Effect of vacuum pressure on small crack propagation in Ti-6Al-4V

Abstract

In this study, small crack propagation tests are conducted on artificially defected specimens of Ti-6Al-4V under air and vacuum pressures of 100 Pa, 10−2 Pa, 10−4 Pa, and 10−6 Pa. Fracture surfaces between these environments are compared, and the effect of vacuum pressure on the crack propagation process is investigated. The similarities between surface cracks in vacuum and internal cracks in the very high cycle regime are also discussed from the perspectives of crack propagation rate and fractographic characteristics. From the results, it was observed that there was a tendency for the crack initiation life to increase and the crack propagation rate to decrease with a decrease in the vacuum pressure, although no notable distinction was observed in air and under 100 Pa. The fractography exhibited an angular surface with sharp edges in air and under 100 Pa. However, a granular feature with rounded edges was observed below 10−2 Pa and became more pronounced as the vacuum pressure decreased. The window periods, which formed an oxidized surface coverage, were believed to account for the vacuum effect on crack propagation rate and fracture surface features. Both the crack propagation rate and fractographic features of surface cracks under 10−4–10−6 Pa resembled those of internal cracks; therefore, the presumably working vacuum environment around internal cracks was considered to be at a high vacuum level.