Continuous Welded Railway Rails: Residual Stress Analyses, Fatigue Assessments and Experiments
Sammanfattning: Railway rails are today often continuously welded. This type of joint generally requires less maintenance than is needed for the bolted joints. It has also reduced wheel damage and made the ride more comfortable. However, these joints are subject to damage. The severity of the fatigue problem is likely to increase since a trend towards higher train speeds, higher axle loads and heavier traffic density is probable even in the future.The thesis deals with the finite element simulation of two manufacturing processes, flash butt welding of rails and laser cladding of railheads, and the subsequent service life of the welds under operating conditions. Welding of rails, elasto-plastic material modelling, in particular for material subject to cooling from a melted state, fatigue and general issues in computational welding mechanics are discussed. Experimental data and materials characteristics were collected from full-scale measurements of production welds, at the welding site, and from materials testing of smaller specimens. The microstructure and the material characteristics of both the base and welded materials, in the interval -40 °C to +600 °C, for the rail steel R260 (900A) were investigated. The emphasis is on the method of joining rails with flash butt welding and the coating of surfaces using laser cladding, which were simulated with material models that handle the recovery behavior in the vicinity of the melting point. In addition, the interaction between residual stresses from the manufacturing operations, and service loads from the wheel-rail contact was studied. The influence of residual stresses on fatigue initiation, of a twin-disc specimen simulating laser cladded rail, and crack growth, from defects in a flash butt weld, was examined. The results showed, in both cases, that the residual stresses caused by the processes were high enough to raise the risk of fatigue failure above that of the base material. The calculated manufacturing residual stresses were validated and found to be in good agreement with experimentally determined residual stresses in both the twin-disc and the welded rail.
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