オーステンパ処理球状黒鉛鋳鉄の高サイクル引張疲労寿命分布特性 : 長寿命域におけるフィッシュアイ形破壊の影響

抄録

We carried out uniaxial tension fatigue tests on a large number of austempered ductile iron (ADI) specimens. As a result, it was observed that the fatigue life distributions at low stress levels were divided into two groups : a short-life group below 2×10⁶ cycles and a long-life group fractured at near 1×10⁷ cycles. Weibull plots of these fatigue data showed the distinctive feature that the gradient of distribution rose at near 1×10⁷ cycles. After Scanning Electron Microscopy observation of the fracture surface, it was clarified that the short-life group was mainly characterized by stage 2b crack propagation from casting defects and that the long-life group was characterized by fish-eye-type crack propagation. Using a competing risk model composed of these two fracture modes, we could well explain the characteristic fatigue life distributions at low stress levels.