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Asce's survey shows that 80%--90% bridge damage were associated with fatigue and fracture problems. With the
operation of vehicle weight and traffic volume increases constantly, the fatigue of welded steel bridge is becoming more
and more serious in recent years. A large number of studies show that most prone to fatigue damage of steel bridge is part
of the welding position. Thus, it's important to find a more precise method to assess the fatigue life of steel bridge.
Three kinds of fatigue analysis method is commonly used in engineering practice, such as nominal stress method, the
local stress strain method and field intensity method. The first two methods frequently used for fatigue life assessment of
steel bridge, but field intensity method uses less ,and it widely used in fatigue life assessment of aerospace and
mechanical. Nominal stress method and the local stress strain method in engineering has been widely applied, but not
considering stress gradient and multiaxial stress effects, the accuracy of calculation stability is relatively poor, so it's
difficult to fully explain the fatigue damage mechanism. Therefore, it used strain field intensity method to evaluate the
fatigue life of steel bridge.
The fatigue life research of the steel bridge based on the strain field method and the fatigue life of the I-section plate
girder was analyzed. Using Ansys on the elastoplastic finite element analysis determined the dangerous part of the
structure and got the stress-strain history of the dangerous point. At the same time, in order to divide the unit more
elaborate introduced the sub-structure technology. Finally, it applies K.N. Smith damage equation to calculate the fatigue
life of the dangerous point. In order to better simulating the actual welding defects, it dug a small hole in the welding
parts. It dug different holds from different view in the welding parts and plused the same load to calculate its fatigue life.
Comparing the results found that the welding defect in different parts had different influence on the fatigue life.
Simultaneously, it based on S-N curve the I-shaped beam and combined with Palmgren - Miner linear cumulative
damage theory to calculate the fatigue life of the dangerous part. The corresponding calculation results proved the
superiority of the strain field intensity method.
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