Numerical And Experimental Study On Wheel/Rail Adhesion Characeristics At High Speed

Wheel/rail adhesion at high speed is one of the biggest issues for wheel/rail tribology research. The traction and braking ability of high-speed trains is limited by wheel/rail adhesion. Since China has massive land and high-speed railway lines of more than ten thousands kilometers are open air, railheads are inevitably contaminated by "the third body" such as water, oil and so on, which can change wheel/rail adhesion characteristics and cause wheel/rail rolling contact fatigue. In recent years, Chinese railway has a developmental trend of high speed and heavy haul, which makes more demands of the exertion of wheel/rail adhesion ability. Since wheel/rail adhesion is a complicated tribological behavior and affected by many factors, the study on wheel/rail adhesion at high speed is rare. Although some work has been done by experiments, the testing speed is low, which is not able to clarify the mechanism of wheel/rail adhesion at high speed. In order to make better use of wheel/rail adhesion and ensure high speed railway running safely, it is necessary to further study wheel/rail adhesion characteristics at high speed. Theoretical reasons will be found for the selecting the measures for further adhesion control and enhancement.Through the numerical and experimental work, the present paper conducts the following work:(1) A brief overview of the study on wheel/rail adhesion at high speed at home and abroad and the necessarity of the study are presented. And the thesis framework on the wheel/rail adhesion is also briefly introduced.(2) A three-dimentional model of thermal full elastohydrodynamic lubrication is developed to study wheel/rail contact characteristics under water and oil lubrication. The effect of train speed and axle load on interfacial liquid film thickness is investigated. The difference of film thickness between considering and without considering temperature effect is obtained. Through fitting twenty groups of numerical results, the empirical equations for the center and minimum water film thickness related to train speed, material parameters, axle load and ellipitical ratio are obtained. These will provide the theoretical basement for the following simplified models.(3) The elastic-plastic micro-contact models based on stasticastic method are systematically investigated. The relationship of gap/contact-force and contact-force/contact-area with plastic index change obtained by using the three micro-contact models is discussed from the micro aspect. Using the Newton-Raphson method, wheel/rail contact pressure distribution is obtained with different contact loads and surface roughness parameters from macro aspect. The contact pressure and contact width under different loads are investigated. The above comparisons and application conditions of the three models can provide the the theoretical basis for selecting the most avalible model prediction wheel/rail adhesion.(4) For the small film thickness ratio (A<0.5) caused by the small wheel/rail surface roughness under water lubrication, the two-dimensional and three-dimensional models of high-speed wheel/rail adhesion based on the Grubin theory and micro elastic-plastic contact model are developed. The inlet heating, elastic-plastic micro contact, temperature caused by the moving roughness are firstly coupled into one model. For the general film thickness ratio (A>0.5), firstly, a three-dimensional model considering surface roughness and interfacial liquid is developed. Then a two-dimensional high-speed wheel/rail adhesion model which firstly takes the temperature, elastic-plastic behavior of asperity contact and rheology property into one model is developed. At last, the above two models are extened to a comprehensive three dimensional model which can take temperature and elastic-plastic asperity contact behavior into account. The traditional thermal EHL simulation method is successfully extended to be avalible to wheel/rail contact pair. The effect of train speed, surface roughness, axle load, surface roughness pattern, temperature, rheology property, slip ratio et.al on wheel/rail adhesion characteristics are quantitively explained using the above three models.(5) The high speed wheel/rail adhesion tests under water, oil and water/oil mixture contamination are conducted to study the wheel/rail adhesion characteristics by JD-2 wheel/rail rolling contact facility. The maximum simulation speed of the test machine is as much as 300 km/h. The velocity-adhesion coefficient curves are obtained from the present tests under the three different contaminations. The present experimental results can clarify the adhesion mechanism from the other point of view and enrich the test data of the adhesion coefficient over 200 km/h. The numerical model is validated and compared with the high speed experimental results. The present numerical and experimental results will the theoretical foundation for the wheel/rail adhesion control and the choice of adhesion enhancement measure.