Failure analysis of automatic coupler SA-3 in railway carriages

In the present work, failure analysis of an automatic coupler in railway carriages which failed in service and caused derailment, was carried out. An evaluation of the failed coupler was undertaken to assess its integrity, including visual examination, photo documentation, fractography, chemical analysis, hardness measurement, tensile and charpy impact testing and microstructural characterization. Results show that some casting defects (e.g. mis-run, cold lap, hot crack and inclusion) resulted in poor mechanical properties. Mis-runs were also responsible for fatigue crack nucleation from two different locations. Beach marks can be observed on the fracture surface. According to the results of fracture mechanics, the load of final fracture was not higher than the standard value. In other words, the fracture was not due to employing improper mechanical loads but mostly because of casting defects.