Influential Factors on the Formation of Initial Crack Propagation Region in Sub-surface Fracture of Ti-6Al-4V Alloy

To determine the influential factors on the formation of initial crack propagation region in the very high cycle fatigue (VHCF), uniaxial fatigue tests and K-decreasing tests were conducted with Ti-6Al-4V alloy. The test data indicated that the crack growth rate in vacuum was much slower than that in air, which corresponded to longer fatigue lives of sub-surface fractures. The fracture surface of sub-surface fracture and that obtained in the crack propagation test in vacuum showed similar features that are characterized by a unique granular concavo-convex microasperity. Since the granular fracture surface was never observed in surface-initiated cracks, the granular concavo-convex pattern was considered as clues to an elucidation of the initial crack propagation mechanism in the sub-surface fracture. The granular features also resembled so-called ODA (Optically Dark Area) in high strength steels. Based on this observation, a common mechanism of sub-surface crack growth regardless of materials was discussed for the VHCF phenomena.