Wheels, Rails and Insulated Joints - Damage and Failure Probability at High Speed and Axle Load

Doktorsavhandling, 2011

The thesis deals with some fatigue related problems in railway mechanics related to increased axle loads and speeds. The focus is on defects and discontinuities in the wheel--rail system that affect the risk of fatigue and fracture of railway components such as wheels, rails and insulated joints. Numerical simulations are performed to study plastic deformation and fatigue impact on an insulated joint. The simulations feature a sophisticated constitutive model able to model multi-axial ratchetting which is indicated as the main damage mechanism. Effects are quantified for increased vertical and longitudinal load magnitudes and insulating gap width. High longitudinal loading is found to be severely deteriorating for the rail material. The risk for rail breaks is investigated from mechanical and statistical points of view with focus on impacts of flatted wheels. The influence of wheel flat impacts on rail cracks is quantified from a dynamic load analysis that evaluates rail bending moments. Stress intensity factors for rail head cracks under bending and temperature loading are then evaluated. A stochastic relation is established between the crack and the wheel flat impact positions. Finally rail crack growth and fracture are predicted using standard methods. To evaluate the risk of subsurface cracking in a wheel, the Dang Van equivalent stress under Hertzian contacts is evaluated. Two methods for this evaluation are found to be useful when considering accuracy and efficiency. Subsurface initiated rolling contact fatigue cracks initiate in the vicinity of material defects. As these exist randomly in the material, fatigue will appear randomly under otherwise constant conditions. Further, corrugation of the rail adds to the randomness. A combination of statistical methods, contact mechanics and fatigue analysis is employed for the analysis. Statistical properties of the material defects, contact geometry and contact load or the output from a full train--track simulation are taken as input. For the failure analysis, the damage accumulation for random amplitude loading is evaluated. The results show how a combination of rail corrugation and high train speeds has a significant impact on the probability of fatigue. A sensitivity analysis reveals a strong influence of the fatigue strength and the material defect distribution.