The Growing Of Micro-Crack On Wheel/Rail Contact Surface

Since the beginning of the new century, with the increase of the speed of passenger train and the axle load of freight train in our country, it proposed a series of challenges on the railway technology research. Many of the key technical issues need to be resolved. And the wheel-rail rolling contact is one of them. Wheel-rail rolling contact fatigue and wear has been the old problem for railway transport and hard to solve. The fatigue crack growth and fracture are very important in these problems. In this thesis, two dimensional finite element model of the surface micro-cracks was established by using finite element program ANSYS. The contact elements were used on the crack surfaces to simulate the opening and closing of crack. The singular elements were introduced at the crack tip to analyze the stress intensity factors at the crack tip. It was considered that the material nonlinearity and the nonlinearity of contact boundary. The stress intensity factor at the crack tip and the stresses was obtained for the surface micro-crack vertical to rail surface. By using the crack propagation criterion and crack fracture criterion of Fracture mechanics, the surface micro-crack for U71Mn rail is analyzed. On this basis, the fatigue life for rail is estimated through the Paris formula. Specific research works carried out as follows:1. Using the Hertz contact theory, without the action of friction, the stress intensity factor of the crack tip and the stresses were calculated for the surface micro-crack on the rail by using ANSYS finite element model. The results showed that, the axle load has a significant influence on the stress intensity factor, especially on KI; The stress intensity factor of surface micro-cracks reaches its maximum at the edge of contact patch.2. The finite element model of the surface micro-cracks on rails is established. Considered the real wheel-rail contact friction, the stress intensity factor at the creak tip and the stresses of "worn profile tread-new rail" and "worn profile tread-worn rail" were calculated for different lengths of surface micro-cracks and different axle loads. The results show that:the stress intensity factor of "worn profile tread-new rail" is much larger than "worn profile tread-worn rail", and rail surface micro-cracks propagate more easily, so the new railway track should avoid heavy vehicles running; with the wheel rolling, the crack tip stress intensity factor first increase and then decrease, the peak take place near the edge of the contact patch; As the axle load increase, the stress intensity factor at crack tip increases significantly; The length of the crack increases, the stress intensity factor at the crack tip increase and then decrease; In the wheel rolling process, the propagation of micro-cracks only took place near the edge of contact patch, at other location it was closed.3. Considered the real wheel-rail contact friction, the stress intensity factor at the crack tip and the stresses of "conical profile tread-new rail" was calculated for different lengths of rail surface micro-cracks and different axle loads. Comparing the result with the worn profile tread, we can see that the surface micro-cracks are easier to propagate under conical profile tread.4. According to the Paris formula, the expression to estimate fatigue life was obtained, and the fatigue life of the rail surface micro-crack for "worn profile tread-new rail","worn profile tread-worn rail" and "conical profile tread-new rail" were estimated though this expression. The results showed that:the axle load has an important influence on the fatigue life of surface micro-crack, and the wheel-rail contact tread has an important influence on the fatigue life of surface micro-crack also.