Design review of a freight railway axle: fatigue damage versus damage tolerance

Railway axles are designed for an infinite life with admissible stress levels which correspond to generous safety factors applied to full-scale fatigue properties of materials. Nevertheless, in order to keep an adequate safety level for such a long-lasting component (an axle can typically run for 36106 km) subjected to surface deterioration or corrosion, the design is complemented by “damage tolerance” analyses, in which it is assumed that a flaw could grow under service loads, in order to define an appropriate inspection plan. The ultimate “damage tolerance” approach is to design an axle so that there is no need for periodic NDT inspections except those carried out at overhauls (the so called “one million miles axle”). The aim of this paper is to describe the application of this concept to the axle of a freight train comparing this new design concept with the traditional fatigue design.