DESIGN REVIEW OF A FREIGHT AXLE: ACHIEVEMENT OF A MILLION MILES AXLE

摘要

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 [1]. Nevertheless, some failure occurs due to corrosion, ballast impacts [2-3]. In order to avoid these failures, which are related to a number of factors not easily predictable, a suitable inspection strategy is supported by the "damage tolerance" analyses. For the inspection plan the choices to be made are: i) the inspection periodicity; ii) the NDT inspection method. Failure probability for crack propagation depends on the stress spectrum and the NDT method [4] and the determination of best inspection periodicity is based on the compromise between costs (for a failure and the inspections) and failure probability [5]. One of the aims of the WIDEM (EU supported project) [6] was to develop new design methods based on these concepts. WIDEM activity has shown that another feasible approach to the problems of long lasting axles, exposed to different damages able to trigger fatigue failure, is to design the axle so that service stresses are low enough to decrease the risk of fatigue crack growth from accidental defects: if the crack growth is slow, there is the possibility to avoid periodic NDT inspections and only to rely on the integrity inspections which are carried out at overhauls (the so called "million mile axle") [7]. The aim of this paper is to describe the application of this concept to the axle of a freight train.