2014 年 2014 巻 59 号 p. 19-28
It has been clarified that surface fracture and fracture accompanying a fish-eye on its fracture surface occurred under short and long fatigue life regions, respectively, in the stepwise S-N curve of an austenitic stainless steel obtained at 700℃. In order to clarify the effect of oxidation on fracture morphology, fracture surfaces obtained already for SUS321-B steel were examined in detail by EPMA. After a small fish-eye, of which origin was located at the subsurface of the specimen, had reached the specimen surface, two types of fracture processes occurred. One type was that the fish-eye became a small surface crack and propagated as a StageⅡ crack. The other type was that fracture surfaces of the fish-eye were covered with oxides and the fish-eye became a non-propagating occluded crack. In the latter case, initiation of a new crack at the specimen surface was induced by the occluded crack and then fracture occurred. Oxidized semi-circular area was formed near the fish-eye in both types, which was brought by behavior of oxide-induced crack closure. After a large fish-eye, of which origin was located at the interior of the specimen, had reached the specimen surface, the fish-eye became a large surface crack and propagated as a StageⅡ crack. In this case, oxidized semi-circular area was not formed near the fish-eye. The depth of fish-eye origin from the specimen surface and stress intensity factor at the tip of fish-eye when it reached the specimen surface were pointed out as factors controlling which type of fracture processes occurred.
