抄録
The purpose of this study is to demonstrate the plain strain fracture toughness (KIC) test method by using the small round bar specimen with a circumferential notch. Specimens with different notch diameter of 1.5,2.0 and 2.5 mm were prepared. In this study, the two types heat treated steel, normalized and quenching/tempering (QT) were prepared. These specimens were tested under tensile load at room temperature and fracture surface was observed by SEM. Distributions of stress near notch tip were calculated by FEM. Test results showed that the K_<IC> value obtained by the specimen made by normalized steel were invalid due to its plastic deformation until fracture. On the other hand, when QT specimen was used, the obtained K_<IC> value were allowable. Calculated stress distribution near the notch tip showed that the small scale yielding condition was satisfied when QT specimen was subjected to tensile load. The SEM observation also suggested that the uniform ductile fracture surface was appeared at QT specimen. It is suggested that valid K_<IC> value would be obtained by using the small round bar specimen with a circumferential notch when the appropriate hardness and notch diameter.
