| With the increasing application of steel structure bridges,the repeated loading of components in trains or automobiles during service makes the problem of fatigue damage more and more important.Aiming at the stress cycle caused by the dynamic load of the steel structure bridge,this paper establishes a non-linear cumulative damage formula that can take into account the mean stress and the variable amplitude.Through calculations of steel truss bridges and steel box girder bridges,the fatigue failure characteristics of these two types of bridges are systematically analyzed.The main research results are as follows:(1)For the numerical simulation of the variable amplitude,different stress range have different nonlinear cumulative damage rates,resulting in a situation where the fatigue life is not easy to calculate.Based on the continuum thermodynamics and dissipated energy theory,by accurately calculating the non-linear cumulative damage degree corresponding to each stress range,the non-linear cumulative damage formula that can accurately consider not only average stress,multiaxial stress coefficient,initial damage,and variable amplitude is established.By considering the proportion of each stress range in the total damage in the cyclic block,this formula avoids the problem that replacing the stress range with equivalent stress range leads to low actuarial accuracy,and the problem of excessive calculation cost caused by the cumulative damage recurrence one by one.By considering the proportion of each stress range in the total damage in the cyclic block,this formula avoids the problem that replacing the stress range with equivalent stress range leads to low accuracy,and the problem of excessive calculation cost caused by the cumulative damage recurrence one by one.(2)By analyzing the parameters in the nonlinear formula,it is found that the degree of nonlinear accumulation is negatively related to the stress range.When the stress range is extremely large,the damage accumulation becomes a linear accumulation.When analyzing the fatigue life at the stress concentration point,the multiaxial stress coefficient should be considered.If it is simply considered as uniaxial stress fatigue damage,the error is about ± 40%.When analyzing the residual fatigue life,it can be seen that when the cross-sectional area is reduced by only 5%,the residual fatigue life is less than 1% of the original.(3)For the steel truss arch bridge,the fatigue life is calculated based on the nonlinear cumulative damage formula.It is found that the fatigue damage of various types of bridge members is as follows: bottom chord bar > vertical web member> tilted belly poles > suspender > top chord.The fatigue life of different sections of a member is quite different,and the fatigue life at different corners of the same section is also quite different.Taking vertical web member as an example,the fatigue life of such members at the section where they are connected to the bottom chord bar at the side corners of the railway is the lowest,while there is no fatigue damage away from the railway side.(4)For UHPC orthotropic steel box girder bridges,the fatigue life was calculated based on the multi-axis nonlinear cumulative damage formula.The analysis found that the fatigue damage of this type of bridge mainly occurred in the components near the vehicle load location,and the fatigue life of the components at the wheel pressure was about 10% of that of the non-acting components.From a structural point of view,fatigue cracks are mainly concentrated at the welded holes of the diaphragm,the weld of the roof and the stiffeners,and the weld of the welded holes of the stiffener and the diaphragm.The multiaxial stress coefficient has a high degree of influence on the life at the stress concentration point.The fatigue life considering this coefficient is 142.8% ~ 66.7% without considering the fatigue life of this coefficient. |
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