To establish the fatigue crack initiation analysis method for U rib-cover plate welded joints of steel bridge decks,the conversion of cohesive parameters in the mixed loading mode was considered based on the Roe-Siegmund cyclic cohesive zone model,the secondary development of ABAQUS was carried out,and the VUMAT subroutine reflecting the cumulative fatigue damage was formed.The cohesive parameters of the material in the welding zone corresponding to the Q345 steel were obtained from the experimental data.Based on the Voronoi diagram method and the grain microstructure and mechanical characteristics of the welding zone,the microscopic grain structure at the U rib-cover plate weld toe was established.In addition,combined with the macroscopic 2D plane strain model,the multi-scale fatigue crack initiation was simulated.Combined with the equivalent structural stress method and the crack propagation theory of linear elastic fracture mechanics,the accumulated cohesive lengths under different stress levels were backpropagated considering the initial defect morphology and critical criterion of fatigue fracture,and then the calculation method for the crack initiation life was obtained.Analysis results show that when using the proposed method to simulate the crack initiation behavior at the weld toe of U rib-cover plate welded joint,the cracks initiate at the weld toe and propagate perpendicular to the top plate surface,forming a transcrystalline fracture mode.The stress distribution of the microscopic grain structure changes with crack initiation and short crack propagations,and as the microscopic grain structure distribution and mechanical characteristics change randomly,the details of short crack propagation paths and critical cycle numbers in the simulation results are not the same.The backpropagated accumulated cohesive length varies with the initial defect morphology ratio,the critical depth of long crack propagation,the distribution and mechanical characteristics of microscopic grain structure,and the stress amplitude.The fitted curves of accumulated cohesive length and equivalent structural stress amplitude obtained by considering the above factors are capable of obtaining the corresponding crack initiation lifes.Therefore,the established multi-scale fatigue crack initiation simulation analysis method can provide a new solution to obtain the fatigue crack initiation lifes of steel bridge decks.3 tabs,17 figs,31 refs.
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