Combined thermal and mechanical loading of railway wheel treads. A numerical study of material response and cracking under braking conditions

Licentiatavhandling, 2011

The scope of the thesis is prediction of the material response owing to combined thermal and mechanical loading of a railway wheel tread. The long-term objective is to improve models for prediction of surface-initiated Rolling Contact Fatigue (RCF) under the combined effect of mechanical loading due to rolling (and sliding) contact and thermal loading due to braking and/or wheel--rail friction. Paper A and Paper B present elastic-plastic Finite Element (FE) simulations to evaluate the impact of simultaneous thermal and mechanical loadings of the wheel tread. Thermal and mechanical loads are combined in a three-dimensional (3D) sequentially coupled analysis where nodal temperatures from a transient thermal analysis are applied as predefined fields in a structural analysis. The mechanical contact load is prescribed as a moving Hertzian contact stress distribution on the wheel tread. The interfacial shear stress distribution corresponds to full slip or partial slip conditions. Paper A outlines the necessity of 3D analyses when dealing with the combined effect of thermal and mechanical loading for realistic predictions of the material response. It is further quantified how a combination of high traction and thermal loading has a very detrimental influence on the material damage. Paper B identifies feasible mesh discretizations and load application schemes that lead to sound predictions of the material response at reasonable computational efforts. Paper B also quantifies differences in predicted material responses under full slip and partial slip conditions. For a given total tangential force, partial slip conditions result in larger plastic strain magnitudes in a thin layer near the contact surface. Paper C presents a numerical analysis of the formation of tread cracking due to severe tread braking. The analysis combines two-dimensional (2D) FE-simulations to evaluate thermal stresses with an analytical evaluation of resulting Stress Intensity Factors (SIFs). A criterion for static fracture is used to identify critical crack size for when existing surface cracks are prone to propagate and resulting crack lengths after propagation. Conclusions are that fully functional brake systems are not likely to induce thermal crack propagation under normal stop braking. However with pre-existing defects a severe drag braking due to malfunctioning brakes may cause very deep cracking. Results from the 2D analysis are compared to full 3D FE-simulations. 2D results are found to be on the conservative side.