Evaluation of fatigue-crack initiation at inclusions in fully pearlitic steels

The effect of inclusions in rail steels on fatigue-crack initiation was investigated. A comparative study of fatigue-crack initiation in four-point bending specimens obtained from the rail head perpendicular to the rail direction was carried out. Four rail steels with different properties and inclusion type and distribution were considered. Acoustic emission was used to detect the onset of crack initiation during fatigue testing. Following testing, detailed microstructural observation and composition analysis were carried out by scanning electron microscopy to characterize the crack-initiation sites. The results reveal that crack initiation occurs at elongated inclusions. It was observed that there are three different mechanisms for initiation of fatigue cracks. They are: (1) microcrack initiation in a local deformation band ahead of the inclusion, (2) interfacial surface decoherence, and (3) the inclusion breaks and acts as a crack. The difference in the fatigue properties among the four rail steels was explained by these three fatigue-crack initiation mechanisms.