Crack path investigations in a pearlitic rail steel after pre-deformation under cyclic Mode-II loading

Abstract

The crack path under cyclic Mode-II loading in undeformed and pre-deformed pearlitic rail steel R260 is investigated. To produce large-scale specimens with pre-deformed microstructures two methods are applied. Firstly, cylinder bars are processed using an axial-torsion machine resulting in a moderate deformation. Secondly, a new high pressure torsion (HPT) setup is developed to achieve a more strongly deformed microstructure. In the fatigue tests, a distinctive change in the crack path is observed. In the undeformed and moderate deformed specimens, the cracks bifurcate and turn into a Mode-I dominated mixed-mode. In the HPT material, the cracks grow in Mode-II without mixed-mode propagation. For rolling contact fatigue cracks in rails these experimental results indicate a strong connection between the propagation mode and the anisotropy of the highly deformed pearlitic microstructure. This microstructure is characteristic near the wheel-rail contact surface. In future, these findings are important to improve numerical-based crack growth prediction models.