Comparison Of Short Fatigue Crack Behavior For LZ50 Axle Steel Under Two Different Loading Frequencie

The initiation and propagation process of the fatigue short crack occupies the most part of the fatigue life for smooth surface components. LZ50 axle steel is one of the materials widely used in speed-up and heavy freight rolling stocks. The variation of loading frequency in service may lead to the changes of the fatigue property consequently. While the viewpoint on the influence of the loading frequency on fatigue property is not consolidated, the research of this thesis is of great value both in theoretical study and engineering guidance. Based on the replica observations on surface short fatigue cracks of hourglass shaped specimens of LZ50 axle steel under two different loading frequencies, i.e.,15Hz and 180Hz, following studies are performed:1. With the analysis of replica test results and tests of metallographic, random hardness and mechanical property, the mechanism of short fatigue cracks initiation and propagation under two loading frequencies was studied. The results show that all of the fatigue cracks initiated in ferrite grains, which are softer compared with pearlite grains, or grain boundaries. In microstructure short crack (MSC) stage, the growth of dominant effective short fatigue crack (DESFC) decreased twice, the lengths of DESFC according to the two decelerations are equivalent to the average diameter of ferrite grains and the average interval of rich pearlite banded structures respectively. In the PSC stage, the dominant crack propagates continuously with the increase of its size and is not significantly subject to the micro-structures any more.2. Comparison of curves for DESFC growth rates and effective short fatigue cracks (ESFCs) density under two loading frequencies reveals that most of curves are overlapped and have the similar variation trend. Combined with the roughly same morphology of fatigue fractures, conclusion can be drawn that the loading frequency has no significant effects on the short fatigue behavior of LZ50 axle steel. However, in the MSC stage, the DESFC growth rates of six specimens under 15Hz were slightly higher than that of the specimens under 180Hz. Random microstructure hardness analysis of before and after fatigue tests showed that the material under lower loading frequency is more likely to be affected by the influence of cyclic load, which is conducive to the cyclic hardening process. That may lead to the decrease in the regional material toughness and form more conducive conditions for crack propagation. Therefore, it may be one of the reasons why the DESFC growth rates under lower loading frequency in the MSC stage is slightly higher than that under higher loading frequency.3. Three characteristic parameters, i.e., the DESFC sizes, the fatigue life fraction and the ESFCs density, were compared on three aspects, i.e., total fit effect, consistency with the relevant fatigue physics and safety of design evaluation. The results revealed that there is no significant difference in statistical evolution for LZ50 axle steel under two loading frequencies.4. Based on the short fatigue crack growth model which includes multiple micro-structural barriers, the typical test data under two loading frequencies were fitted. From the fitting effect, the short fatigue crack growth model conforms to the experimental data well, and can reflect the periodic effect of different micro structural barriers on short fatigue crack behavior.