Fatigue Crack Growth Performance Of Bridge Steel:Effect Of Stress Ratio,Microstructure And Single Overload

Fracture failure of components due to material fatigue during the service of steel bridges is a significant problem.Understanding the fatigue crack growth(FCG)behavior of materials is crucial for guiding the design of structures to withstand fatigue.The stress ratio and microstructure are the most important and prevalent factors affecting FCG behavior,and single overload is commonly used to investigate variable-amplitude FCG behavior.Hence,this paper combines experimental and theoretical methods to investigate the FCG behavior of four commonly used steel bridge plates,including Q345 q C,Q370 q E,Q420 q E,and Q500 q E.The main studies are as follows:(1)FCG tests were conducted on Q345 q C bridge steel under different stress ratios(R = 0.1,0.2,0.3,0.4,0.5,and 0.7).Probabilistic Paris and Walker formulas with five survival probabilites(P = 1%,10%,50%,90%,and 99%)were proposed using the method of probabilistic statistics.The theoretical FCG life at P = 50%,90%,and 99% was calculated based on the integration formula of Paris and Walker models and compared with the test values.The results show that as the increase in stress ratio,the range of stress intensity factor(SIF)amplitude included in the stable expansion stage is smaller,and the amplitude of stress intensity factor at the end of the stage is also smaller.When the survival probability P = 90%,the FCG life calculated using the integral formula based on the Walker model has a high safety tolerance and low error.(2)FCG tests were performed on the surface-and middle-layer materials of Q370 q E,Q420 q E,and Q500 q E bridge steel.The effects of the equivalent grain size and volume fraction of the second hard phase on the FCG behavior of bridge steel surface-and middle-layer materials were investigated.Fatigue fracture morphology of the surface-and middle-layer materials of bridge steel was examined using a scanning electron microscope(SEM).The results show that the FCG rate decreases with increasing strength grade for a specific sampling location(surface or intermediate layer).Materials with smaller equivalent grain sizes or higher volume fractions of the second hard phase exhibit lower FCG rates.Fatigue steps were observed in Q500 q E bridge steel with the lowest fatigue crack growth rate,indicating that fatigue steps significantly contribute to FCG resistance.(3)The effects of single overload(OLR = 1.5,2.0,and 2.5)under different stress ratios(R = 0.1,0.3,and 0.5)on the FCG behavior of the Q500 q E highstrength bridge steel were investigated.The distribution of the crack tip strain field was systematically analyzed by the digital image correlation(DIC)test technique.Plastic zone sizes and residual strain at the crack tip were clarified.The crack opening and closing response of the crack tip wake region was examined using the virtual extensometer technique.By studying the deformation near the crack tip,the mechanisms of FCG retardation are revealed,and the mechanisms of fatigue crack arrest are clarified.Finally,a modified formula,based on the modified Wheeler model of the plastic zone theory and experimentally measured plastic zone size at the crack tip,was proposed to predict the FCG rate after a single overload,yielding predicted values that are in good agreement with the test values.