EVALUATION OF FATIGUE-CRACK INITIATION AT INCLUSIONS IN FULLY PEARLITIC STEELS

The effect of inclusions in rail steels on fatigue-crack initiation wa s investigated. A comparative study of fatigue-crack initiation in fou r-point bending specimens obtained from the rail head perpendicular to the rail direction was carried out. Four rail steels with different p roperties and inclusion type and distribution were considered. Acousti c emission was used to detect the onset of crack initiation during fat igue testing. Following testing, detailed microstructural observation and composition analysis were carried out by scanning electron microsc opy to characterize the crack-initiation sites. The results reveal tha t crack initiation occurs at elongated inclusions. It was observed tha t there are three different mechanisms for initiation of fatigue crack s. They are: (1) microcrack initiation in a local deformation band ahe ad of the inclusion, (2) interfacial surface decoherence, and (3) the inclusion breaks and acts as a crack. The difference in the fatigue pr operties among the four rail steels was explained by these three fatig ue-crack initiation mechanisms.