Crack Propagation Analysis And Fatigue Life Evaluation Of Rib-to-deck Welded Joints Of Orthotropic Steel Deck

With the widespread use of orthotropic steel bridge deck in large and medium span bridges,people pay more and more attention to its fatigue problems.The fatigue cracks originating from the rib-to-deck welded joints are the most difficult to observe and the most dangerous fatigue cracks.For the rib-to-deck welded joints witch mainly subjected to compressive stress dominated cyclic loading,there are some differences in how to consider the stress amplitude in design codes of different countries,and there is very limited fatigue test data of steel used for bridges under the negative stress ratios.To cope with the situation,this thesis focuses on solving fatigue problem of steel bridge by fracture mechanics and the nominal stress approach based on specific S-N curves.The major toipics in the thesis are as follows.(1)Evaluation of fatigue stress analysis model for rib-to-deck welded joints.Based on the Haihe Bridge in Tanggu,Tianjin,the FEA model of standard steel box girder segment,the simplified FEA model and the submodel of steel box girder are established to analyze the fatigue stress characteristics at the rib-to-deck welded joints of orthotropic steel deck.The results show that the 3-span submodel proposed in this thesis is an efficient and accurate FEA model for the fatigue stress analysis of the rib-to-deck welded joints at any position of the orthotropic steel deck,as it has the computational accuracy of the FEA model of standard steel box girder segment and the computational efficiency of the simplified FEA model.(2)Three-dimensional crack propagation analysis for the mix-mode fatigue cracks of rib-to-deck welded joints.Based on Schwartz-Neuman alternating method,a local three-dimensional fracture mechanics analysis model of the rib-to-deck welded joint is established,and the dynamic crack propagation analysis of the weld toe and weld root of the rib-to-deck welded joints are realized,which is in good agreement with the experimental phenomena observed in the open literature.The results show that fatigue cracks at rib-to-deck welded joints are of the mixed mode cracks of Model I,II and III,but dominated by Model I,which will no longer remain flat during the propagation process.The fatigue life of the rib-to-deck welded joints is controlled by the crack propagation along the deck thickness at the weld toe.(3)Fatigue crack growth rate test of Q345 qD steel and weld zoon under different negative stress ratios.Fatigue crack growth tests of Q345 qD specimens under three stress ratio loading cases(R= 0,R=-0.5 and R=-1),and of Q345 qD weld zoon under four stress ratio loading cases(R= 0,R=-0.5,R=-1 and R=-1.5)have been conducted.The single specimen data approach and group data approach are both used to verify that the compressive stress in cyclic loading really plays a great role in driving the fatigue crack growth.The experimental data of this paper are fitted based on different crack propagation rate models.The results show that the single crack propagation driving force parameter based on Paris formula is no longer suitable for fatigue crack propagation analysis under the cyclic loading with negative stress ratio.The research in this paper can provide a certain research basis for the safety design of bridges and the establishment of a more accurate crack propagation rate model describing the effect of R-ratio.(4)Fatigue life prediction for rib-to-deck welded joints of Haihe Bridge.Based on the measured traffic data of Haihe Bridge,five typical fatigue vehicles were identified.The equivalent stress ranges of the rib-to-deck welded joints are calculated by using the fatigue load vehicle recommended in China specification for Design of Highway Steel Bridge and the actual traffic axle weight.The calculation results show that the equivalent stress range obtained by the fatigue performance evaluation method adopted in China specification for Design of Highway Steel Bridge is lower than the fatigue strength level of this structure detail,while the equivalent stress range calculated according to the actual traffic axle weight is larger than the fatigue strength level of this structure detail.It is indicated that using the fatigue evaluation method in China specification is not safe to predict the fatigue life of the Haihe Bridge...