Study On Rail Weld Defect

Continuously-welded rail (CWR) has replaced bolted rail joints as the preferred method of joining rails in track all over the world. The CWR efficiently reduces the level of impact forces between wheel and rail at rail joints and extends the service life of the wheel and rail with respect to bolted rail. However, with the increment of train speed and axle load, the phenomenon of rail weld defect (RWD) shows a tendency to be severe. The RWD leads to the higher operating costs and represents one of the main risks for safety of railway operation. Therefore, from the practical and cognitive points of view, studies on RWD are very important. In this thesis, based on the vehicle-track coupling dynamics theory, wheel-rail rolling contact theory, elastic-plastic mechanics and finite element method, the defects of rail weld with different type are investigated through the numerical simulation.(1) In order to better characterize the wheel-rail interaction at high speed rail weld, a vehicle-track coupling dynamics model is developed. In the model, a Timoshenko beam is used to model the rails which are discretely supported by sleepers. The sleepers are assumed to move backward at a constant speed to simulate the vehicle running along the track at the same speed, and therefore such a track model can consider the effect of the discrete support by sleepers on the coupling dynamic behaviour of the vehicle and track in the simulation. The normal contact forces between wheels and rails are decided by nonlinear Hertzian elastic contact theory. The coefficient of the normal contact stiffness is decided by the instant contact condition of the wheel and rail surfaces. The tangential wheel-rail contact forces are calculated by Shen-Hedrick-Elkins' model. The wheel-rail creepages are deduced on the basis of the normal velocity difference of the contact planes of the wheel and rail is always equal to zero. The rail weld irregularities are modeled as the local track vertical deviation described with some ideal cosine functions. The dynamics model is used to analyze wheel-rail impact loading caused by compound irregularity, single irregularity, the long and short wavelength rail weld irregularity. The loading between the rail and sleeper and the accelerations of the wheel and the track components are also investigated. The effects of the train speed, the axle load, the wavelength and depth of the irregularities, and the weld center position in a sleeper span on the wheel-rail impact loading are analyzed. The results show that the short-wavelength irregularities of rail weld easily excite the higher resonant frequencies of the track and can lead to a severe damage of the wheel and track components. Hence, the phenomenon of short-wavelength irregularities of rail weld should be avoided in the on-site construction and maintenance. Moreover, the ratio of the depth to wavelength is more reasonable to control the rail weld irregularities. An irregularities-controling table is developed, which provides a reference when the rail weld is ground.(2) Based on the above vehicle-track coupling dynamics model and three dimensional elastic-plastic finite element model, a stress analysis model of rail weld is set up. In the model, the rail base metal, weld metal and heat affected zone have different properties of materials. The vehicle-track coupling dynamics model is applied to calculate the dynamic wheel-rail contact force and the size and location of the contact area. The contact force, the size and location of contact area obtained by the vehicle-track dynamics simulation are utilized as the inputs to FASTSIM program to obtain the normal and tangential force distributions. Then the sizes and locations of contact areas and normal and tangential force distribution are used as the inputs to finite element model. The Mises equivalent stress, residual equivalent plastic strain and residual displacement near the rail weld at the tangent and curve tracks are investigated. Results show that the rail weld irregularities lead to high wheel-rail impact loading and high localized plastic deformation. The mechanical properties of the weld metal should be equal to those of the rail base metal in the on-site construction to decrease the impact loading caused by the non-continuity deformation and rail weld irregularities.