圧延異方性材の中・高繰返し数ねじり疲労

平滑材およびき裂材の疲労過程に及ぼす層状組織および介在物列の影響について

抄録

The effect of two metallurgical factors, i.e., inclusions and the rolling-induced ferrite-pearlite laminated structure, of SB56M carbon steel on the fatigue process of specimens with or without a slit was examined under cyclic torsional stressing. Specimens with different directions of anisotropy relative to the specimen axis were prepared. A half of them were annealed, and the rest were spheroidized in order to eliminate or decrease the effect of the laminated structure. Micro- and macroscopic observations on processes of fatigue damage were made on the surface of the specimens subjected to either low- or high-cycle fatigue stress.
Both the inclusions and the laminated structure were responsible on the micro- and macroscopic scales for anisotropic features of fatigue damage and failure, degree of which was affected by the applied stress levels, although the two factors played different roles in the fatigue process.
In the case of smooth specimens, resistance of the pearlite bands in the laminated structure to propagation and interlinking of slip-band type microcracks initiated in the ferrite bands controlled the rate of damage and the paths of crack propagation at high stress levels. At low stress amplitudes the pearlite resistance to propagation of microcracks originated at inclusions were the most influential factor in the failure process of annealed specimens, while in spheroidized specimens, in which the laminated structure was diffused to a great degree by this heat treatment, the macroscopic anisotropy in failure appearances almost disappeared.
In the case of slitted specimens, anisotropy in the crack propagation rates was observed for annealed specimens: the cause of this anisotropy may be interpreted in terms of the pearlite resistance to crack propagation. This anisotropy was almost diminished by spheroidizing treatment, although an anomaly was observed for a particular (45°) combination of the direction of the lamination and the specimen axis. The inclusions hardly affected the crack propagation process for both annealed and spheroidized specimens, except for the final stage of rapid crack propagation.