Observation of the initial process of internal fracture in very high cycle fatigue in Ti-6Al-4V by synchrotron radiation μCT imaging

Abstract

The initiation and growth of internal small fatigue cracks with around ten or several dozen μm in Ti-6Al-4V were nondestructively examined by using synchrotron radiation μCT at the large synchrotron radiation facility SPring-8. Lots of grain-sized internal cracks were observed roughly evenly in the observation volume in the specimen; in contrast, only one surface crack was detected. The initiation lives of the internal cracks were widely different for each crack and had no significant correlation with the crack initiation site nor the initial crack size. The internal cracks propagated microstructure-sensitively with several crack deflections, and the growth rates were very small, less than 10-10 m/cycle. The crack growth rates just after facet formations showed large variability and had no apparent relationship with the crack initiation life nor the initial crack size. This variability can likely be attributed to microstructural inhomogeneities around the crack initiation facets. The estimated facet formation rate indicated that most facets formed rapidly compared with the following internal crack growth rate.

Keywords

• gigacycle fatigue
• fatigue crack initiation
• fatigue crack growth
• non-destructive inspection
• synchrotron radiation