Fatigue crack initiation in rail steels.

摘要

Specimens were machined from the rail heads transverse to the rolling direction. The specimens were tested in the as received condition and in a condition with a shot peened surface. The specimens were fatigued in a four point bending configuration using two different loading programs until crack initiation occurred. Crack initiation was detected using acoustic emission techniques. The specimens were examined using optical and scanning electron microscopy to determine the crack initiation mechanism.; The fatigue performance of each steel was determined by the nature of the elongated inclusions. The crack initiation mechanism was found to be a function of these inclusions. Differences in the initiation mechanism determined the fatigue resistance of each steel. The presence of brittle, easily damaged, inclusions was most detrimental to the fatigue performance of the rails. Inclusions which were ductile and resisted becoming fatigue damaged were less harmful. Elongated inclusions that were ductile and had a strong cohesive strength were the least damaging to the fatigue performance.; A head hardened steel with the finest pearlitic microstructure had the poorest fatigue performance because of the brittle inclusions present. By contrast the steel with the coarsest pearlitic microstructure and the largest inclusion content had the best overall fatigue performance.; After shot peening the fatigue performance of all the steels is closely matched, except for the head hardened rail. The greatest increase in the fatigue performance occurs at low stress levels close to the endurance limit. (Abstract shortened by UMI.)