(9) 放射光X線CTと真空疲労試験による内部起点型疲労破壊の評価

抄録

In recent years, it has become widely recognized that high-strength materials, such as high-strength steels and titanium alloys, can undergo fatigue failure originating from internal sites at stress levels lower than those expected from their static strength. This review introduces an evaluation methodology combining synchrotron radiation X-ray computed tomography（CT）and vacuum fatigue testing to clarify the mechanisms of internalinitiated fatigue fracture. The former technique enables the elucidation of the initiation and propagation behavior of small internal cracks, while the latter simulates the environment surrounding internal cracks that is not exposed to the atmosphere. The proposed approach was applied to two titanium alloys,（α+β）-type Ti–6Al–4V and β-type Ti–22V–4Al. The results demonstrate that, although both are classified as titanium alloys, the initiation sites, propagation paths, and growth rates of small internal cracks differ significantly between them. One of the primary factors responsible for these differences is considered to be the unique characteristics of internal cracks propagating in environments not exposed to air （i.e., high to ultra-high vacuum conditions）． Furthermore, the sensitivity of internal crack growth to vacuum environments may vary depending on the material, particularly in terms of adhesion and cohesive interactions at the crack tip and fracture surfaces. To properly evaluate internal-initiated fatigue fracture in metallic materials and to develop effective mitigation strategies, it is essential to incorporate interdisciplinary insights from adhesion and bonding science.