Factors Affecting Strength in Surface and Sub-Surface Fracture of Ti-6Al-4V Alloy

Abstract

To reveal influence factors on the very high cycle fatigue properties of Ti-6Al-4V alloy, uni-axial fatigue tests and fatigue crack propagation tests were conducted. In the uni-axial fatigue tests, specimens with artificial defects of several sizes were used. Both tests were performed in air and vacuum environments. The effects of environments around fatigue cracks on crack propagation properties were investigated. As a results, fatigue crack propagation rates in vacuum were lower than those in air, and the results correspond to the fact that the sub-surface fractures occur in longer life regime than surface fractures. The threshold stress intensity factor range showed size dependencies, in addition, the difference of environments affected the size dependencies. The dependence in vacuum was stronger than in air. The result indicates that a small crack (size of a crack origin) can propagate in lower stress intensity factor ranges in vacuum, and this can relate to the reason why the sub-surface fractures occur in lower stress levels than the surface fractures. Fracture surface observations revealed that fine convex-concave granular pattern (Granular region) was formed around the origins of surface fractures in the vacuum environment, and the aspect resembled the fracture surface of sub-surface fractures. The result showed that the crack propagation mechanism in vacuum and sub-surface of materials were similar. The effects of vacuum environment are almost same as those of environment around the sub-surface crack. The effects of environments around fatigue cracks can well explain the very high cycle fatigue properties of the material.