Study On Short Fatigue Crack Behavior Of LZ50 Axle Steel Under Torsion Loading

Fatigue is the main cause of failure of engineering structures and components,accounting for approximately 50% to 90% of various failure behaviors.With the increase of the speed of railway transportation and the increase of the loading in our country,the accidents caused by fatigue failures of various components of railway vehicles occur from time to time.As the key components to withstand the weight of rolling stock,the axles are often subjected to alternate or multiple combinations of different loading modes.In the past,studies on the fatigue properties of axle materials mainly focused on axial tension & compression and rotation bending.However,the influence of the torsional loading on the fatigue behavior of axles has rarely been studied.Therefore,taking the axle material LZ50 steel of railway vehicles as the object,the short fatigue crack behavior under the action of torsional loading is studied,which is of great significance and value for the reliability management and safety evaluation of axles.This article focuses on the short fatigue crack behavior of LZ50 axle steel under torsional loading and carries out the following work:1.The metallographic test of the LZ50 axle steel material was carried out and it was shown that ferrite and lamellar pearlite were the basic metallurgical structures of the material,and the ferrite and pearlite exhibited periodic intervals along the rolling direction of the axle.2.The fatigue short crack replica test of LZ50 steel under torsional loading is completed.The observation results show that under the torsion loading mode,short cracks initiate in the ferrite grains and expand along the axial direction under the action of shear stress.After a certain length,the bifurcation starts,and then it is expanded and connected in the circumferential direction until the sample breaks.In the process of short crack propagation,the expansion rate has two significant decelerations.For the first time,it encounters ferrite grain boundary barriers,and for the second time,after it extends along the axial direction to a certain length,it encounters pearlite grain boundary barriers.3.The observation of the fracture morphology of the torsion fatigue specimen was completed.Under the torsional loading,the shear wear morphology and the tension & compression wave morphology of the fatigue fracture coexist,and at the same time,it has a transitional morphology that combines the two parts.The transient breaking zone is a dimple fracture under normal stress.4.The dominant effective short fatigue crack size and the fatigue life fraction under torsional loading were analyzed,and the minimum hypothesis distribution(EMVD1)was determined as a good hypothetical distribution of the two characteristic statistical parameters.5.Based on the short crack growth rate model considering the retardation effect of multimicrostructures,the experimental data were fitted and a good fitting effect was obtained,which reflected the influence of two kinds of microstructure structural obstacles on the short crack propagation rate.