Study On Fatigue Properties Of Rib-to-deck Structural Detail Under The Influence Of Wedding Initial Micro-crack

Orthotropic steel bridge deck has the advantages of high load-bearing capacity and strong spanning ability,but it requires a large number of welding during the manufacturing process,which makes the location of the welding seam prone to fatigue cracking.There are two main factors that affect the fatigue properties of the weld position: one is the stress concentration caused by the discontinuous geometric structure of the weld under the external load,and the other is the initial defect and residual stress introduced during the welding process.In view of the structural details of the rib-to-deck,the microscopic inspection of the welded joint section found that the initial microcracks of different sizes and directions generally exist at the weld toes and roots.The microcracks at different fatigue vulnerable positions will inevitably produce corresponding fatigue properties.According to the details of the welding structure of the rib-to-deck,by obtaining the size and distribution of the initial micro-cracks in the actual inspection test,this thesis analyzes the fatigue failure mode transfer of the structural details caused by the difference of the initial micro-cracks at the weld toe and the weld root based on the theory of fracture mechanics.The problem is that it is necessary to consider the dominant failure mode of structural details and the fatigue properties evaluation of vulnerable details considering the initial micro-crack size.Aiming at the specific fatigue damage details,the influence of the initial microcrack angle is analyzed,and the relationship between the fatigue life distribution caused by dimensional randomness and the fatigue properties dispersion problem in the actual test is determined.The main research contents and conclusions are as follows:(1)Based on the actual bridge welding process,a full-scale model of the top plate and longitudinal ribs is designed and manufactured,and a large number of metallographic specimens of the welded joint are cut and processed.The initial micro-crack detection is carried out on the welded joint specimens,the initial micro-crack size of the fatigue vulnerable position of the structural details under modern automatic welding technology is determined,and the probability distribution characteristics of the initial micro-crack size at the weld toe and root of the structural details are counted.Based on the test results,the geometric characteristics and size distribution differences of the primary microcracks at different vulnerable locations are compared;(2)According to the problem of experiment that the transfer of the weld toe failure is designed to be the root failure problem.Based on the ANSYS software and the sub-model mesh reconstruction method,the finite element model of the weld toe and the root of the weld with different characteristic sizes is established and the fatigue life is calculated.A method for evaluating structural details-dominant failure mode that comprehensively considers the stress levels of fatigue vulnerable details and the degradation effects of micro-cracks is proposed,and it is clear that the initial micro-cracks have an important influence on the fatigue properties of vulnerable details;(3)In view of the randomness of the initial micro-crack angle at a specific fatigue vulnerable position,based on the interaction integral method and the principle of maximum circumferential stress,the effect of different initial micro-crack angles at the weld toe on the stress intensity factor and cumulative fatigue life during the fatigue crack growth process is explored.The influence indicates that the final propagation direction of the crack has nothing to do with the initial crack angle,and is only affected by the actual stress state.The initial micro-crack angle difference has a limited effect on the final fatigue life;(4)In view of the randomness of the initial microcrack size at specific fatigue vulnerable positions,based on the measured initial microcrack size distribution of the weld toe,2000 sets of random initial microcrack parameters are obtained.The fatigue life and equivalent fatigue strength distribution of vulnerable details under the influence of random initial microcracks are calculated.Comparing the calculated theoretical results with the fatigue properties dispersion of the specimen in the actual test shows that the randomness of the initial microcrack size is one of the important factors leading to the fatigue properties dispersion.