Experimental research on dominant effective short fatigue crack behavior for railway LZ50 axle steel

Behavior of dominant effective short fatigue crack, the crack results in the final failure of specimen, was experimentally investigated by a replica technique with seven smooth hourglass shaped specimens of railway LZ50 axle steel. Character of two-stages, i.e. the micro-structural short crack stage and the physical short crack stage, was revealed for the crack initiation and growth. Most importantly, the crack growth rate exhibited decelerations twice in micro-structural short crack stage. This behavior was related to the ferrite grain boundary firstly and then to the pearlite banded structure. The boundary appeared a barrier because there were pearlites around with significant higher micro-hardness values. Also because of the higher hardness values, the layered pearlite banded structure acted as another barrier for the crack to overcome. In physical short crack stage, the crack propagated with a decreasing resistance of micro-structural barriers as the crack length increased. Hence, the process of crack initiation and growth are subjected to the competition between the intrinsic resistances from the barriers and the increasing driving force from the growing crack size. Finally, a new short crack growth model is presented to describe the periodic influence of micro-structural barriers. Description to the test results of LZ50 steel has indicated the availability and reasonability of present model.