Crack closure in near-threshold fatigue crack propagation in railway axle steel EA4T

The paper is focused on fatigue crack propagation in the threshold area of the EA4T railway axle steel. Railway axles represent one of the most critical components for safety of trains. The main aim of this paper is to describe fatigue crack closure mechanisms in EA4T, which significantly influence the threshold value - a crucial parameter for determination of residual fatigue lifetime of railway axles. Investigated fatigue crack closure mechanisms are: plasticity induced crack closure, roughness-induced crack closure and oxide-induced crack closure. One part of this paper is focused on comparison of crack closure determined by NASGRO relationship (including plasticity induced crack closure mechanism only) with the experimentally obtained one. Difference between these two approaches is explained by presence of roughness and oxide induced crack closure. Based on theoretical models different sources of fatigue crack closure mechanisms was separated and evaluated. The significance of oxide-induced crack closure was confirmed based on experimental measurement of fatigue crack propagation in low humidity air. Finally, the effect of a reduced threshold value in low-humidity air on residual fatigue lifetime of railway axles is demonstrated.