Fatigue cracking and stresses in yokes in heavy haul service

Yoke failures are a significant issue in heavy haul rail systems, and can cause delays in operations. Failures often occur as fatigue cracks at the rear radius location, with the majority of fatigue cracking on either the upper or the lower of these radius locations in the yoke. Such cracks can propagate through the entire section, with minimal if any fatigue cracks developing at the other rear radius. When combined with strain data collected during normal service conditions using IRT's instrumented ore cars, it is evident that considerable vertically oriented bending may be present in the draft system, in addition to the nominal tensile loads in these yokes. The bending in yokes is most likely due to misalignment in the draft system, which can lead to high bending stresses on either the top or the bottom rear radius locations in the yokes. Such misalignment is important to identify, as it could affect the overall service life of yokes. Based on these data, heavy haul operators need to understand and control bending loads to manage fatigue cracking and failures in yokes.