Features of the initiation and development of internal fatigue cracks considered on the examples of rail failure

This paper describes an attempt to study the problem of internal fatigue-crack development in a railroad rail by using a model in which the cyclic load, combined with other types of action, causes a fatigue fracture to initiate inside the metal. Previously conducted studies are reviewed that show that the initiation of cracks inside the specimen is usually observed in the presence of a strain-hardened (by rolling, surface heat treatment, chemico-thermal treatment, etc.) surface layer. Such cracks usually initiate just below the strain-hardened layer. Typical examples of transverse and longitudinal cracks are shown. The experimental work performed in this study involved subjecting rail specimens to static and cyclic loading and impact compression. Experimental results show that the white zone interlayers consist of martensite and austenite and that their structures and properties are similar to those of the white zone found in the deep layers of railhead metal. The author concludes that: (1) the formation of internal fatigue cracks conforms to the general laws of the fatigue process and the formation of internal cracks, like the formation of cracks initiated on the external specimen surface, takes place in the presence of a free surface; (2) during complex loading, the formation of the internal, slightly constrained surface is facilitated by the changes in the condition and structure of the metal due to the application of the load components and to their interaction; (3) under correct loading conditions a fatigue crack can act as a free surface for other fatigue cracks.