Fatigue crack behaviour in pearlitic railway rails subjected to large shear deformation

Doktorsavhandling, 2024

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 one of the safest and most energy-efficient modes of transportation, offers a sustainable alternative to fossil-fuel-based transportation. There are, however, many challenges that must be addressed for rail transportation to be a more competitive option. Safety, functionality, reliability, and economic feasibility must be ensured. The major challenge related to materials is rolling contact fatigue, which impairs safety and economic reliability. The imposed loadings from the wheel/rail contact induce severe deformations in the near-surface region of the rail, leading to the formation of an aligned and anisotropic microstructure. Rolling contact fatigue cracks are often initiated in this region, and crack propagation is affected by the direction of the microstructure alignment.

The aim of this thesis work is to better understand how the anisotropy developing in service changes the fatigue and fracture characteristics of rail steels. Fatigue crack propa-gation experiments under uniaxial, pulsating torsional, and non-proportional multiaxial loading, on both virgin 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 used rails and was obtained by bi-axial large shear deformation under compression.

The uniaxial and torsional fatigue crack propagation experiments showed that the fatigue life is dependent on the material state, with predeformed material exhibiting a longer fatigue life. The effect of predeformation on the crack growth direction was limited in uniaxial loading but dependent on the material state in torsional loading. For the multiaxial fatigue crack propagation experiments, the crack growth direction was significantly influenced by predeformation. The early crack growth rate was found to be similar for both the undeformed and predeformed material states. In addition to these experiments, in-situ fatigue crack propagation experiments were also conducted on single edge notched specimens machined from predeformed test bars along two different directions. This was a first attempt to characterize the effect of microstructure alignment locally on the crack growth and crack paths. Lastly, a method for in-field railhead crack detection using digital image correlation was proposed. The method was evaluated under laboratory conditions, and the detected cracks correlated well with the crack network in the analyzed rail section.