Research On Rolling Contact Fatigue Damage Mechanism Of Train Wheel Tread

Rail transportation,as one of the important transportation modes for land transportation,plays an irreplaceable role in promoting the development of national economy.Wheels are the load-bearing and moving components which are critical to the safety,effectiveness,and smooth running of the train.However,rolling contact fatigue and spalling damage often occur during the service of train wheels which seriously affects the safety,economy and comfort of train running.Although researchers have carried out lots of research on wheel tread damage,the problems of rolling contact fatigue and spalling damage still exist and it is more frequent with the increase of train axle weight and running speed.Consequently,the mechanisms of the rolling contact fatigue and spalling were carried out in this thesis.Firstly,the failure analysis of an actual service wheel with tread rolling contact fatigue and spalling damage was carried out.The causes and mechanisms of the rolling contact fatigue and spalling damage were analyzed.Secondly,the damage behaviors and mechanisms of the rolling contact fatigue specimens were studied by conducting the standard rolling contact fatigue test in the laboratory,which was compared with the actual service wheel to analyze the reasons,similarities and differences of the damage behaviors and mechanisms.The name and length of the rolling contact fatigue crack were defined and a quantitative evaluation method for rolling contact fatigue crack damage degree was proposed.Finally,the influence of the creep rate on actual wheel tread stress and contact spot shape was analyzed through the finite element analysis.The stress distributions and contact spot shape of the standard rolling contact fatigue specimen was also studied and compared with actual service wheel’s.An actual service wheel suffered rolling contact fatigue damage.Macro observation,micro observation and mechanical performance test were carried out on the actual service wheel to reveal the behaviors and mechanisms of the rolling contact fatigue damage using the failure analysis method.Macroscopic observations showed that the rolling contact fatigue damage of the wheel tread occurred in a distance range of 90～130 mm from the inner side of the wheel,showing a very strong regional clustering distribution characteristic.The top angle of the rolling contact fatigue crack pointed to the outer side of the wheel.Microscopic observations and mechanical performance tests showed that the surface structure of the rolling contact fatigue damage zone undergone the plastic deformation and grain breakage and refinement.The causes and mechanisms of the rolling contact fatigue damage of the wheel were revealed by the comprehensive analysis of wheel tread damage.Simultaneously,the relationship between wear and rolling contact fatigue damage was revealed.The standard rolling contact fatigue tests were carried out in the laboratory to compare and analyze the similarities and differences of the rolling contact fatigue damages between the standard rolling contact fatigue specimens and the actual service wheel and then further reveal the behaviors and mechanisms of the rolling contact fatigue damage.The results showed that the mechanism of the crack initiation and propagation was the same,but the crack propagation angle,the thickness of the plastic deformation layer and the depth of the spalling pit were different,which was mainly related to the shear stress on the wheel tread.The greater the shear stress,the smaller the crack propagation angle and the greater the thickness and spalling depth of the plastic deformation layer.The damage of the actual service wheel was greater than that of the standard rolling contact fatigue specimen.The rolling contact fatigue crack was named "triangular directional crack" according to its morphological characteristic and the method of expressing the crack length was defined.In addition,a quantitative evaluation method for evaluating the degree of rolling contact fatigue crack damage was proposed.The actual service wheel and the standard rolling contact fatigue specimen in the laboratory were modeled using the finite element simulation calculation method to carry out the stress field analysis of the wheel-rail contact under typical service conditions and the laboratory conditions.The effect of creep rate on wheel tread stress magnitude and distribution law under actual service conditions was studied and the difference in stress magnitude and distribution law between actual service wheels and rolling contact fatigue test wheels was compared and analyzed.The results showed that the creep rate has no effect on the compression stress of the wheel tread,but has a significant effect on the shear stress of the tread.The greater the creep rate,the greater the shear stress and the larger the adhesion area.The contact spot shape of the actual service wheel tread was an ellipse,while the standard rolling contact fatigue specimen’s was a rectangle.The stress distribution rules of the two were exactly opposite,which resulted in the differences in damage morphology and degree of the both.However,the damage mechanisms were the same.