Statistical evolution of short fatigue crack growth rate for LZ50 axle steel
Initiation and propagation of short fatigue crack occupy most of the fatigue life of metal components with smooth surface. To characterize the statistical evolution behavior of short fatigue crack growth, totally 9 hourglass shaped specimens were fatigued under a loading frequency of 15 Hz by short fatigue crack replica test method. Afterwards, according to the “effective short fatigue crack criterion”, replica test results were observed. Growth rate of DESFC for all specimens were obtained and analyzed. Combined with metallurgical test of LZ50 axle steel, conclusions can be drawn that it is the restraints of ferrite grain boundary and rich pearlite banded structure that cause the twice decline of DESFC growth rate. DESFC growth rates of each specimen at 10 given crack lengths and fatigue life fractions were obtained using linear interpolation method. The dispersion of DESFC growth rates was large and not steady in early cycles of MSC stage. But with the increase of loading cycles, the overall dispersion come down in later cycles of PSC stage gradually. Furthermore, seven commonly used distributions, i.e., 3PWD, 2PWD, ND, LND, EMVD1, EMVD2 and ED, were compared on three aspects, i.e., total fitting effect, consistency with the relevant fatigue physics, and safety of design evaluation. The results show that the ND, instead of 3PWD and 2PWD, is a good distribution for the growth rates of DESFC.