Back Rim Face Wear Damage and the Conditions that Can Lead to Failure in Feight Car Wheels

Back rim face wear damage from special trackwork such as guard rails can result in localized heating and a thin, surface layer of martensite. Cracks usually initiate in the martensite layer and propagate where local tensile stresses exist from localized heat input and phase transformations. These small cracks usually terminate when they reach the martensite/heat-affected zone/base metal interface, or propagate nearly parallel in or near this interface. Even though approximately 30% of wheels on cars inspected exhibit some form of this wear damage, there is a very low failure rate caused by this condition. This illustrates that wear damage alone is not sufficient to cause fatigue crack growth that leads to a critical fracture condition. However, when back rim face damage occurs in combination with a sufficiently high thermally-induced circumferential tensile stress from the high-thermal input of a brake problem, fatigue crack growth can lead to catastrophic wheel failure. The metallographic and residual stress results presented show that back rim face damage is cause for concern but is not sufficient alone to cause wheel failure.