Search

資料番号: AA0063716010

金沢大学高度モビリティ研究所 / 金沢大学理工研究域機械工学系, Blue brittleness is exhibited by some steels after being heated to an elevated temperature. Although at high temperature a given strain is associated with a lower stress level, at the temperature at which blue brittleness is observed there is a pronounced work-hardening effect. Furthermore, the temperature at which blue brittleness occurs is a function of the strain rate. Therefore, to improve the accuracy of numerical simulations of hot or warm working which utilizes the thermal softening of steels, a dynamic constitutive equation which can model the blue brittleness phenomenon is needed. In this paper we present such a constitutive equation, which has two parts : one models the ordinary constitutive relation and the other the phenomenon of blue brittleness. Unknown material constants in the equation are determined by a non-linear least-squares method from uniaxial impact and static compressive test results. The equation is then evaluated by comparing it to experiments over a wide temperature range.

The degree of diminution of fracture ductility varies with prestrain level. This behavior is investigated in the present study by using carbon steel and 70/30 brass. Transition of the fracture ductility appears at about a pretorsional strain of unity in the case of carbon steel. On the other hand, in the case of 70/30 brass, fracture ductility decreases smoothly and this transition behavior is not observed. However, the main cause of the loss of fracture ductility of the two materials is that the surface layer of the material becomes brittle due to the shear strain, and tensile fracture starts from surface cracks initiated by tensile deformation. The difference in diminution behavior of ductility between the two materials is thought to be due to differences in crystal structure and thus in the brittleness of the surface layer. The ductility of this surface layer is greatly reduced after the transition in the case of carbon steel., 論文

The diminution of fracture ductility of smooth specimens and small cracked specimens on surfaces subjected to torsional prestrains was investigated. For the smooth specimens, the transition of tensile-fracture ductility after a critical torsional prestrain is independent of the existence of microcracks and attributed to the embrittlement of the surface thin layers of the specimens due to the formation of anisotropic helical structures. For the cracked specimens, the degree of fracture ductility depends on the area of the crack and the magnitude of torsional prestrain. In the case where the torsional prestrain is lower than a critical value, the surface crack starts in the direction close to the maximum shear stress, and ductility remains within the specimen. If the torsional prestrain exceeds a critical value, the specimen is broken in a brittle manner, and the crack extension shows the helical pattern. This behavior is due to the embrittlement of the specimen surface layer.