To study the mechanisms of fatigue crack nucleation and of growth in the early stage of fatigue crack in low carbon steel, observations were made on the surface, the longitudinal sections and the fracture surfaces of specimens using optical and scanning electron microscopes. Then, crystallographic investigations were made on the fracture surfaces by the use of each pits.
The main results obtained are as follows;
(1) The fatigue crack nucleation occurred in a few grains which existed at the surface of specimen, and the fracture surfaces were mainly made of the slip planes ({110}). The slope of fracture surface to the specimen surface corresponded to the direction of maximum shear stress in the specimen.
(2) The traces of pencil glide slips were observed on the fracture surfaces of fatigue cracks in the early stage. The slip directions of pencil glide slips corresponded to both the slip direction (<111>) in iron and the direction of maximum shear stress in the specimen.
(3) Therefore, the mechanism of fatigue crack nucleation and of growth in the early stage can be explained by the extrusion mechanism that the slip planes intrude into the inner part of specimen by repeated stresses.