessment methods.The main research contents,methods and research results are as follows:1.The research status of fatigue life assessment methods of steel bridges were collected and elaborated.Then the fatigue design criteria and methods were introduced,and the influencing factors of fatigue performance were summarized.(Chapter 1)2.The fatigue performance experiments of 15 fatigue specimens of the girder's new structural details were designed and carried out.The loading strain and fatigue life of these fatigue specimens were also obtained,and the fatigue failure process and development history of fatigue damage were analyzed.The S-N curves of the welded detail with the cope hole and the welded detail with a half-arc plate filled in the cope hole were obtained on the basis of the fatigue test results.(Chapter 2)3.The three-dimensional finite element models of the girder's new structural details were established by ANSYS.The strain test results of static loading and finite element calculation results were compared and analyzed.The results show that there is a small difference between the test results and the finite element calculation results.The influencing factors of local stress concentration coefficients of welded details were introduced.According to the geometrical parameters and welding parameters of the girder's new structural details,the effect of the plate thickness,filler plate thickness,butt weld parameters,plate gap width,weld penetration parameters and initial geometric imperfection on the stress concentration coefficients of flange plate,web and weld were analyzed.The results show that the main factors influencing the local stress concentration coefficients for the welded detail with the cope hole among the girder's new structural detail are the plate thickness and the radius of the cope hole.Significant factors for the welded detail with a half-arc plate filled in the cope hole are the filler thickness,fillet weld size,plate thickness,cope hole radius and plate gap parameter.Remarkable factors for the welded detail with staggered butt welds are the initial geometric imperfection,weld reinforcement height and plate gap parameter.(Chapter 3)4.The principal theory of hot spot stress method was introduced,and the surface extrapolation method,thickness linearization method,Dong method,1 mm method and equivalent hot spot stress method among the hot spot stress calculation methods were summarized.According to three-dimensional finite element models established by ANSYS,the influence factors of hot spot stress were discussed,and the hot spot stress of the girder's new structural details was calculated by surface extrapolation method,Dong method and 1 mm method.Fatigue life prediction of the girder's new structural details was carried out,and prediction results of each method were compared.The results show that the element type and the local structural treatment method of welded details have a significant effect on the extrapolation calculation results of hot spot stress.Two-point extrapolation method and SOLID45 element are recommended to calculate the hot spot stress of the girder's new structural detail,and the extrapolation point of hot spot stress should be selected outside the 0.5t range.It is preferable to use surface extrapolation method to predict the fatigue life of the girder's new structural details.Then,the thickness linearization method and Dong method can be selected.(Chapter 4)5.The basic theory of linear elastic fracture mechanics and the stress and displacement field of the crack tip were introduced.The finite element models of three-dimensional fracture mechanics for the girder's new structural details were established by ANSYS,and the stress intensity factors of the crack tip were obtained by the displacement method.The effect of geometric parameters and crack parameters on the stress intensity factor was discussed.Crack shape correction coefficient functions with different crack shape ratios were obtained,and the fatigue crack growth life of the girder's new structural details was predicted by the Paris formula.The results show that the crack shape ratio and the half-crack length have a significant influence on the stress intensity factor of the welded detail with the cope hole and the welded detail among the girder's new structural detail.The geometrical parameters of the girder's new structural detail also have some influence on the stress intensity factor,and the maximum value of the stress intensity factor K_I was obtained at the deepest point of the crack front.(Chapter 5)6.Three fatigue reliability analysis models were introduced,and then the distribution type and parameters of random variables in fatigue reliability analysis of the girder's new structural details were expounded.The fatigue limit state equation of the girder's new structural details based on the calculation results of linear elastic fracture mechanics and the crack shape correction coefficient function was established.The fatigue reliability indexes of the girder's new structural details were calculated by the JC method.The effect of the parameter values of random variables on the fatigue reliability index was discussed.According to the specified target reliability index,the inspection cycle and maintenance strategy for the welded detail with the cope hole among the girder's new structural detail were established.The results show that the fatigue reliability indexes of the welded detail with the cope hole are more sensitive to the average value of C and initial crack size,variation coefficient of C and crack shape ratio,which is not influenced by the variation coefficient of initial crack size.The inspection cycle is the corresponding service time of cumulative loading 1.983 million times under fatigue load for this welded detail.(Chapter6)... |
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