| Fatigue assessment on structural details of steel bridges is usually implemented through the nominal stress approach according to the current design codes of different countries. The assessment is based on specific S-N curves and determined loading history for various structural details. This approach cannot be conducted for new types of connections, joints or structural details made by new types of steels. In such situation, fatigue tests usually will be employed, but the tests are extremely time and money consuming while the results generally cannot be entended to other cases. To cope with the situation, this paper focuses on solving fatigue problem of steel bridges by Fracture Mechanics. The major research work and achievements are as follows.(1) Fatigue crack growth rate tests under three different stress ratio loading cases have been conducted on 54 Q345 qD specimens of three thicknesses and 18 WNQ570 specimens of 10.0mm thickness. The standard value of fatigue crack growth rate parameters has been proposed for Q345 qD and WNQ570 base metal and butt welds, based on group data approach and single specimen data approach, respectively. Suggestions on how to choose parameters for fatigue assessment are presented in the meantime. This work can provide essential data support for the fatigue assessment of related engineering projects.(2) Fatigue tests on 78 specimens have completed involving three typical welded structural details, which are butt welds connecting plates with different thicknesses, cruciform joints with load-carrying fillet welds and cruciform joints with non-load-carrying fillet weld. The geometric profile, initial fatigue crack location and strain data for these specimens are carefully measured. The nominal S-N curves used in current design codes are proven to be appropriate.(3) The mix-mode fatigue crack propagation analysis approach is proposed based on the interactive platform of FRANC3 D and ABAQUS. The approach has been verified to be accurate and reliable by the fatigue test results. On the basis of the validated approach, the reasonability of the analytical solutions for the three typical welded structural details has been investigated and the influence factor analysis on their fatigue lives is fulfilled. Recommendation on the choice of initial cracks for the three typical welded structural details is also provided.(4) The mix-mode fatigue crack propagation analysis approach has been employed in multi-scale finite element(FE) models furthermore. The accuracy and reliability are verified by the existing fatigue tests, while the validated approach is applied in the fatigue assessment of a typical railway steel truss bridge. The fatigue risk ranking of all possible structural details in this bridge is generated by conducting the nominal stress approach, during which the most dangerous two structural details are recognized. The multi-scale FE model and local FE model with pure solid elements are established for these two details respectively. Both the models contain three dimensional cracks. The residual fatigue life of the two details is predicted. Through comparing the differences of the results carried out by multi-scale model and local model, this paper conducts a discussion on the necessity of fatigue crack propagation analysis in multi-scale FE model for real structures. The remarkable influence of increasing train axle load is analyzed.This paper proposes a validated fatigue assessment approach with excellent generalizability based on Fracture Mechanics, which also has been applied in practical engineering projects. The approach can overcome the shortcomings of the traditional fatigue assessment approaches effectively and serve for the development of steel bridges in China. |
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