Study On Fatigue Crack Propagation Behavior Of U71Mn Welded Rail

With the development of economy,China’s railway passenger and freight volume are increasing rapidly.Rail is one of the important basic bearing parts of the railway system.Its anti-fatigue crack growth ability is related to the formulation of reasonable grinding and replacement cycle of rail,and also the basis of residual life evaluation of rail bearing structure.Therefore,it is very important to accurately obtain the basic data of sensitivity of rail bearing structure to fatigue crack propagation.In this thesis,fatigue crack growth rate tests are carried out for rail head,rail waist and rail bottom of base metal and weld of U71Mn welded rail to study the difference in anti-fatigue crack growth ability and reasonable explanation combined with the microstructure was given.At the same time,the plastic zone of crack tip was quantitatively studied by digital image correlation technique（DIC）.Based on the finite element software,the sub-surface fatigue crack propagation of rail was simulated and analyzed,and the influence of different crack morphology ratios and inclination angles on the residual life of rail was discussed.The main contents of this thesis are as follows:CT samples were taken from rail head,rail waist and rail bottom of weld and base metal of U71Mn rail,and constant amplitude and overload fatigue crack growth rate tests were carried out.Microhardness test and metallographic observation were conducted on the samples after the test.The results show that the hardness of rail waist is close to that of rail bottom,and the hardness of rail head is lower than rail waist and bottom,regardless of base metal or weld metal.Under the same position,the weld hardness is lower than the base metal,and the weld hardness presents a V-shaped distribution,and the weld center’s hardness is the lowest;The structure of the base metal is single pearlite,but there are more reticular ferrite in the weld center and the reticular ferrite in weld center of rail head and bottom is more than that of rail waist.Comparing the fatigue crack propagation rate of different parts of rail with different materials,it is found that the base metal is stronger than the weld in terms of the anti-fatigue crack propagation ability of rail head,while the variation trend of rail waist and rail bottom is opposite.For the anti-fatigue crack propagation ability of base metal,rail head,rail waist and rail bottom decrease successively,and the anti-fatigue crack propagation ability of weld rail waist is the strongest,and the anti-fatigue crack propagation ability of rail bottom and rail head decrease successively.Comparing the test results under different stress ratios,it is found that the fatigue crack growth rate increases with the increase of stress ratio when?K is equal.In addition,Paris fitting of da/d N-?K curve is carried out,and Paris parameter values of each sample are obtained.The analysis of overload test data shows that overloading can obviously reduce the crack propagation rate.Based on the digital image correlation technology,the displacement field of the crack tip was obtained,and the stress-strain field of the crack tip was obtained based on the theoretical formula of fracture mechanics.Then,the region where the equivalent stress was greater than the yield strength was extracted,and the plastic zone profile of the crack tip was obtained.The r_pvalue of unidirectional plastic zone size was calculated according to Irwin theoretical formula.It was found that with the increase of crack length,the r_pvalue was larger and the increase rate was faster and faster.Two methods were used to locate the crack tip position accurately.The results show that the error of the crack tip position obtained by using DIC displacement data is large,but the coordinates of the crack tip located by combining DIC displacement data and r_pvalue are more reliable.The crack opening displacement data were analyzed and the CTOD measurement position was located under a certain crack length.The exact values of plastic?CTOD(?CTOD_pl)and elastic?CTOD(?CTOD_el)at the crack tip are extracted by analyzing the loading and unloading data of overload samples.Based on the crack opening displacement data,the crack opening force(F_open)was accurately captured,and the crack closing effect was considered.The Elber model was used to re-fit the crack growth rate data.Compared with Paris fitting parameters,it was found that the introduction of crack opening force only had an effect on parameter c,but had no significant effect on parameter m.Based on the finite element software,the fatigue crack propagation test of rail head of base metal was simulated and analyzed,and the feasibility of the simulation method was verified.A three-dimensional model of wheel-rail contact was established,and the measured load spectrum was applied.The calculation results show that the Von Mises stress of wheel-rail contact points on the dangerous section in the following 3 mm is the largest.Therefore,initial cracks with different morphologies and angles were introduced to study the influence of different initial crack parameters on the residual life of rail.The results show that the cracks on the subsurface of rail belong to type II+III composite cracks.What′s more,The smaller the crack morphology ratio is,the closer the crack inclination is to 50°,the faster the crack expands,and the shorter the residual life of rail is.