Research On Cooperative Propagation Behavior Of Double Fatigue Cracks Of Diaphragm Arc Cutouts In Steel Bridge Deck

Orthotropic steel bridge deck has been widely used in long span Bridges since its development.However,because of its complex structure,it is prone to fatigue cracking,which has a great influence on the safety of bridge operation.Fatigue cracking at the arc opening at the connection between diaphragm and U rib is one of the most common fatigue diseases.There are many cracks in this part,among which the cracks in the arc-shaped opening part of diaphragm are more serious.In practical Bridges,there are several fatigue cracks in the arcshaped openings of some diaphragmals.The propagation characteristics of multiple cracks are different from those of single cracks.After initiation,multiple cracks will expand independently in their respective regions and affect each other,causing great harm.Most scholars at home and abroad focus on the single crack,but there is still a lack of in-depth and systematic research on the coupling propagation behavior of double cracks,which fails to fully reveal the fatigue performance of the structure details.Therefore,in this paper,the arc-shaped opening of diaphragm plate in typical fatigue vulnerable parts of orthotropic steel bridge panel is selected as the focus part.Combined with the theory of linear elastic fracture mechanics and ABAQUSFRANC3D interactive modeling technology,a numerical analysis model of arc-shaped opening of diaphragm plate is established.The stress intensity factor and propagation characteristics of multiple fatigue cracks in arc-shaped openings of steel bridge deck diaphragm under vehicle loading were studied.The main research contents are as follows:(1)The stress intensity factor was calculated by ABAQUS-FRANC3D combined modeling based on M-integral method.A flat plate model with a semi-elliptic initial crack was established and Franc3d’s calculation results were compared with the theoretical values calculated by the formula method in Newman manual.The maximum relative error does not exceed 2.4%.Thus,the accuracy and reliability of ABAQUS-FRANC3D interaction technology are verified.(2)The orthotropic steel bridge deck which is widely used in China is selected as the research object and numerical simulation is carried out by using finite element software.Multiscale fracture mechanics finite element model of steel bridge deck is established.The loading mode and loading condition of segment model were determined,and the ABAQUS user subroutine Dload was used to load the moving load.The longitudinal history of static stress intensity factor of single crack and double crack at arc opening of diaphragm is analyzed.The amplitude of type I stress intensity factor under the double crack simulation is obviously smaller than that under the single crack simulation.The influence of different initial morphology ratio on stress intensity factor is also revealed.The maximum KI values of different morphological ratios all appeared in the same loading condition.(3)Based on the existing research,and referring to the relevant regulations of the structural dimensions of orthotropic steel bridge deck given by national codes,two sensitive parameters of steel bridge deck size parameters and fatigue crack geometry parameters are selected for analysis.The interaction factor λ was introduced to characterize the coupling effect of double cracks.The effects of fatigue crack geometry parameters(fatigue crack depth,fatigue crack morphology ratio and fatigue crack Angle)on static stress intensity factor of double cracks at curved openings of steel bridge deck diaphragms are discussed.(4)Based on the theory of linear elastic fracture mechanics,the initial crack was inserted into the finite element model by ABAQUS and Franc3d software.The propagation process of single and double cracks at arc-shaped openings of diaphragm is simulated.The cooperative propagation behavior of multiple fatigue cracks was analyzed.The influence of coupling effect of double cracks on crack growth rate was also discussed.In the simulation of double crack growth,one of the two single cracks that can continue to grow stops growing after a certain number of cycles,and the other one develops into a main crack.By calculating the stress intensity factors corresponding to each expansion step,the characteristics of double fatigue crack propagation at the arc opening of steel bridge deck diaphragm under vehicle load were revealed from the initial morphology ratio and initial depth of fatigue crack.