Experimental Investigation On The Damage Behaviors Of Rolling Wear And Contact Fatigue Of Wheel Materials

The rapid development of railway transportation brings diversification of railway wheel materials and operation condition. Issues of railway wheel damages become increasingly apparent. In order to improve the safety of the railway vehicle operation, higher requirements are demanded on wear and fatigue performance of railway wheel. Therefore, it is necessary to conduct the experimental investigation on wear and fatigue mechanism of railway wheel materials. In this dissertation, the influence of material compositions, operation speeds and lateral forces on the rolling contact fatigue and wear behivours of railway wheel materials have been investigated using a JD-1 wheel/rail simulation facility. The main achievements and conclusions are as follow:1. The comparative analysis of wear and fatigue characteristics on four wheel materials with different chemical composition has found that the wear resistance of wheel material increases with the carbon content, while the anti-fatigue performance shows contrary, and the enhanced effect of carbon content on rolling contact fatigue sensitivity is stronger than the reduced effect of the manganese content. In addition, the chromium can effectively improve the corrosion resistance. The hardness of near-surface of wheel specimens after test apparently increases and its distribution can be described by an exponential function. The index set of the harden ratio, the thickness of harden layer and the area of radius curvature function can well describe the hardenability of the wheel specimens at near-surface.2. The comparative analysis on wheel damage behivour between low and high speed conditions has been performed. Results indicate that both specimens under two conditions have irregular wear. It shows that smooth and rough region appear alternately along rolling direction on the worn surface. Both the maximum hardness value and the thickness of harden layer of rough areaare bigger than those of smooth area. However the difference of damage between two areas at low speed is significantly bigger than that at high speed. Both the Fitting Deviation Method (FDM) and the Fixed Ratio Method (FRM) can effectively evaluate the thickness of specimen near the surface damage layer, however the FDM has more practical significance and provides some beneficial references for later analysis the damage thickness of near surface layer of rolling contact parts.3. As the material accumulation phenomenon often occurres at one side of wheel roller in previous experiments, it is similar with the tread rollover which occurres in practical operation, thus the damage behavior of wheel material under lateral force has been investigated. Rresults indicate that scars of wheel rollers under lateral force could divide into plastic accumulation region (PAR), fatigue region (FR) and worn region (WR), which the damage of PAR is dominanted by plastic deformation and fatigue cracks, that of FR is dominanted by plastic deformation and wear, while that of WR is dominanted by mild wear. Drastic changing in stress and the stress direction is from the high stress region to the lower one, there will appear the material accumulation and fatigue cracks, some metals extend from contact area to non-contact area and form a mental tongue, which corresponds with the tread rollover damage. Grains of wheel material become elongated obviously under high stress, with fibrous shape showed in cross section.The plastic deformation at the near-surface of wheel roller could be described by shear strain, which has very strong consistency with hardness distribution. However this characterization method shows more convenience than hardness measurement, thus it has better applicability.4. Aimed at the the fluctuation of hardness value data and the special regular of fatigue cracks initation, the inhomogeneity of wheel material in microscale has been investigated, and on this basis, the mechanism of fatigue crack initation has been further investigated. The results indicate that the hardness distribution of wheel material shows very obvious inhomogeneity in microscale which isrelated to the measurement scale:the smaller measurement scale, the more obvious inhomogeneity. New fatigue indexes based on material inhomogeneity can further describe the mechanism of fatigue crack initiation. Adequate consideration should be given to the inhomogeneity of wheel material when its fatigue performance is evaluated.Furthermore the minimum of shear stress limit should be treated as its fatigue limit.