Issue 47

T. Kawabata et alii, Frattura ed Integrità Strutturale, 47 (2019) 416-424; DOI: 10.3221/IGF-ESIS.47.32 419 studies on the brittle fracture initiation mechanism of ordinary structural steel materials [17-20], today we know that carbides cause brittle cracks. Many application studies have been adapted to these findings on the background of industrial usefulness. The history of the investigation into pure iron that does not include a second phase is older than that of ordinary structural steel materials. There are two micro-crack generation models. One is due to dislocations crossing or colliding with obstacles on the slip plane [21-23] and the other is due to twin crystal intersection [24, 25]. Fig. 6 shows SEM images of the brittle fracture trigger point in this specimen. A clear brittle fracture trigger point was observed, and the centre appeared to be the intersection of a block pattern due to twin deformation. According to Matsuda et al. [26], a twinning deformation develops on the {110} plane. Microscopic defects along the twin deformation band, which seemed to have served as initial cracks, were observed. It was confirmed that the defects were void-like and grew to a length of approximately 40 μm until they reached the critical condition of brittle fracture initiation (Fig. 7). Figure 5 : Appearance of the fracture surface Figure 6 : Observation of the crack initiation site in the bend specimen of 3%Si steel.