North American freight car axles are one of the most robust mechanical components in railway service. Axles can be in service for many years (several decades in some cases) and undergo millions of fatigue cycles under heavy axle loads. Although for rotating bending fatigue the central axis is a zero stress location, larger internal central axis discontinuities will experience finite cyclic stress levels as the radial distance increases from the central axis. Larger internal discontinuities at the axle central axis, caused by forging bursts or unhealed pipe, are not a common occurrence; however they are irregularly shaped and represent a possible source of fatigue crack initiation. This paper describes efforts to quantify stresses at central axis discontinuities of various different sizes using finite element analysis (FEA) computer simulations. Also, the paper reviews the radial ultrasonic inspection of a large group of axles and subsequent destructive testing to examine the physical size of indications found using the ultrasonic techniques. Recommendations for radial ultrasonic testing of newly manufactured and second hand axles are provided, and changes to appropriate Association of American Railroads (AAR) axle specifications are outlined.

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