Abstract: Short fatigue crack replica tests of LZ50 axle steel were carried out under seven different tension-torsion stress ratios, λ, at the same equivalent stress amplitude. The results show that, the change of tension-torsion stress ratio does not change the general trend of the short fatigue crack initiation and growth. The short fatigue cracks mostly initiate in the inner or at the boundary of the ferrite grains. At the microstructurally short crack(MSC) stage, there are two obvious drops of crack growth rate due to the inhibition of ferrite grain boundary and pearlite banded structure, while at the physically short crack(PSC) stage, the microstructure of the material has a less effect on crack growth and the growth rate increases continuously. Meanwhile, with the decrease of the stress amplitude ratio, both the crack growth rate at the same crack size and the angle between the fatigue fracture surface and the specimen axial direction gradually decrease. The initiation of the crack transforms from a single-crack-source mode to a multi-crack-source mode, and the fatigue life is increasing. Furthermore, by constructing a stress amplitude ratio and fatigue life curve equation, it is proved that with the same equivalent stress amplitude, the axial tensile-compression load contributes more to the fatigue damage of the material than the torsion load. As the stress amplitude ratio decreases, the increasing trend of the fatigue life increases.

Key words: short fatigue crack, tension-torsion load, stress amplitude ratio, LZ50 steel