Abstract
The torsional fatigue test were carried out with specially prepared mild steels containing silicate inclusions.
In order to investigate the initiation and propagation of fatigue cracks, the testing machine was stopped at intervals after additional cycling, and microphotographs were taken in the same area on the surface of the specimen.
The results obtained are as follows:
(1) The average ratio of torsional fatigue limits to rotating fatigue limits obtained at previous report is about 0.7.
(2) The first cracks are formed within the first 5per cent of the lifetime. As is known, the fatigue cracks initiate at an earlier stage of the life to fracture.
(3) The initial cracks are very often started from the surface at which no defects such as inclusions or cavities produced by the crumbling of these inclusions are microscopically found.
It is thought that most of the cracks appeared on the surface of the specimen started from inclusions which are just below the surface.
(4) The initial cracks mostly appear in the axial direction of the specimen.
