Effect of large shear deformation on fatigue crack behavior in pearlitic rail steel

Sammanfattning: The impacts from global warming and climate change continue to rise and securing the needs of future generations requires a transition to a climate-neutral society. Rail transportation, as being one of the safest and most energy efficient modes of transportation offers a sustainable alternative to fossil-fueled based transportation. There are however many challenges that must be addressed for rail transportation to be a viable option. Safety, functionality, reliability and economic feasibility must be ensured. The major challenge related to materials is rolling contact fatigue which impairs the safety and economic reliability. The key factor to mitigate the effect of rolling contact fatigue is to understand how the material behavior changes when the material is subjected to repeated rolling contact loading. The imposed loadings from the wheel/rail contact induces severe deformations in the near-surface region of the rail and leads to the formation of an aligned and anisotropic microstructure. Rolling contact fatigue cracks is often initiated in this region and crack propagation is promoted by the direction of the microstructure alignment. The shape of typical head checks for example, correlates well with the anisotropy infracture toughness. The aim of this thesis work is to better understand how the anisotropy developing in service changes the fracture and fatigue characteristics of rail steels. Fatigue crack growth experiments under uniaxial and pulsating torsional loading, on both undeformed and predeformed pearlitic rail steel R260 have been conducted. The material state of the predeformed material is similar to the material state in the near surface of deformed rails and was obtained by large shear deformation under compression. The fatigue crack propagation experiments showed that the fatigue life is dependent on the material state where predeformed material have longer fatigue life. The effect of predeformation on the crack growth direction was limited in uniaxial loading whilst dependent on the material state in torsional loading.