SS400炭素鋼のモードII,モードIII疲労き裂の進展挙動
2001 年 50 巻 10 号 p. 1102-1107
詳細
2001 年 50 巻 10 号 p. 1102-1107
Fatigue characteristics are of fundamental importance when choosing a structural materials, especially when deciding upon the materials for reactor structure where long-term safety is primary importance. In this field, recently, the propagation behavior of mode II and mode III fatigue cracks has attracted special interest. The mode II and mode III fatigue testings involve greater difficulties compared with the mode I testing and the testing procedure has not been established. SS400 carbon steel was fatigue tested by a torsion fatigue testing machine. In the torsion test, fatigue cracks can propagate in all modes. To identify their behavior, surfaces of the specimen were observed by CCD camera and the behavior of surface cracks were recorded in a time-lapse video recorder which can record up to for 40 days. To reveal the morphology of the mode II and mode III crack propagation, some of test interrupted specimens were sliced and observed by a two dimension measuring microscope.
To investigate the crack morphology three dimension crack demonstrate computer program was developed. In the torsion test of plain surface specimen the mode II crack initiates first, but it transits to mode I in the early stage. However, according to the 3D view a large mode III crack propagates into inside in this stage. Persistent slip band like patterns were observed on the surface and the cross section of the fatigue cracked specimen and all mode III cracks propagate in the band. Some of them grew across the center of the specimen. The da/dN-ΔK diagram of the surface mode II fatigue crack propagation was compared with that of mode I by CT type specimen of the same material. mode II propagation rate agreed with mode I propagation rate where the stress intensity factor range was converted as ΔKI=2ΔKII. It means that to design the structure by mode I crack propagation data alone may be acceptable under above conversion. To establish this relationship and to elucidate the behavior of mode III fatigue cracking, more investigations are required.
