Influence Of Loading Path On Short Fatigue Crack Behavior Of LZ50 Steel

With the development of rail transportation towards the direction of heavy haul and high speed,the service environment of axle,which is an important component of bogie,is getting worse and worse.Among many failure modes of railway axle,fatigue is the most common one,in which the initiation and propagation of short cracks account for about 80%of the entire fatigue life.While in service,the load condition of axle is affected by many factors such as turnouts,rail joints and track irregularities,so it is difficult to describe it by simply using a single load waveform or phase.In this thesis,LZ50 steel is taken as the research object to study its short fatigue crack behavior under different waveforms(rectangular-wave,triangular-wave)and phase differences(0°,30°,60°,90°).1.The short fatigue crack replica tests under five different loading paths of waveforms and phase differences were carried out.The experimental results show that the short fatigue crack originates at ferrite grain boundary or inside the ferrite.In the process of short crack propagation,the phenomenon of periodic deceleration and acceleration occurs,and the two obvious deceleration processes are obviously related to the two kinds of microstructure characteristics of LZ50 steel.2.By comparing the test results under different waveforms and phase angles,it can be seen that the fatigue life obtained using the rectangular-wave loading path is lower than that of the triangular-wave while ensuring all the other loading parameters are consistent.While under the rectangular-wave loading,with the decrease of the phase difference,the fatigue life shows a downward trend.3.According to the SEM observation of the fracture,the fractures under different loading paths all showed multi-crack sources;compared with the rectangular-wave,the spacing of fatigue fringes is significantly smaller and more secondary cracks in the propagation region of the triangular-wave.By comparing the fracture morphology of the rectangular-wave at different phase differences,with the increase of phase difference,the white veins in the crack source region are gradually worn out;the spacing of fatigue steps and the scratches increases in the propagation region;and the tear strips appear in the instantaneous fracture region.4.Two typical characteristic parameters,i.e.the dominant effective short fatigue cracks(DESFC)size and the life fraction,were statistically analyzed.It is determined that for these two parameters in rectangular-wave loading path condition,the extreme minimum value distribution(EMVD1)is a good hypothetical distribution to describe them.However,in triangular-wave loading path condition,EMVD1 is a good hypothetical distribution for the life fraction,while the extreme maximum value distribution(EMVD2)is the one for the DESFC size.5.A growth rate model including the multiple microstructure barriers is adopted to fit the short crack test data under five loading paths,which shows a good fitting effect.The fitted growth rate curves can reflect the periodic changes in the process of short crack growth.