Issue 47

T. Kawabata et alii, Frattura ed Integrità Strutturale, 47 (2019) 416-424; DOI: 10.3221/IGF-ESIS.47.32 418 propagation behaviour of brittle fracture could be observed by a high speed camera [15]. Normalizing was conducted at 1300°C to obtain a coarse grain size of 4-5 mm. The microstructure is shown in Fig. 2. Figure 2 : Chemical composition [mass%] and microstructure of 3%Si-2%Al steel A bending test piece with a length of 120 mm, width of 40 mm, thickness of 5 mm, and electric discharge notch of 5 mm was prepared (Fig. 3(a)). A strain gauge that included 5 gauges (Fig. 3(b)) was attached to one grain in the test piece, and a three-point bending test was conducted at room temperature with a span length of 90 mm. As a result, a brittle crack occurred under a load of 22.3 kN that propagated to the middle of the test piece and arrested. Through visual observation of the side surface of the specimen, it was confirmed that the strain gauge was attached to a region close to the end of the crack propagation (Fig. 4). After testing, the specimen was divided into two by brittle fracture by applying additional load at ambient temperature. Observation of the fracture surfaces was conducted using an optical microscope, as shown in Fig. 5. As with the fracture surface of the 3% Si steel previously studied by the authors [15], numerous traces of twinning deformation were observed on the fracture surface. (a) specimen configuration (b) multiple-strain gauge Figure 3 : Test specimen configuration (Units: mm) and strain gauge used Figure 4 : Appearance of the test specimen after the bend test at ambient temperature Micromechanism of brittle crack initiation in the 3%Si-2%Al steel In the bending test, brittle fracture occurred within a grain adjacent to the notch bottom as shown in Fig. 5. Through observation of the brittle fracture triggering area, the fracture micromechanism was investigated. As the result of many C 0.001 Si 3.02 Mn 0.01 P 0.002 N 0.0017 Al 1.88 O 0.001