高圧小丸棒引張試験を用いた590MPa級高張力鋼板の延性破壊挙動の解析

抄録

This study focuses on ductile fracture in a low-stress triaxiality. Fracture in a low-stress triaxiality does not occur in a common tensile test, because necking induces a high-stress triaxiality. Therefore, for fracture strain evaluations in a low-stress triaxiality, a variety of sheet-type specimens are employed, so that their shapes induce a low-stress triaxiality on the ductile fracture site. Such analyses provided fracture strains in a low-stress triaxiality through numerical simulations, showing irregular inflection points in the fracture strain locus. However, these irregular inflection points are not consistent with the fracture strain locus of bulk metals evaluated in a Bridgman's round bar tensile test, i.e., tensile test in a high-pressure fluid. Therefore, the validity of fracture strain measurements with a low-stress triaxiality must be discussed. The compact tensile machine developed in Osaka university enables tensile testing of a small round bar under hydrostatic pressures of up to 700 MPa. We used this machine in this study. Fracture strains under hydrostatic pressures of 200 MPa and 400 MPa were evaluated in two types of 590 MPa-class steel sheets with 1.2 mm thickness. The small round bar specimens with minimum diameters of 1.0 mm were cut from these thin steel sheets. These tensile tests revealed a monotonic increase of fracture strains with respect to the imposed hydrostatic pressures. The results are consistent with those in Bridgman's tensile tests. Conversely, they are not consistent with the fracture strains evaluated by using sheettype specimens.