Fracture mechanics is a technical discipline that assumes the presence of defects in the material and studies their propagation under loading. It allows to mathematically estimate the relevance of a defect and its period of stable propagation before catastrophic failure. Crack propagation calculations are therefore an established engineering tool to ensure operational safety of components. This approach is often called “damage-tolerant design”. In the railway industry, particular attention is paid to the wheelset axle: a component that in the past has been object of catastrophic failures due to unexpected critical crack propagation. The goal of crack propagation calculations is ensuring sufficiently short non-destructive testing (NDT) intervals to detect the presence of potentially dangerous cracks before they become critical.