Study On Evolutionary Process Of Corrosion Pit And Its Effect On The Fatigue Property Of Corroded Steel

The plastic and fracture toughness of steel structure are reduced by the corrosion pits on its surface,so the crack extension rate would increase,which can undoubtedly impact on subsequent fatigue performance of steel structure.Because the fatigue fracture caused by corrosion is a big disaster for of steel structure,so the effectively predicting the corrosion residual fatigue life is an important aspect of durability of steel structure.Then,this study researches the fatigue performance and crack growth rate of corroded steel under alternating load based on theoretical derivation,experimental studies and finite element simulations,and the mainly contents and results are included in this paper as following:(1)Based on the morphology data of steel surface with different corrosion time,the depth(a)width(c)and margin distance(S)of corrosion pits on corroded steel surface at neutral salt fog environment were made the statistical analysis,respectively.The results show that all of the depth,width and margin distance of corrosion pits are obey normal distribution,with the increasing of corrosion degree,the average value and maximum value of depth and width of corrosion pit increased,the variation coefficient gradually reduced,moreover,the average value,maximum value and variation coefficient of margin distance gradually reduced,the irregular degree of corroded steel surface gradually reduced.Analysis results of the fractal theory show that,with the increasing of the average value and maximum value of depth and width of corrosion pit,the fractal dimension of corroded steel surface gradually increased,and its non-uniform degree of corroded steel surface is gradually outstanding;In addition,with the increasing of average and maximum value of S,the fractal dimension of corroded steel surface firstlydecreased,after increased,and the uneven degree of corroded surface firstly raised,after reduced.(2)According to the results of monotonic tensile test and surface topography measurement of corroded Q235 steel,the relationship between the yield strength,modulus of elasticity,strong flexor ratio and elongation with the depth,width and adjacent distance of corrosion pits was researched,the elastic modulus of these parameters was established;in the view of microscopic,material degradation mechanism of the performance was explained by using the stress concentration factors of different specimens from numerical method.(3)According to the analysis results of etch-pit parameters of corroded steel surface and fatigue fracture,the relationships between corrosion degree respectively and the type of fatigue fracture source,fatigue life were researched.the results showed that the single fracture source mainly lead to the failure at the initial stage of corrosion,the corresponding types of typical pit was deep and narrow(D-n);the interaction fracture source mainly lead to the failure at the last stage of corrosion,and the corresponding types of typical pit was shallow-wide(W-s),the residual fatigue life of corroded steel was associated with parameters of corrosion pit.By using the results of fatigue tests under different fatigue stress levels,the fatigue life degradation model based on fractal dimension of irregular corroded steel surface was established,and the model can predict the residual fatigue life of corroded steel with a certain irregular surface.(4)Using elastic-plastic mechanics theory,and combining the micro-shape effect of corrosion pit,the model for calculating the stress intensity factor range based on shape parameters of pits was established.Based on the small crack theory,threshold stress intensity factor range was revised on effect of pit.Based on the FE calculation results of corroded steel specimens,the influences of the average and maximum value of a,c and S on interaction between adjacent pit were analyzed,the calculation method of the interaction degree of adjacent pit was established,the condition criterion of interaction starting for adjacent pit was set up for different corroded steel.(5)Based on the short cracks theory,prediction model(FCGR-MPI)of fatigue crack growth rate of corroded steel was established to characterizing the crack closure effect.Compared with fatigue crack propagation test,FCGR-MPI model can achieve ahigh precision for predicting the fatigue crack growth rate.By using the finite element method,the location of the macroscopic corrosion pit causing the crack initiation was determined,and based on morphology characterization of the key corrosion pit.The prediction methods for fatigue life of corroded steel had been established.