Abstract
Uniaxial tension-compression fatigue tests using low-temperature-tempered Ni-Cr-Mo steel "SNCN439" were conducted over (10)^7 cycles. Based on the fractographic analysis around fracture origins and the data arranged by using stress intensity factor range, following results were obtained; (1) Even though fatigue tests were carried out until (10)^8 cycles, the fatigue limit of interior-originating fractures was not found. (2) Interior cracks grew from small defects whose initial stress intensity factor ranges ((ΔK)_<ini>) were not only higher but also lower than growth limits of surface cracks. (3) ODA (Optically Dark Area) was only observed around interior origins. ODA was formed even in a short life region where surface-originating fractures usually occurred. ODA was formed around the interior origin whose (ΔK)_<ini> was not only lower but also higher than the growth limit of the surface crack. (4) The formation of a fine bumpy pattern which was a characteristic of ODA's fracture surface was affected by cyclic load, and the fine bumpy pattern became clear as number of cyclic load increased.
