抄録
The authors' method for breaking a high carbon steel rod into billets is characterized by a process in which cross-sectional cracks develop from a slight peripheral notch by rapid local heating and cooling.
In the previous paper, the residual stress distribution in the notched or cracked section of SUJ-2 notched specimen was calculated by F.E.M., based on the measured residual stress distribution of the unnotched specimen caused by rapid heating and cooling. And it was shown that the length of cracks developing in actual rod breaking tests could be precisely estimated by comparing the calculated distribution of residual stress after cooling near the crack tip in the notched section of the specimen with the true fracture strength of material. However, in this rod breaking method, crack development was found to occur during cooling.
In the present study, the change of stress distribution in the central cross-section of the unnotched specimen was examined with the lapse of time during heating and cooling in order to clarify the stress at crack initiation. Elastic-plastic FEM analysis was adopted for the specimens subjected to the heating and cooling conditions including the optimum one for breaking notched SUJ 2 φ 55 rods by a single heating process.
The results obtained are summarized as follows.
(1) The stress of specimen during heating was compressive near the surface and tensile at the core in both axial and tangential directions. During cooling, the stress at the surface layer changed to tensile, the compressive stress adjacent to the surface layer remained, and the tensile stress at the core gradually changed to compressive. The tensile stress at the surface layer of specimens under the optimum heating condition was higher in value and present in the deeper portion than those under other conditions.
(2) The period when the crack started to develop in the actual rod breaking coincided with the calculated lapse of time when the stress at the surface layer reached the highest and deepest level.
