Research On Fatigue Crack Propagation Behavior Of U75V Rail Steels

As the railway transportation in China is developing toward high-speed and heavy haul, fatigue fracture due to the wheel-rail rolling contact will become the major damage pattern which replaces the traditional wear failure. U75V is one of the most widely used rail steels, which distributes through the high speed passenger lines, heavy freight lines and the mixed passenger and freight lines. Therefore, it has important significance for the safe operation of the railways to study the fatigue crack propagation behavior of the U75V rail steels.In this paper, the fatigue crack growth behavior of U75V rail steels is studied. The main works and conclusions are as follows:(1) The fatigue crack growth threshold values ΔKth of U75V steels under different R-ratios are measured by the stepped unloading method. The R-ratios are0.03,0.1,0.2,0.5, respectively. The fracture surface morphologies are carefully studied with SEM to explore the fracture mechanism in this region. It is found that the threshold values ΔKth decrease with the R-ratio increasing. The fracture surface is covered with ridges and transgranular facets are also observed.(2) In order to research the R-ratios and specimen directions effects to the fatigue crack growth, the fatigue crack growth tests with different R-ratios are conducted. The compact tension (CT) specimens are obtained from the rail head part of an U75V rail steel, which had a normalizing heat treatment. The sampling directions are L-T, S-T and L-S, the R-ratios are0.03,0.1,0.2and0.5. The results show that the L-T and S-T specimens display sensitivity to the R-ratios, the crack growth rates of these orientations increase with increasing stress ratio. The L-S specimens show no relationship with the R-ratios. Although the Paris constant n of the L-S direction is slightly larger than that of the other two directions, the difference amang the values of n of the three directions are still tiny. The SEM shows that the lower growth rate regime appears to be parallel arrays of ridges while curving and fragmental ridges accompanied with fan-shaped rivers like cleavage fracture as well as secondary cracks are found in the higher growth rate regime.(3) A series of fatigue crack growth experiments are conducted using the non-normalizing heat treatment specimens, their R-ratios are also0.03,0.1,0.2and0.5. The experiments indicate a scattered result, the crack growth rates are not identical even the specimens have the same R-ratio. Some of the data show that the crack growth curves are not the straight lines in the double logarithmic coordinates but having fluctuation. The SEM to these specimens appear to fan-shaped rivers like cleavage fracture existing in the entire fracture surface formed by fatigue crack growth.