| Long span bridges are subjected to the long-term fatigue loads such as vehicles,wind,waves and currents and exposed to the natural environment,therefore,the corrosion fatigue failure caused by the combined action of corrosive medium and fatigue load is one of the typical failure modes of high-strength steel wire used for bridge cables.On the basis of summarizing the tests and numerical simulation in the field of corrosion fatigue at home and abroad,the S-N curve of fatigue,S-N curve of corrosion fatigue and size of fatigue crack-cycle times curve were obtained through fatigue tests,corrosion fatigue tests and fatigue crack propagation tests of steel wire with defects,and the influence rules of different corrosion factors and surface defect morphology characteristics on the fatigue performance of steel wire were analyzed.With the combination of theoretical analysis and numerical simulation,the evolution process of fatigue damage,corrosion fatigue damage and fatigue crack propagation of cable wire can be visualized.The specific research contents and results of this paper are as follows:(1)A set of corrosion fatigue test device for steel wire used for bridge cable is designed to realize multistage synchronous pulsating fatigue loading of multiple steel wire and greatly shorten the duration of the test.(2)In order to simplify the electrochemical formation process of initial pit on the surface of steel wire,a machine notch was made on the surface of steel wire.The corrosion fatigue coupling test was carried out on steel wire with prefabricated notch at different concentrations and p H values to obtain S-N curves under various working conditions,and the influence of solution concentration and p H value on the life of steel wire was analyzed.The results show that the life of steel wire with prefabricated notch under the coupling of corrosion medium and fatigue load is significantly lower than that under fatigue load alone.As the concentration of Na Cl changed from 3.5% to 0.5% and the acidity of the solution increased,the life of steel wire was significantly shortened.(3)The effect of notch size and shape on the life of steel wire was studied by corrosion fatigue coupling test.The initial pits of steel wire due to environmental erosion is simulated by cutting different sizes and shapes on the surface of steel wire.The effect of notch size and morphology on the life of steel wire was studied by using the corrosion fatigue coupling test device.The results show that the deeper the notch,the smaller the width,the sharper the shape,and the shorter the life of steel wire.(4)The effect of notch size and morphology on fatigue crack growth rate was studied by crack front marking technique test.The fatigue crack growth test was carried out,and the "beach-like patterns" was made in the steel wire section based on crack front marking technique,and the fracture morphology was copied by Auto CAD and the crack size was measured accurately.By observing the fracture morphology,the propagation behavior of fatigue crack is analyzed and judged.The growth rate of fatigue crack was determined based on the quantitative relationship between fatigue crack size and cyclic loading times.The results show that the deeper and narrower the notch,the sharper the shape,and the higher the fatigue crack growth rate.(5)In order to accurately obtain the damage evolution of steel wire under fatigue loading,the theories of continuous damage mechanics and fracture mechanics were combined,and commercial software ABAQUS was used to realize the visualization of fatigue damage evolution and corrosion fatigue damage evolution of steel wire with prefabricated notch.The damage evolution rule was analyzed,and the fatigue life and corrosion fatigue life of steel wire obtained by numerical calculation were compared with the results of the fatigue test and corrosion fatigue coupling test to verify the accuracy of the numerical simulation.The results show that the calculated fatigue life and corrosion fatigue life are basically consistent with the test results.With continuous evolution,the maximum stress appears at the bottom of the notch and the minimum stress at the edge of the notch.With the increase of fatigue loading times,more and more damage elements appear in the steel wire.The damage accumulates continuously and the stress distribution of the steel wire changes with the fatigue loading process.The life of steel wire under the coupling action of corrosion medium and fatigue load is lower than that under the simple fatigue load,the damage evolution rate of the former is also higher than that of the latter,and the damage element appears earlier under the coupling action.At the later stage of damage evolution,due to the action of corrosive medium,the damage difference between near and far notch of steel wire is smaller than that without corrosion.(6)In order to accurately obtain the rule of fatigue crack propagation,the visualization of crack propagation process was realized based on the Extended Finite Element Method and commercial software ABAQUS.The changes of stress and displacement in the steel wire during crack propagation were studied,and the simulation results were compared with the above fatigue crack propagation test results to verify the accuracy of the numerical simulation.The results show that the rule of fatigue crack propagation predicted by the Extended Finite Element Method is in good agreement with the experimental results,and the crack propagates in the direction perpendicular to the tensile load,which conforms to the characteristics of type I crack propagation.The maximum stress always appeared at the leading edge of the crack and moved forward with the growth of fatigue crack.The stress gradient in the tip of the crack is quite dramatic.On the whole,the numerical simulation results are close to the experimental results,which verify the reliability and practicability of the simulation method in this paper.The new fatigue testing device designed in this paper is based on the traditional fatigue testing machine,the multistage synchronous pulsating fatigue loading of multiple groups of steel wires and the coupling of corrosion medium and fatigue load is realized.It makes up for the traditional fatigue test which takes a long time and is difficult to be coupled with the corrosion environment,thus greatly shortening the duration of the corrosion fatigue test.At the same time,the method based on the combination of continuous damage mechanics and fracture mechanics can realize the visualization of fatigue damage evolution and corrosion fatigue damage evolution of steel wire,the Extended Finite Element Method can realize the visualization of fatigue crack propagation process of steel wire,the numerical simulation is in good agreement with the experimental results. |
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