Potential improvements of the presently applied in-service inspection of wheelset axles

Abstract

Wheelset axles are safety relevant components because they are subject to high dynamic and static loads during operation. Therefore, they have to undergo periodic non-destructive examination to find potential defects before they can lead to failure. Minimizing the risk of fatigue failure for 3.5 million axles, potentially in service in Europe, was the objective of the 3.5 years R&D project EURAXLES which ended in April 2014. EURAXLES aimed to propose a global concept approach for axles design, production, and maintenance. An important objective within Work Package 5 was the analysis of the present status of in-service inspections. The strengths and weaknesses of the presently applied inspection systems and practices were evaluated with the aim to identify potential improvements provided by new technologies. That evaluation was firstly based on the results of a questionnaire which was distributed to the main European railway operators with the request to describe the testing procedures and equipment applied in their maintenance facilities. Secondly, the results of benchmark tests with presently applied ultrasonic inspection equipment were exploited for the analysis. These tests were performed at hollow axles and solid axles with implemented artificial defects.

Introduction

The purpose of non-destructive testing of wheelset axles is the detection of defects which are caused by static and dynamic loads during operation. These loads lead to torsional and bending stresses. Torsional stresses can cause longitudinal cracks in the axle, bending stresses might lead to transverse cracks. The maximum bending stresses occur in the center of the axle shaft. By the superposition of torsional and bending stresses critical stress peaks can occur at certain locations. Torsional stresses appear predominantly in disk-braked axles at locations between wheel seat and brake disk seat.

Typically, possible cracks propagate from the surface of the axle. Potential causes are corroded areas emanating from damages at the protective coating or notches due to ballast pick-up. Possibly, also non-metallic inclusions very close to the surface, particularly at spots of corrosion or notches can lead to crack initiation.

The EURAXLES project which was launched within the FP7 Framework Program of the European Community aimed to minimize the risk of fatigue failure of wheelset axles proposing a global concept approach for axles design, production and maintenance. Non-destructive testing was addressed in Work Package 5 of EURAXLES. An essential objective of this work package was to define optimal technologies and methods for reliable detection of critical defects in railway axles during in-service inspection. They are ranging from improvements of existing technologies and methods to the development of new ones. In this context, predictive techniques and methodologies for on-board continuous measurement of physical parameters, enabling the establishment of reliable and effective condition based maintenance, had to be considered.

Following a brief overview of the state of the art of the presently applied technology for in-service inspection of hollow axles and solid axles, the results of an analysis of the strengths and weaknesses of the presently applied inspection systems and practices are outlined. The essential shortcomings are addressed with the aim to propose improvements of the detection sensitivity and reliability of non-destructive testing.