Comparative Study On Short Fatigue Crack Behavior Of EA4T Steel Under Different Combined Tension-torsion Loading Paths

With the rapidly changing in science technology and social economic today,the rail transportation has entered a new stage of high speed and heavy haul,and the research on the safety and reliability of relevant structures in railway vehicle has become more important than ever.As an important part of vehicle operation and load bearing,the axle mainly bears multiaxial alternating loads in service.Its failure mode is mostly the fatigue fracture,and the proportion of short fatigue crack initiation and propagation in the failure process is usually very high.Therefore,conducting researches on the initiation and propagation of short fatigue cracks under multiaxial loading can provide a theoretical basis for evaluating the axle safety properly.In this thesis,EA4 T steel,a commonly used material of high-speed train axle,is taken as the research object to study its short fatigue crack behavior under different combined tension-torsion loading paths.1.The short fatigue crack replica tests under six loading paths were carried out.The results show that under the action of tensile and torsional multiaxial loads,short cracks basically originate at the tempered sorbite grain boundary or inside the ferrite grain,and then propagate along the direction of maximum shear plane or in the direction perpendicular to the specimen’s axis.In the process of propagation,the short crack growth rate decreases several times because of the obstacles of grain boundary,inclusion and banded structure.The results of typical specimens show that when the dominant effective short fatigue crack(DESFC)length is below the critical length of 500 μm,the crack usually grows slowly.The process of the initiation and propagation of short cracks accounts for 70% to 80% of the entire fatigue life.2.By comparing the results obtained using the six loading paths,it can be seen that with the increase of phase angle,the material gradually exhibits non-proportional additional hardening effect,which reduces the fatigue life.The growth rate of short crack decreases in early MSC stage,and then increases in the later MSC stage and in PSC stage.The fracture analysis shows that as the phase angle increases,the number of crack sources decreases,the area of scratches in the extension region increases,and the number of dimples in the final fracture region decreases.3.For the two typical characteristic parameters,i.e.the DESFC length and the life fraction,statistical analysis shows that under 0° proportional loading path,the normal distribution(ND)is a good hypothetical distribution to describe them,while for 90°non-proportional loading path,the extreme minimum value distribution(EMVD1)is a good hypothetical distribution.4.Based on the short fatigue crack growth model that includes the influence of multiple microstructural barriers,the test data under six different loading paths are fitted.The fitting effect is good and the curves effectively indicate the periodic effect of microstructure on short crack growth.