Effect Of Loading Sequence On Short Crack Behavior Of LZ50 Steel

During the actual service of railway vehicles,the mass of empty and loaded vehicles varies greatly,resulting in the load borne by the axle arranged in a certain order of load spectrum.In the process of operation,the eccentric load caused by line conditions,curve passing and other factors will also cause the axle to bear different nominal rotational bending loads.LZ50 steel,as a common material for the manufacture of railway vehicle axles,often shows different fatigue resistance under the action of large and small loads applied in different order,which is closely related to the short crack stage behavior occupying 70% ～ 90% of the fatigue life of smooth surface structure.Taking LZ50 steel as the object and based on the fatigue replica test of rotating bending specimens under different load sequences,this paper carried out a comparative study on the initiation and propagation behavior of short cracks under different load sequences:Firstly,the rotary bending fatigue tests of LZ50 steel samples under constant amplitude load(300 MPa low load and 335 MPa high load)and high-low load and lowhigh load conversion at two different reference life fractions(0.3 and 0.65)were completed,which provided a reference for the subsequent design of short crack behavior research scheme.Secondly,the fatigue short crack replica tests of high-low and low-high load conversion under two reference life fractions were carried out respectively.The results of replica observation show that the fatigue short crack behavior of LZ50 steel is greatly affected by the microstructure barrier in the early stage,showing the typical stage characteristics of microstructure short crack(MSC).At this time,the effect of the transformation of loads on the crack growth rate is not obvious;After entering the stage of physical short crack(PSC),the influence of macro conditions on the crack growth rate began to dominate.In the later stage,the crack growth rate of the sample under high load was significantly higher than that under low load.The fracture analysis shows that under the two loading sequences,the cracks are multi-source initiation;In the early stage of fatigue life(reference life fraction 0.3),the fatigue bands in the crack growth zone and the number of dimples in the transient fracture zone of the high-low loading sample with load conversion are narrower than those of the low-high loading sample with variable load at the same time,indicating that the crack growth behavior is different under different load sequences.Thirdly,the two typical statistical characteristic parameters of dominant effective short crack scale and life fraction are analyzed.It is determined that the minimum value distribution(EMVD1)and three parameter Weibull distribution(3PWD)are good hypothetical distributions of dominant effective short crack scale and life fraction of low-high loading samples respectively,while for the two characteristic parameters of high-low loading samples,the good hypothetical distributions are 3PWD.Finally,based on the modified multi microstructure barrier short crack growth rate model,the short crack growth rates of samples with different loading sequences are fitted.The model can better characterize the short crack propagation and reflect the periodic deceleration phenomenon in the process of propagation,and the variable load of different reference life fractions has an obvious influence on the location and amplitude of deceleration in the growth rate curve.The main research content of this paper is the influence of load action sequence on the short crack behavior of LZ50 steel,which is of great significance to obtain a more reasonable basis for reliability analysis and safety evaluation.