According to the Federal Railroad Administration (FRA), between 2013 and 2015, accidents associated with wheel failures represented 11% of all equipment-caused accidents, a figure based on November 2016 data from FRA’s Office of Railroad Safety. “Although rare, broken wheel derailments tend to be more catastrophic than other derailment types due to sudden fracture, which can occur at revenue train speeds,” FRA said. Coupled with increased movement of hazardous material (including crude oil), this scenario poses an increasing risk to public and railroad safety.”
FRA’s report, Wheel Failure Investigation Program, Phase 3 (download below), summarizes the results of the third phase of a multi-phase study to determine the underlying mechanisms for catastrophic wheel failures, such as shattered rims and vertical split rims (VSRs), and potential solutions and strategies to minimize derailments due to these types of failures. FRA contracted with ENSCO, Inc. (now contract operator of the USDOT’s Transportation Technology Center, Pueblo, Colo.) to lead the study, with significant contributions made by Steven Dedmon and SimuTech Group, as well as an industry stakeholder working group (SWG) with members from the Association of American Railroads (AAR), car owners, wheel suppliers and researchers.
The FRA Office of Railroad Safety (RRS) was tasked with arriving at a better understanding of VSRs and other wheel failure modes. Objectives include improved insight into failure causes, development of detection and prevention methods, and establishment of approaches to minimize wheel failure-related derailments.
“Past studies and research supported by FRA have focused on developing an understanding of wheel failures from various perspectives,” FRA noted. “A definitive study to determine the underlying mechanism(s) for wheel failures, such as shattered and vertical split rims, and potential solution(s) and strategies to minimize derailments due to these types of wheel failures was needed. In response to a request from RRS, FRA’s Office of Research, Development & Technology (RD&T) established a multi-phase research program in February 2016 to develop a comprehensive understanding of various wheel failure mechanisms, identify major contributing factors to these failures, arrive at potential strategies to mitigate them, and consequently improve safety of rail network operations.” This program included the following phases:
- Phase 1: Problem Definition and Size Analysis.
- Phase 2: Review and Analysis of Tests and Analytic Studies on Investigation of Wheel Failure Mechanisms.
- Phase 3: Modeling and Analysis of Underlying Wheel Failure Mechanisms and Failure Prevention and Mitigation Strategies.
“Due to the complicated nature and multi-cause origin of VSR failures, Phase 3 focused on the investigation of its contributing factors through Finite Element Analysis (FEA),” FRA said. “This included analysis of wheel failure mechanisms, recommendations for improvements to industry data collection efforts, and identification of potential strategies to mitigate the failures, consequently improving rail network operations safety and reducing risks.”
The research team drew the following conclusions from Phase 3 and prior research activities:
- “The likelihood of a VSR increases with increased brake heating. Without brake heating the probability of a VSR is diminished.
- “In order of importance, the contributing factors to the development of a VSR are brake heating, subsurface crack size, and distance from the tread contact point with the rail to the subsurface crack tip.
- “A large percentage of subsurface cracks will break out to the tread, creating an out-of-round condition and high impact readings from wayside detectors. Those which turn away from the tread (and generate a VSR) need high thermal gradients to generate the thermal stresses necessary to drive crack growth.
- “Manufacturing residual compressive stresses prevents the subsurface crack from breaking out to the front rim face, instead forcing a crack path to the rim inner diameter.
- “Wheel-rail contact loads cause crack initiation (likely in the presence of a material anomaly) and propagation. The larger the crack gets, the less important wheel-rail contact becomes, since the distance to the crack tip gets larger and stresses decrease.
- “Unstable crack growth occurs when the crack tip progresses away from the tread surface and into a tensile residual axial stress pool resulting from heat treatment.”
