Don Eadie
Eadie Consulting
Abstract:
Rail Integrity: What Really Matters, and What Can Be Done About It
Rail break derailments are an area of focus and concern for both the railroad industry and regulators, particularly in light of several high profile accidents involving for example crude by rail. This paper takes a holistic approach to the collective knowledge in this area, as well as for suggested opportunities and priorities to reduce rail break derailments, with a focus on failures related to transverse defects. A number of different modelling approaches have been used to identify major influences on rail fracture propensity. In some cases these models are used to suggest either rail wear or wheel load limits. The rationale is that there will be some critical level of rail wear or wheel impact load when a rail with defects of a sufficient size will fracture. These various approaches use tools such as fracture mechanics and FEA. Major factors identified include wheel impact load, defect size and growth rate, rail tensile stress, rail fracture toughness, and track stiffness. The overall rail fracture process in the case of transverse (or detail) fracture can be considered as comprising four main stages: 1) Crack initiation caused by wheel/rail contact stresses 2) Crack growth influenced by contact stress and liquid inclusion 3) growth beyond the contact stress influenced by flexural stresses with cracks becoming three dimensional, and finally 4) rail fracture under the influence of high wheel impact and high rail tensile forces.
Empirical analysis of railroad operating data is reviewed to examine and validate the relative importance of these different factors. For example, the influence of cold weather conditions confirms the importance of rail tensile stress and perhaps track stiffness as major influencing factors. Further data shows the importance of wheel impact loads in determining the number of rail breaks. In two separate cases, analysis of rail wear profiles shows no clear correlation of increased wear with rail break derailments, within current typical Class 1 railroad rail wear limits.
With a combination of the modelling and real world validation, we can consider a safety management system approach towards driving down broken rail derailments. This can include improved management of all the stages of crack and defect growth:
Improved management of the wheel/rail interface will address initiation and growth of cracks near the surface. For example, enhanced rail grinding will remove cracks before they grow excessively. Management of wheel rail profiles reduces peak pressures and crack initiation, and use of top of rail friction modifiers will reduce tractive forces that lead to rail surface ratcheting.
For cracks that progress into the rail and out of the influence of wheel rail contact forces and below the work hardened zone, management of tensile rail stresses will be of particular importance, especially when considered from the point of view of seasonal temperature changes. Improvements in rail "cleanliness" may be an area that should receive renewed industry focus. Finally, tightening of wheel impact load limits can be considered.
Improvements in flaw detection should be considered through either increasing test frequency and / or managing the timing of inspections especially with respect to the times and locations of high break frequency. Improved UT technology should be pursued. In addition management of cracks in the early stage can be improved through use of new eddy current and magnetic flux measurement technologies.
Given an absence of real world data supporting more stringent rail wear limits, it is suggested that this area would benefit most from a harmonization of Class 1 railroad standards in this area, coupled with clearer definition of required actions as rail wear limits are approached.
The paper concludes by suggesting that new targets and metrics be considered which look at the frequency and geographical distribution of Rail Service Failures/mile/year, with a focus on the "tail" of this distribution at or above 0.1. Specific remedial actions should be developed to target these problem areas.