| Fatigue problem of steel bridges are usually evaluated by the method based on the S-N curves,but the nominal stress method,hot spot stress method,and notch stress method in this type of method have their own limitations.Fatigue testing consumes a lot of time and expensive costs.Therefore,it is of great significance to develop a suitable fatigue performance evaluation method to study the fatigue cumulative damage and crack growth of orthotropic steel decks（OSDs）weld details during the entire service period.Focusing on the theme of the fatigue performance of the typical weld details of OSDs,the steel box girder of the suspension bridge of Taizhou Bridge was taken as the research background.Based on the theory of linear elastic fracture mechanics,the fatigue crack propagation problems of typical weld details in OSDs were studied.The major research contents and results are as follows.（1）Fatigue experimental study on deck-to-rib weld details in OSDS.A full-scale model with two U ribs was designed.The fatigue experiment is carried out.A solid finite element model was established.The hot spot stresses of the weld toe of the deck were calculated by the IIW 3-point extrapolation method.The hot spot stresses of the U-rib weld toe were calculated by the DNV2-point extrapolation method.The stresses at the weld root of the deck were calculated by the nominal stress method.The results of static load test show that the errors between the measured hot spot stresses and those calculated by the finite element model are less than 10%.The errors between the actual nominal stresses at the welding root and those calculated by the finite element model are within 8%.When the fatigue test results were loaded to 720,000 cycles,the fatigue damage at the Weld Z3 was more serious than that of the other three welds.The depth of the fatigue crack at the Weld Z3 along the the deck was 2mm.The crack growth process was similar to that of a real bridge.（2）The barycentric rational collocation method（BRICM）combining a regular domain method and a domain decomposition method for solving plane elastic problems and plane crack problems was proposed.Firstly,the barycentric rational interpolation formulas and differential matrices in 1D and 2D were introduced.Secondly,the governing equations for solving plane elastic problems and plane crack problems,BRICM combining a regular domain method,BRICM combining a regular domain method and a domain decomposition method and the calculation method of stress intensity factor were also given.Thirdly,combining a regular domain method and a domain decomposition method,the collocation method based on BRICM was proposed for solving plane elastic and crack problems.Finally,numerical examples of plane elastic problems and plane cracks were given.Numerical results show that the proposed method can effectively solve the problem of plane cracks.The matrix-vector calculation formula is convenient for programming.The calculated stress intensity factor accuracy is as high as 10-3.（3）Numerical simulation of crack growth of the fatigue specimen based on linear elastic fracture mechanics.Based on the linear elastic fracture mechanics method,the BRICM combining a regular domain method and a domain decomposition method and the finite element/boundary element interactive method（ABAQUS software and Franc3D software interactive platform）were used to simulate the crack growth at the end of the deck-to-rib weld root of the fatigue test specimen and evaluate the life of it in 2D and 3D,respectively.The fatigue life results of the numerical simulation and the experiment results of the fatigue specimens were compared and analyzed.The analysis results show that the flatter the initial crack shape is,the shorter the fatigue life is.Compared with the results of 3D numerical simulation results and test,the fatigue crack growth life calculated by the 2D strain model is too short.In order to solve this problem,according to the test and three-dimensional simulation results,the modification Paris formula for evaluating fatigue crack growth life could be applied to the deck-to-rib of the fatigue specimen was proposed.The maximum error of the fatigue life calculated by the two-dimensional strain model based on the modification Paris formula of the specimen is 13.2%.The error of the fatigue life calculated by the three-dimensional model of the specimen is about 10%.（4）Fatigue crack growth analysis and life assessment for OSDs in the Taizhou Yangtz River Bridge.Firstly,a multi-scale finite element model of the OSDs in the Taizhou Yangtz River Bridge was established and the stress history of the deck-to-rib welding details was obtained.Secondly,the the equivalent stress ranges at the weld root and weld toe were both calculated.Thirdly,based on the linear elastic fracture mechanics theory and the two-level method,the above two numerical simulation methods were used to analyze the crack growth and evaluate the fatigue life in 2D and3D of the cracks starting at the weld root and weld toe,respectivly.Finally,different local fracture mechanics models in 3D were established,and the influence of deck thickness,welding depth and stress range on fatigue crack growth was discussed and analyzed.Based on linear elastic fracture mechanics theory,two numerical simulation methods were proposed to carry out fatigue crack growth analysis and life evaluation in 2D and 3D,respectively.The reliability and accuracy of the proposed numerical methods were verified through fatigue tests.The proposed numerical methods were also applied to the Taizhou Yangtz River Bridge.This method provides a reference for the effective application of fatigue crack growth analysis and life assessment technology in long-span orthotropic steel deck bridges. |
