Research On The Fatigue Performance Of Corroded Steel Wire And Evaluation Method Of Fatigue Reliability For Bridge Cables

Cables are the safety lifelines of bridges,and play key roles as connecting links.In recent 20 years,the frequent cable breakage accidents and a number of cable-replacement cases of through and half-through arch bridges have aroused widespread concern on the the damage and healthiness assessment of bridge cables.Corrosion and fatigue are the two main factors that affect the safety and durability of cables.However,there has been no effective method to make a reasonable evaluation of cables safety performance during their service under these two factors.Based on the theory of linear elastic fracture mechanics and the means of fatigue tests,this paper carried out a systematical study on the corrosion damage evaluation,fatigue fracture mechanisms and damage tolerance analysis of steel wires.On the basis of material research,the method of residual life prediction and fatigue reliability evaluation on corroded cables were established.The main contents and conclusions of this study are as follows:1.Taking in-situ corroded wires from two existing bridges for research object,the corrosion morphology of corroded wire was characterized and analyzed based on the profile measurement data,and the maximum pitting evaluation model for corroded wire was established.The findings showed that the scale index and self similarity index of corroded wire both increase with the increase of corrosion degree.But with a stronger randomness,there is only a statistical correlation between the self similarity index and the corrosion degree.The maximum pitting depth of corroded wire conforms to the Gumbel extreme value distribution and the pitting effect of free segment of cable is more significant than that of the anchorage segment.2.Fatigue tests under different stress amplitudes were carried out on in-situ corroded wires and artificial accelerated corroded wires.Through the fracture surface analysis,the fatigue fracture mechanisms of corroded wires were investigated,and the calculation methods of fatigue limit,fracture toughness and crack shape evolution law of corroded steel wire were proposed.The results showed that the fatigue life of corroded wire is of great randomness,and the pitting morphology is not the only factor affecting the fatigue life of wires.The crack initiation of corroded wire is primarily determined by the elastic crack threshold condition,and the fatigue limit of corroded wire can be well estimate by the Murakami formula.The critical fatigue fracture condition of corroded wire accords with the brittle fracture criterion,and the crack shape evolution model established by fracture analysis is more secure than the existing model.3.By conducting a series of single edge cracked sheet fatigue tests,the crack propagation characteristics of high-tensile steel wire in steady-state propagation region and near threshold propagation region were obtained.The crack growth model of steel wire with considering stress ratio was established by Walker formula.In view of the depth dependence of crack propagation characteristics caused by the material inhomogeneity,fatigue tests were carried out on round bar notched specimens from the same batches to verify the effectiveness of sheet test results for the analysis of macro damage tolerance of wires.On this basis,a review was made on the investigations of crack propagation characteristics of cold drawn eutectoid steels,and the similarity laws of fatigue crack growth in different tests were summarized,and the reasons for the difference of test results were discussed.4.The research on probabilistic damage tolerance performace analysis of corroded wire was carried out,and the effectiveness of the linear elastic fracture mechanics method for predicting the residual life of corroded steel wire was verified from both theoretical and experimental aspects.On this basis,through the theoretical and numerical studies,the probabilistic damage tolerance analysis method of corroded cable based on the relevant failure simulation was proposed,and a measured data of tested cables was analyzed by using this method.The results showed that a conservative prediction can be obtained by probability damage tolerance analysis when the material fatigue resistance is unknown.5.For the service safety of corroded cables,a fatigue reliability evaluation system was established,which is suitable for inert and time-varying corrosive environment.A case study was carried out on hangers of through and half-through arch bridges to analyze the effect of corrosion degree,load grade and environmental aggressiveness on fatigue life and service reliability of cables.The results showed that,in inert corrosion environment,the fatigue reliability of cables has different tolerances to the corrosion degree under different load grades.For the cables suffered from miner corrosion damage and low grade load,there is still some safety stock in fatigue life during routine inspection period.In time-varying corrosive environment,although cables mainly fail in fatigue mode,corrosion damage takes the largest proportion of total damage in the whole life.