Special Issue Paper
(Received 11 April 2016; accepted 14 July 2016)
Published Online: 22 December 2016
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Cite this document
Knuckle connectors on railcars are required to sustain thousands of cyclic stress events. Manufacturers continue to develop, test, and improve the life of these castings, which are now designed to exceed 400,000 cycles when subjected to tests involving cumulative cyclic sinusoidal load range settings varying between 6800 and 127,000 kgf at room temperature. This paper addressed real-world difficulties of differentiating between component failures due to overload and casting defects, and those that result from fatigue damage. Fatigue fractures in castings are rarely planar and comprise areas where solidification kinetics are different, and locations where porosity or large inclusions are present. The cumulative effect these factors have on fracture is to generate a rough topography, which may exhibit no beachmarks or other outward sign by which a fatigue fracture is commonly recognized. In this paper, an advantage was gained over a field failure because the fracture was produced in a laboratory test. Cycles to failure was recorded and details of the applied cyclic stress were also known. The difficulty regarding scanning electron microscope (SEM) analysis was that classic fatigue features normally visible to the naked eye, such as beachmarks, thumbnail initiation sites and planar areas, were not evident on the fracture face. In addition to documentation of numerous defect features on the fracture surface, SEM analysis revealed a rare but proof-positive form of evidence for propagation of the early stages of fracture by fatigue. The feature has been described in the literature as “tire tracks,” alluding to their similarity to ATV tire impressions or Lunar Rover tracks on the moon. A brief review of earlier literature examples of tire track features is provided, and the generally accepted mechanism for their formation is assessed.
MCS Associates, Inc. and Product Evaluation Systems, Inc., Latrobe, PA
Holp, C. R.
FirstEnergy Nuclear Operating Co., Beta Lab, Mayfield, OH
Moorhead, W. J.
Product Evaluation Systems, Inc., Latrobe, PA
Stock #: MPC20160047