We were asked to perform a fatigue analysis on a rail coupler, in accordance with AAR Specification M1001.
The AAR Type “F” Coupler F70DE was evaluated for fatigue loading in accordance with AAR Specification M-1001, “Design, Fabrication, and Construction of Freight Cars”. The coupler is a casting made from Grade E steel.
The maximum stresses were assumed to occur in the shaft portion of the coupler. The cross-sectional area in the shaft portion of the coupler was determined for five different sections. Due to the contours, holes, and other features of the casting, the calculation of the cross-sectional areas were approximated.
Since the shaft of the coupler includes several holes and the cross-section of the shaft varies along its length, there exist features in the coupler that result in local stress risers. Stress concentration factors were determined using Peterson’s “Stress Concentration Factors”. The cross-sectional area and corresponding stress concentration factor that produced the largest axial stress in the coupler shaft for a given load was identified and used in calculating the stresses.
Axial stresses were calculated for the maximum and minimum loads for each cycle in the force spectrum from Table 7.16 of AAR Specification M-1001. This spectrum is for a fully loaded 110-ton high-side gondola in unit train service. The corresponding stress range and alternating stress were calculated for each of the cycles in the spectrum.
A strain-life curve for Grade E steel derived from AAR M-216 test results by Transportation Technology Center, Inc. (TTCI) was used for the fatigue evaluation.
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