Rotating bending fatigue tests were made using the specimens cut out from a rolled steel for structures in rolling and thickness directions. The fatigue processes on specimen surface were observed successively by an optical microscope, and then the fracture surfaces of the same place were observed by a scanning electron microscope.
The main results obtained are as follows:
(1) It was clarified that the shape of an inclusion in this material was a circular-plate type. The boundary between the inclusion and the matrix was smooth when observed from the thickness direction.
(2) Circular-plate type inclusions in the thickness directional specimen could be regarded as cracks; that is, the total life of the thickness directional specimen was almost equal to the life of crack propagation. This is the main cause of the anisotropy of fracture life, that is, the difference in life between the rolling and thickness directions.
(3) The fatigue cracks in the rolling directional specimen originated along grain boundaries in most cases and the fracture surface at the boundaries was very smooth.
