抄録
Induction hardened axles have been used since the start of the Shinkansen (Japanese Bullet train) service in 1964. Axles are subjected to cyclic loading, and induction hardened axles are used under cyclic compressive stress conditions because of the large compressive stress state caused by induction hardening along the axle surface. Japanese Railways regularly inspect its axles, and if any crack larger than 0.15mm in depth is found, the axle is taken out of service. The compressive stress is around -500MPa, and magnetic particle inspection is conducted at intervals of 2.2×10^8 and 4.4×10^8 rotation. The issue of whether or not a crack more than 0.15mm in depth will propagate under cyclic large compressive stress conditions is of great importance from the viewpoint of railroad safety. From the above mentioned background, an experiment of fatigue crack extension under a cyclic large compressive stress condition was conducted. The result shows that the crack did extend in the range of around 3.6〜5.2×10^8 cycles under the condition of -500〜0MPa cycle. An elastic perfectly-plastic stress analysis was also conducted, and shows 89MPa tensile stress origination at the crack tip when unloading. This can explain such a crack extension. Whereas, the analysis of -560〜-440MPa cycle for actual maximum stress in the field shows compressive when unloading.
