Vertical Split Rim Research Articles

Failure analysis of a heavy-haul train wheel

The failure analysis of a fractured wheel from a train travelling in a heavy-haul line was carried out. The microstructure of the wheel was studied by Light Optical Microscopy (LOM) and the chemical composition was analyzed by Optical Emission Spectrometry (OES). Tensile tests were performed in samples extracted from the fractured wheel to evaluate mechanical properties. The results from the material’s characterization combined with the fractographic examination and the stress analysis by finite-element modeling (FEM) allowed to conclude that the wheel failed due to fatigue originated in a heavily deformed surface layer produced by rolling contact fatigue (RCF), which was not timely removed in maintenance interventions. The main crack propagated from the surface to the rim and caused a failure known as Vertical Split Rim (VSR).

Effect of fracture toughness on vertical split rim failure in railway wheels

Vertical split rim (VSR) cracks are the most common cause for the removal of broken or cracked heavy haul car wheels in North America. This failure mode is characterized by rapid, unstable crack propagation in the hoop or in the tangential direction. To investigate the methods to prevent VSRs, an understanding of how the service loads correlate to their occurrence is necessary. In this study, a finite element analysis is conducted to evaluate the rim stresses below the tread surface during rolling contact between the wheel tread and the rail. Then, the stress intensity factor is calculated using the stress distribution in the wheel rim obtained using the finite element analysis. The results of the analysis show that axial residual tensile stress, which contributes to unstable vertical crack propagation, increases at deeper positions below the tread surface owing to cyclic rolling contact. These results indicate that a vertical crack can propagate rapidly below the tread surface. The fracture toughness of the wheel steel is measured using Charpy impact tests. The results of the test show that Weibull distribution enables the approximation of the relationship between fracture probability and fracture toughness. To evaluate the effect of the fracture toughness of wheel steel on the differences in VSR crack probability, a comparison between high fracture toughness wheels and conventional Class-C wheels is made. The VSR rate can be predicted from the stress intensity factor and Weibull equation for fracture toughness. The prediction results show that the VSR rate of high fracture toughness wheels is only 5% of that of conventional Class-C wheels. Therefore, high fracture toughness steel wheels are considered to have a higher resistance to VSR cracks compared with conventional Class-C wheels.

Failure Mode Changes with Successive Component Improvements

The relatively recent appearance of formerly rare vertical split rim wheel failures provides a good case study of how a shift in failure mode occurs when a material improvement is made to solve an existing problem. The mode of railroad wheel failures have changed three times over the past 40 years when the dominate failure mode has been identified and corrected. When the principal failure mode has been corrected the service life increases, but a new or previously minor failure mode assumes the role of principle failure cause. Because of the large population of railroad wheels in North American service, failure trends can be easily observed. This paper will discuss the evolution of failure mode with successive material and structural improvements.

Modeling of vertical split rim cracking in railroad wheels

Vertical split rim cracking, due to rapid unstable propagation of a shallow sub-surface crack parallel to the front rim face, is one of the dominant mechanisms of railroad wheel failure. Wheel impact load is believed to be a trigger for this unstable crack growth. This rapid crack growth rate depends on several factors, such as wheel geometry (wheel diameter and rim thickness), load magnitude, load location, residual stresses in the rim, worn tread profile, and material defects in the rim (size, shape, location, and orientation). This paper develops a computational methodology to investigate the effect of these parameters on vertical split rim cracking, using finite element analysis and fracture mechanics. Vertical split rim cracking is modeled using a three-dimensional, multi-resolution, elastic–plastic finite element analysis. Material defects are modeled as mathematically sharp cracks. Wheel impacts are simulated by applying high mechanical loads on the tread surface. The residual stresses and wheel wear effects are also included in modeling vertical split rim cracking. The proposed computational methodology can help to predict whether a vertical split rim failure might be triggered for a given set of parameters, such as load magnitude, load location, wheel diameter, rim thickness, residual stress state, crack size, crack location, and crack orientation.