Abstract
The mechanism of the initiation of fish eye failure was examined by stereography of the fracture surface of SCr 420 carburized steel tested under rotating bending. Three modes of fish eye nucleation were observed: mode I crack initiation from nonmetallic inclusion (ni), mode II crack initiation from ni followed by the transition to mode I crack, and mode II crack initiation at the martensitic lath interface.
The mode II crack plane was close to or less inclined than the plane of maximum shear stress. The maximum length of mode II crack was longer when the local stress at the origin was lower. The transition from mode II to mode I occurred at the threshold mode I stress intensity factor, KIth, ≅3MPam1/2. The threshold mode II stress intensity factor for the nucleation was estimated as KIIth≅1.5MPam1/2.
