The Research On Damage Evolution Process Of Steel Wires Used In Bridge Cables Under The Coupling Of Corrosion And Fatigue

Corrosion fatigue(CF)damage is one of the typical failure modes of the high strength steel wire used in the bridge cables because of its exposure to the heavy polluted environment and the cyclic loads in a long term.In this paper,a simulation method was proposed by summarizing the domestic and foreign studies on the laws of CF failure and performance degradation of steel wires.Finite element(FE)models of steel wire with initial random corrosion pit were developed by cellular automaton(CA)technology.Based on the CF damage evolution model,a user material subroutine(UMAT)was compiled to simulate the CF damage accumulation process.Using the proposed simulation method,the CF damage state of steel wires was evaluated,the CF life of steel wires was predicted and the CF failure mechanism of steel wires was analyzed.The main works and results of this paper are as follows:1)The CA technology was utilized to simulate corrosion process and generate initial corrosion pit model.Based on the electrochemical essence of metal corrosion,the system of metal,passive film and corrosive media was divided into arranged cells in a three-dimensional cylindrical CA space.In the CA space,cell transition rules were set to simulate corrosion process of steel wire from the mesoscopic level.Finally,the morphology and position of random irregular corrosion pit were obtained by the CA method.By using the programming interfaces between MATLAB、AutoCAD、RHINO and ABAQUS software,the data obtained by the CA method in MATLAB were inputted into the above software to produce the three-dimensional grid model,the surface model and the geometer model successively.Hence,the visualization of high strength steel wire with initial corrosion pit was realized.The obtained geometer model doesn’t pre-assume the initial corrosion pit,comprehensively considers the randomness of metal corrosion process and perfectly presents the distribution of corrosion pit morphology on the surface of steel wire.2)In order to simulate the accumulation process of corrosion fatigue damage related to the cyclic load and the corrosive environment.Firstly,based on the linear elastic finite element method,focusing on the characteristics of corrosion fatigue damage analysis,a solution process based on cyclic block is proposed.Then,according to the continuum damage mechanics model,the user material subroutine(UMAT)was compiled by FORTRAN language.The simulation of corrosion fatigue damage accumulation was realized with reference to the concept of life-and-death element method.Meantime,the experimental results in the literature were used to prove the accuracy of the proposed method.Finally,the script program written in PYTHON language realizes the application process of cyclic loads,which effectively improves the efficiency of modeling analysis.3)The damage accumulation process of high-strength steel wire under pre-corrosion fatigue(PCF)was simulated,and the PCF life of high strength steel wire with initial corrosion pit was predicted.The simulation results show that under the PCF effect,the damaged element appears firstly in the high stress zone at the bottom of the corrosion pit,and the distribution of the damaged zone is affected by the pit morphology.With the application of the cyclic loads,the fatigue damage accumulates and penetrates into the steel wire,the rate of damage accumulation increases gradually,and the stress distribution also changes with the accumulation of damage.With the increase of the initial corrosion degree,the fatigue performance of the steel wire gradually decreases,and the stress concentration is the major reason for the PCF life reduction of corroded steel wire.Due to the presence of sharp angles in the random corrosion pit,the fatigue life of steel wire with random corrosion pit is much smaller than that of steel wire with regular semi-ellipsoid corrosion pit.Under the condition of the same pre-corrosion mass loss rate,the fatigue life of corroded steel wire is positively correlated with the number of corrosion pits,positively correlated with the angle of corrosion pits,and little correlated with the distance of corrosion pits.4)The damage accumulation process of high-strength steel wire under the coupling of corrosion fatigue(CCF)was simulated,and the CCF life of high-strength steel wire with initial corrosion pit was predicted.The simulation results show that under the CCF effect,the corrosive medium and the fatigue load promote each other,which results in the degradation of the fatigue resistance of the material.However,under the different stress levels,the influence of corrosion on the fatigue properties of steel wire is different.In the process of damage accumulation,the mutual coupling effect of corrosion fatigue continuously enhances,and its adverse effect on the mechanical performance of steel wire gradually increases.Compared with regular semi-ellipsoid pit models,damage accumulation process of the random pit model is more complex and its CCF life is shorter.Under the condition of the same initial pit depth,the CCF life of the steel wire is little correlated with the number of corrosion pits,positively correlated with the angle of corrosion pits and positively correlated with the distance of corrosion pits.The numerical simulation method proposed in this paper can make up for the shortcomings of the existing simulation methods that focus on the 2D or 3D regular defect model.The proposed method fully considers the influence of the irregular pit morphology on the mechanical performance,and well predicts the residual life of high strength steel wire with initial corrosion pit.Hence,the proposed method can partially replace the corrosion fatigue test to a certain extent,which greatly saves the experimental time and expense.The proposed method has important theoretical and practical significance for the reasonable determination of the fatigue strength and service safety assessment of bridge typical members.