Abstract
Rotating bending fatigue tests were carried out, using the specimens cut out from a rolled steel with remarkable laminated structures in three different directions-rolling, thickness and 45°. The fatigue process on the specimen surface was observed successively by an optical microscope, and then the fracture surface of the same position was observed by a scanning electron microscope.
The main results obtained are as follows:
(1) The anisotropy of the fracture life is controlled by the difference in the process of crack initiation and propagation up to about 0.5mm.
(2) Cracks in the rolling directional specimen originate at grain boundaries or the inside of grains in ferrite layers, while those in the thickness directional specimen originate at boundaries or near-boundaries between ferrite and pearlite in most cases, and the initial fatigue fracture surfaces are almost parallel to the maximum shearing stress plane.
(3) The origin of fracture in the material with a large grain size is not so clear as in the case of the material with a small grain size.
