Recent work using x-ray diffraction techniques has shown that the axial residual stress pattern within the railroad wheel rim is significantly different for as-manufactured AAR Class C wheels vs. AAR Class C wheels that have failed due to a vertical split rim (VSR), and non-failed AAR Class C wheels that have been operating in service. VSRs almost always begin at areas of tread damage, resulting from shelling or spalling, and cracking propagates into the rim section under load. At the locations tested, the as-manufactured wheels have a relatively “flat” axial residual stress profile, compressive but near neutral, caused by the rim quenching operation, while wheels that have been in service have a layer of high axial compressive stress at the tread surface, and a balancing zone of axial tensile stress underneath. The magnitude and direction of this tensile stress is consistent with the crack propagation of a VSR failure. When cracks from the tread surface propagate into this sub-surface axial tensile zone, a VSR can occur under sufficient additional service loading, such as loads caused by in-service wheel/rail impacts from tread damage. Further, softer Class U wheels, removed from service and tested, were found to have a balancing axial tensile stress layer that is deeper below the tread surface than that found in used Class C wheels.
This paper describes further efforts to characterize the axial residual stress present in failed VSR and used Class C wheels. Axial residual stress results are obtained near the initiation point of several VSR wheels using x-ray diffraction. Sub-surface axial residual stress patterns are also determined at points of high out-of-roundness for a group of wheels that were tested for TIR (total indicated runout) on the tread surface. Residual stress data and a photo are presented for a wheel rim slice containing a second VSR crack. Additionally, wheel rim ultrasonic testing data, collected by the wheel manufacturer when the wheels were new, are discussed for wheels that have failed due to VSRs and these data are compared to ultrasonic data for non-VSR wheels. Chemistry data are also compared. These data show that the driving force for VSRs is axial residual tensile stress, not a material cleanliness issue.