Abstract
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.DOI: http://dx.doi.org/10.5755/j01.mech.23.2.18108
Highlights
Fatigue fracture is the main failure mode of most mechanical components [1]
The behavior of short fatigue crack, which is strongly affected by microstructure in the microstructure short crack (MSC) stage and large scale yielding in the physical short crack (PSC) stage, is not similar with conventional long crack [3,4,5]
The short fatigue crack replica tests of 9 hourglass shaped specimens were performed, and the dominant effective short fatigue crack (DESFC) growth rates were calculated based on obtained DESFC size and corresponding number of loading cycles
Summary
Fatigue fracture is the main failure mode of most mechanical components [1]. the initiation and propagation of fatigue crack for smooth surface components occupy the most part of service life [2]. The behavior of short fatigue cracks, which is strongly affected by microstructure in the MSC stage and by merge propagation mechanism in the PSC stage, shows a large scatter and obvious statistical characteristics. Conclusions were drawn that all data scattered largely in the MSC stage and had a trend to be consistent in the PSC stage These conclusions were based on the unilateral considerations of fitting effect, while consistency with the relevant fatigue physics and safety of design evaluation were not taken into account. Four typical characteristic parameters were analyzed using seven distributions on three aspects, i.e., total fitting effect, consistency with the relevant fatigue physics and safety of design evaluation, by Zhao [2, 7, 13, 14] and the authors [15]. Good distribution for the description of DESFC growth rate data was determined considering three aspects mentioned above
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