微小内部き裂の発生・進展挙動に基づくα+β型チタン合金の疲労寿命分布解析

抄録

Ti-6Al-4V is a titanium alloy frequently used in many fields including aerospace industries. However, in the very high cycle regime, this alloy develops fatigue fractures from within the material even at stresses lower than those generated at the surface. To ensure long-term reliability, a fatigue life prediction method that takes internal fractures into account should be established. In this study, the initiation and propagation processes of fatigue cracks in Ti-6Al-4V were modeled based on the non-destructive observations of small internal cracks using synchrotron radiation X-ray computed tomography. The proposed model is based on the competition of surface and internal cracks in a virtual specimen. We considered the statistical properties of fatigue cracks, such as initiation life, crack size, and the growth behaviors of surface and internal cracks. To describe the initiation and propagation of fatigue cracks, a Monte Carlo simulation was conducted. The simulation results explained the trend in the fatigue life and fracture mode of the experimental data. Specifically, the effects of initiation life and origin size on fatigue life, which could not be obtained from experimental data, were investigated. As a result, the initiation life accounted for more than 90 % of the fatigue life in surface fractures, but only 15% to 70% in internal fractures. Crack initiation sites that were larger than the average α-phase became the origin of main cracks in surface fractures, whereas those that were smaller than the average α-phase were found to be those in the internal fractures. These findings were attributed to the fact that the surface and internal fractures are affected by distinct different factors. In other words, surface fractures are dominated by the crack initiation process and internal fractures were by the crack propagation process at an extremely low growth rate.