Fatigue Life Prediction Of Steel Bridge Under Complex Stress Based On Damage Mechanism

The steel bridge is liable to fatigue failure due to its alternating loading in the service period.In addition,due to the actual demand of the project,there are inevitably stress concentration areas(such as holes,grooves,etc.are collectively referred to as notches),reducing the fatigue strength of the steel bridge.The existence of the notches makes the stress in the root area change from unidirectional stress to two-direction or three-direction stress,that is,multiaxial stress effect.At the same time,the existence of notches will cause local reinforcement,affect the circulating hardening ability,anti-slip ability,dislocation density and movement ability of the plastic material,and induce serious non-proportional additional strengthening effect.Due to the complexity and diversity of loading forms and notch characteristics,there is no complete and qualitative description of the impact of loading path,material characteristics and notch effect on the fatigue life of steel bridges.And the key problem is how to put forward suitable for precise materials constitutive relation under multiaxial loading,accurately characterize the damage evolution process,and improve the fatigue life of steel bridges under complicated working condition,which has become a "bottleneck" for fatigue damage evaluation of steel bridge under multiaxial loading.The aim of this paper is to reveal the fatigue damage mechanism of steel bridge members under the action of complex stress(multiaxial stress),so as to predict the residual life of steel bridge members,improve the anti-fatigue design level,and provide theoretical basis for fatigue damage assessment and health monitoring of steel bridges.Combined with steel bridge with Q345 B steel under uniaxial/multiaxial fatigue test,the fatigue problem of steel bridge is reduced to the detail fatigue problem at the dangerous point in this paper.With the aid of numerical simulation,theoretical analysis and experiment validation,the fatigue damage evolution and life prediction of steel bridge are studied under complicated stress(multiaxial stress).The specific research contents are as follows:(1)Taking a steel structure viaduct near lanzhou city as an example,the mechanical state of the steel bridge under no-load and limit conditions is analyzed.With the help of the analysis of the whole structure of the steel bridge,the most dangerous point is confirmed,and the whole fatigue problem of the steel bridge is transformed into the detailed fatigue problem at the most dangerous point.(2)With the help of uniaxial and multiaxial fatigue tests of Q345 B steel,the influence of strain amplitude on uniaxial fatigue life is analyzed.Meanwhile,the influence of phase difference,notch geometric parameters and loading amplitude on multiaxial fatigue life is studied.The microstructure and morphology of fatigue fracture are studied by means of electron scanning electron microscope(SEM)and the causes of crack initiation are analyzed.At last the fatigue damage evolution mechanism of steel bridge is revealed.(3)Multiaxial proportional/non-proportional cyclic loading is realized using ANSYS load step,and the influence of bausinger effect on each stress-strain component is analyzed.Based on the energy method and the principle of coordinate transformation,a numerical calculation method for determining the position of critical plane under multiaxial loading is developed by taking strain energy density as the fatigue damage parameter.It makes up for the fact that the energy method does not consider the micro-mechanism of crack initiation and propagation,and the defined fatigue damage parameters have more definite physical significance.(4)In the process of non-proportional loading,whether each stress-strain component causes damage to the material and how much it contributes to the damage are analyzed.The influence of phase difference and material slip characteristics on unconstrained additional reinforcement is studied.The unconstrained additional reinforcement factor is put forward considering phase difference and the material cyc DE fatigue properties in order to improve the applicability of the fatigue damage evolution equation,and the life prediction model in this paper.(5)Analyzing the material degradation mechanism and the mechanics state of fatigue specimen,a multiaxial fatigue life prediction model considering additional reinforcement effect and average strain is put forward based on damage mechanics method.The multiaxial fatigue life is estimated from the perspective of material fatigue damage evolution,and the correctness and reliability of the theoretical method and the prediction model are verified by multiaxial fatigue test.(6)Based on the statistical data of a high-speed toll station,the load spectrum of vehicles passing the viaduct is compiled.The multiaxial nonlinear fatigue damage evolution equation is proposed in this paper,which is used to evaluate the theoretical fatigue damage degree of steel bridges during the design service period.And the results obtained by Miner's linear damage accumulation theory are compared to analyze the advantages and disadvantages of the two methods,so as to provide reference for the fatigue damage evaluation and health detection of steel bridges.