Analysis of rail breaks and insulated joint deterioration

Licentiate thesis, 2008

The thesis deals with fatigue related issues in the field of railway mechanics. In particular rail breaks and deterioration of insulating joints are investigated. The study is largely motivated by plans to increase allowed axle loads in Sweden. The aim is to deliver operational results, as well as exemplify how practical problems in railway mechanics can be assessed by computer based simulations. In the first appended paper the influence of wheel flat impacts on the growth of rail cracks and subsequent rail breaks is analyzed. This calls for an analysis of the dynamic train--track interaction combined with a fracture mechanics analysis of crack growth and fracture. To account for high frequency excitations due to wheel flat impacts the in-house code DIFF is employed for the analysis of train--track interaction. Stress intensity factors for rail cracks are derived using FE-simulations. Added loading due to restricted thermal contraction of all-welded rails is included. Risks of rail breaks for specified crack sizes and temperatures are evaluated as functions of impact load magnitudes. In addition crack growth rates are quantified. Simulation results indicate that wheel flats increase the risk of rail breaks, but have a limited influence on rack growth rates. The latter is due to the infrequent occurrence of wheel flats and the fact that these need to impact in a rather narrow region to cause any considerable influence on the crack growth rate. In the second paper deterioration of insulated joints is analyzed by 3D FE-simulations. Repeated negotiations of a loaded wheel over an insulating joint is simulated. To obtain realistic material responses, an advanced constitutive model is employed. The simulations indicate the accumulated strain (ratcheting) to be the dominant damage mechanism. In a parametric study the influence of insulating gap, axel load, frictional force, and coefficient of maximum friction in the wheel--rail interface is assessed. In particular the frictional force (traction/braking) is found to have a major influence on joint deterioration. A limited field study has been carried out and provided some qualitative confirmation of the simulations.