Study On Fatigue Failure Judgement Basis Of Rib-to-deck Welded Joints In Steel Bridge Deck Based On Fracture Mechanics

Orthotropic steel bridge decks have been widely used in bridges at home and abroad due to their advantages of high bearing capacity,large span capability and convenient manufacturing and construction.In addition to many advantages,due to its own structural limitations,there are many welds in this type of structure,and the stress concentration is prominent.Moreover,it partially bears the repeated effect of automobile wheel load,leading that the fatigue problem becomes a key problem that restricts its development and application.The traffic has to be interrupted and costs a lot to repair once the fatigue crack appears,making the performance and service quality of the structure seriously affected.With the rapid development of China's economy,axle load and traffic volume of operational vehicles continue to increase,making the fatigue problem of steel structure Bridges become more and more serious.Therefore,it is of great engineering and economic significance to study the fatigue problem of orthotropic steel bridge deck.In this thesis,the fatigue cracking at the root of the rib-to-deck welding details is taken as the research object.The finite element models were established and the fatigue crack propagation process at the welding details was simulated.Based on the crack model,the stress of the key measuring point and the stress intensity factor of the crack front were extracted.The main contents and results are present below:(1)Review the research progress of fracture mechanics.The theoretical and experimental researches on fatigue problem of rib-to-deck welding details by domestic and foreign scholars are summarized.The principle and method of fatigue problem evaluated by fracture mechanics are described in detail,and the concrete steps of crack propagation are given;For the welding details of the rib to deck,the finite element models with different parameter combinations corresponding to the experimental model in the existing literature was established by the general finite element software ANSYS.The crack propagation paths of the specimen and the finite element model were compared.Extracted the stress intensity factor of the crack front in the finite element model and the coordinates of the free end of the crack,the relationship curve between the crack length and the fatigue life was obtained through theoretical calculation,and was compared with the corresponding measured points in the literature.Through comparison and verification,it was ensured that the finite element model could correctly reflect the actual stress state of the structure.Based on the above analysis,the stress intensity factors at the free end of the crack and at the middle crack in the finite element model were extracted and the fatigue crack propagation patterns at the welding details of the rib-to-deck were analyzed.(2)Based on the established finite element model,the stress of the key measuring points and the length and depth of the crack propagation were extracted.Based on the obtained basic data,the relationship curve between stress change and crack length and curve between crack length and crack depth at different parameter combinations were obtained,and representative curves were extracted from the overall development law of the relationship curve;Taking the crack growth length as the intermediate variable,the curve between stress change and crack length and the curve between crack length and crack depth were combined to obtain the final relation curve between stress change and crack depth of the measuring point.The relationship curves of stress change and crack depth at the measuring points of the toe and root were used as the evaluation basis to unify and modify the fatigue life of the test model in the literature with the same stress drop level.By comparing the correlation coefficient R~2 of the curves fitted to the data points before and after unification,it was found that the dispersion of the data points after the unification was significantly reduced.(3)The method of unifying the fatigue life of the model by the stress change curve of the toe and root was compared.The comparative study found that:after the unification and modification of fatigue life with the stress change curve of the toe and root,the dispersion degree of the modified test points is improved to different degrees;When taking the correlation coefficient R~2 of the modified fatigue strength fitting curve as the discriminant index,the modified results of fatigue life obtained by the stress change curve of the root measurement points are better.(4)The development trend of stress change curve of welding toe and welding root was analyzed,it is suggested that any stress drop level in the range of 10%-45%of measurement points at the toe be selected as the unified standard for welding details of rib-to-deck,and any stress drop level in the range of 15%-65%of measurement points at the root be selected as the unified standard for welding details of rib-to-deck;According to the stress change curve at the welding toe and the welding root,the crack growth depth can be inferred by the stress change at the measured point on the corresponding curve.