Thermal impact on rolling contact fatigue of railway wheels
Sammanfattning: Rolling contact fatigue (RCF) is a very common and costly damage mechanism for rails and wheels. This thesis investigates the influence of combined thermal and mechanical loading on RCF of railway wheels on the basis of numerical predictions. The established computational framework includes heat flux analyses, (two- and three-dimensional) elastoplastic finite element simulations and subsequent RCF life analyses. The computational framework is employed to quantify the influence of various operational parameters and modelling presumptions such as applied heat and tangential stress characteristics, load application schemes, mesh densities etc. Examples of results include quantifications of how partial slip conditions result in higher plastic strain magnitudes in a thin layer at the wheel tread surface, and differences in material responses between accelerating and braking wheels. The numerical model was extended to incorporate surface initiated cracks. With the extended model it is shown that 1 mm deep cracks have a substantial influence on the state of stress and strain in the bulk material between surface cracks. Further, comparisons between radial (thermal) and inclined (RCF) surface cracks show that the deformation of significantly inclined cracks (30 degrees) is more severe than that of radial cracks. Further, acceleration is found to give larger crack face displacements. However braking tends to induce tensile residual stresses that open the crack mouth, thus allowing fluid penetration that can promote crack growth. Also thermal loading is found to cause a significant crack mouth opening that is decreased by subsequent rolling contact. In a final study numerical RCF predictions are compared to full-scale experimental studies carried out at the Railway Technical Research Institute in Japan. Thermal loading tuned towards measurements by thermocameras and thermocouples are introduced in a truncated loading scheme corresponding to the test configuration. Estimated crack initiation life is found to be in good agreement with test results. The investigation also shows the significant influence of the employed material model. In addition to thermomechanical fatigue analyses, the case of purely thermal fracture has been investigated. This study quantified how the risk of fracture and resulting crack sizes depend on braking conditions and initial surface cracks. The results of this thesis are believed to be of importance in defining and enforcing sustainable operational conditions and maintenance actions. Further, this thesis provides tools to establish root causes and pertinent mitigating actions when thermomechanical wheel cracking nevertheless occurs.
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