Failure Analysis of the Semi-permanent Coupler Used in Metro Rail

Abstract

Failure analysis of the semi-permanent coupler which failed in service of Metro rail was conducted in order to know the reason for failure. The examinations include visual, photo detection, chemical analysis, hardness measurements, tensile and v-notch impact test, microstructure and SEM observation of fracture surface. Chemical analysis and tensile test shows that the coupler material can be classified as grade B cast steel in AAR M201 steel casting specification. Although the microstructure of the coupler material shows incomplete homogenization indicating the improper conduction of heat treatment, the v-notch impact results is accepted according to this specification. The fracture surface shows the existence of much porosity at the internal surface of the hollow like pipe- coupler and the fracture started at these sites showing beach marks. SEM observation of the fracture surface shows fatigue striations indicating the occurrence of fatigue started at the porosity defects. The cast defects act as a stress concentration site for fatigue crack initiation. Crack occurred, propagated, and connected the defects to each other. The occurrence of collision resulted in bending of the female part of the coupler (plastic deformation occurs) at the lower thin wall (∼ 5.41 mm thickness). Moreover, opening of the previously formed fatigue crack and accelerating the failure were occurred. Recommendations were explained to prevent the occurrence of this failure.