Initiation and propagation processes of internal fatigue cracks of Ti-6Al-4V in the very high cycle regime

Abstract

Observations of internal small fatigue crack growth in Ti-6Al-4V were conducted using synchrotron radiation μCT imaging at SPring-8, and the just-initiated internal cracks and their growth processes were successively observed. The internal small cracks propagated very slowly with the growth rate of less than 10^-10 m/cycle. The crack growth rate had a lot variability in the smaller crack length regime. In order to discuss the reason for the small growth rate of internal crack in terms of the effect of environment inside the crack, the internal crack growth rate was compared with the surface crack growth rate in air and high vacuum. As a result, the internal crack growth rate was lower than the surface crack growth rate in air; and matched well with that in high vacuum. The vacuum-like environment at the tip of internal crack is considered to be a reasonable cause for the long fatigue life of internal fractures in gigacycle regime.