Abstract
In order to investigate the characteristics of fracture stress curve of steel by notched bar tension specimen, we must clarify first the range of notch shapes which would cause an initial crack not at the contour, but at the center of notched section. Authors carried out extensive experiments and obtained some results, which are summarized as follows:
(1) Initiation of center crack of fibrous type could be observed after sectioning longitudinally the specimens which were strained and unloaded just before their fracture by means of very careful and minute adjustment of load, using a rigid and small tension testing machine of screw type (capacity, 5tons) (cf. Photos. 1-4).
(2) It was proved that “rim effect” could be recognized conspicuously on the flow stress curve of the specimen in which the center crack was initiated. By observing this rim effect, the range of notch shapes that would produce center crack was determined in detail as shown in Fig. 3.
(3) It was also proved that no contour crack was found in the case where center crack would initiate and that this phenomenon might also be observed in the reverse way. Moreover, we observed that contour crack grew slowly in comparatively a long period and was quite stable, and that, on the contrary, center crack was very unstable and accordingly the specimen broke just after its initiation. The fracture mode was shear at the contour in almost all specimens and fibrous at the center.
(4) It was presumed that micro metallurgical defects of steel might influence strongly on the initiation of ductile crack.
