Comparison Of Short Fatigue Crack Behavior For Axle Steel Under Two Different Loading Types

LZ50 axle steel is widely used in axle manufacturing for speed-up and heavy-haul freight rolling stocks.With the railway transport in China having entered a high-speed,heavy-haul era,the axle,as a key part,always serves in harsh environment.Its loading types are complex and varied which may lead to the difference in the anti-fatigue ability one of the same material.When dealing with the case of fatigue reliability and safety analysis,the impact of the loading type should be considered in order to make the results more reasonable and credible.This paper focuses on the study of short fatigue crack behavior of LZ50 axle steel under two loading types which are Tension&Compression and Bending&Bending respectively.1.Basic material properties tests,such as metallographic tests,mechanical performance tests and hardness tests,were performed for LZ50 axle steel,which provided the basis for the further study of short fatigue crack behavior.2.The fatigue short crack replica tests were carried out under two kinds of loading types.6 groups of effective test data were obtained under the tension&compression loading while 7 groups effective test data were obtained under the rotating&bending loading.Test results show that fatigue short cracks generally occur in the ferrite grain boundary or within the ferrite grains which has relatively lower hardness.3.The fracture SEM observations under two different loading types were perfonned.Both the axial tension&compression loading and the rotating&bending loading have typical fatigue fracture morphology,and there are some differences in the macroscopic dimension due to the different loading types.4.It is found that there are two typical decreasing in the short crack propagation rate under both two loading types.When the short crack scale breaks the interval of rich pearlite banded structure,its growth behavior tends to the rule of the long crack.5.Based on the short fatigue crack growth model which includes multiple micro-structure barriers,the typical test data obtained under two different loading types were fitted.The fitting effect was quite well by compare with the test data.The load type effect coefficient CHTC-RB is introduced as a link between the crack growth rates obtained under the two different loading types respectively.6.The statistical results show that the minimum value distribution(EMVD1)is a good assumed distribution for the dominant effective fatigue short crack(DESFC)size and life fraction(f)under the two different loading types.