Wheels, Rails and Insulated Joints - Damage and Failure Probability at High Speed and Axle Load
Abstract: The thesis deals with some fatigue related problems in railway mechanics related to increased axle loads and speeds. It focuses on defects and discontinuities in the wheel-rail system for which modelling of fatigue probability and deterioration of components. Numerical simulations are performed to study plastic deformation and fatigue impact on an insulated joint. The simulations feature a sophisticated constitutive model for multi-axial ratchetting. The results indicate ratchetting as main damage mechanism. Effects are quantified for increased vertical and longitudinal load magnitudes and insulating gap width. Longitudinal loading is indicated as being severely deteriorating for the rail material. The risk for rail breaks is investigated from mechanical and statistical points of view with focus on impacts from flatted wheels. The contribution of wheel flat impacts, on rail cracks is quantified with a dynamic load analysis that evaluates rail bending moments due to wheel flat impacts. A fracture mechanics analysis is employed to establish stress intensity factors for rail head cracks under bending and temperature loading. Stochastic relation established between the crack position and the wheel flat impact position along the rail. In addition rail crack growth is also studied. The equivalent Dang Van stress under Hertzian contacts is evaluated over the full subsurface space using different approaches. An approach consisting in finding the minimum circumscribed hypersphere to the deviatoric stress path gives good trade-off between accuracy and efficiency. It is also shown that a very computationally cheap approximation employing half the peak Tresca shear stress results in good agreements in the entire subsurface space for conditions relevant under wheel–rail contacts. 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. Corrugation of the rail also adds to the randomness. By combining statistical methods with a contact mechanics and fatigue analysis, probabilities of fatigue initiation and failure are evaluated. The developed model takes as input statistical properties of the material defects, contact geometry and contact load or the output from a full train--track simulation. For the failure analysis, damage accumulation for random amplitude loading evaluated. The results show how a combination of rail corrugation and high train speeds has a significant impact on fatigue probability. A sensitivity analysis reveals a strong influence of the fatigue strength and the material defect distribution.
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