Abstract Crack initiation, coalescence and propagation normally occupy more than 70% the fatigue life of most engineering structure in the whole fatigue damage process. Investigation on the influence of final machining methods, such as surface rolling, on short fatigue crack behavior for LZ50 axle is beneficial to the manufacture and maintenance of corresponding railway axles. After rolling, the surface hardness increased from 201.68 HV0.1 to 222.90 HV0.1. Much higher residual compressive stress was also engendered in surface and sub–surface by this machining method. Totally six smooth hourglass shaped specimens with surface rolling were tested by a replica technique. The characteristic two–stages behavior, that is, the micro–structural short crack (MSC) stage and the physical short crack (PSC) stage, during crack initiation and propagation was revealed. In MSC stage, the growth rate of dominant short crack for all specimens decelerated twice clearly due to different microstructural barriers. This behavior was related to the restraint of ferrite grain boundary firstly and then to the constraint of pearlite banded structure. While in PSC stage, the decelerating trend was no longer obvious with the increasing dominant crack size. With a given dominant crack size, the crack growth rates of surface rolled specimens were much slower than those of specimens without surface rolling. This difference could reach 1 order of magnitude in MSC stage. Meanwhile, the average fatigue life of the former was about 5.4 times longer than that of the latter. The effective short fatigue crack density of surface rolled specimens increased in MSC stage, then attained the peak value at the turning point between MSC stage and PSC stage, and finally decreased in PSC stage. At the same time, surface rolled specimens owned much less crack density than specimens without surface rolling during the whole fatigue process. Surface rolling can restrain the nucleation and connection of micro–cracks, improve the local microstructure conditions, push back the transition point between MSC and PSC stages, and thus improve the anti–fatigue performance of material. Finally, the reasonable assumed distributions for three kinds of characteristic parameters, i.e., dominant short crack size, fatigue life fraction and effective short crack density, were determined. In general, the dispersion of above data was high in initial stage and relatively low in later stage during crack initiation and propagation process.
Supported by National Natural Science Foundation of China (Nos.50575189 and 50821063), Fundamental Research Funds for the Central Universities (No.SWJTU11CX075), and Opening Project of State Key Laboratory of Traction Power, Southwest Jiaotong University (No.2011TPL T02)
