Abstract
Fully annealed commercial base pure iron plate specimens were tested under completely reversed bending stress at 60Hz. The crack nucleation and its early growth were observed until about 80% of the fracture life at a stress level just above the endurance limit. Then the specimen was removed from the machine and the grain orientation was determined for grains surrounding cracks by the etch pit method. Observation was also made after removal of surface layers to see the crack growth pattern toward the inside from the surface. It was found that cracks originated at grain boundaries on the surface penetrated into the inside along two or three dominant slip systems in one of the grains adjacent at the cracked boundary in the same manner as the pencil glide which was commonly observed in b.c.c. metals.
