Initiation and growth behaviour of small internal fatigue cracks in Ti-6Al-4V via synchrotron radiation microcomputed tomography

Small internal fatigue cracks initiated in Ti-6Al-4V in the very high cycle regime were detected by synchrotron radiation microcomputed tomography (SR-μCT) at SPring-8 in Japan. The initiation and growth behaviours of the cracks were nondestructively observed, and the da/dN-ΔK relationship was measured and compared with that obtained in a high vacuum environment. SR-μCT revealed that more than 20 cracks were initiated in one specimen. The crack initiation life varied widely from 20% to 70% of the average fatigue life and had little influence on the growth behaviour that followed. The initiation site size of each internal crack detected in one specimen was comparable with the size of the fracture origins obtained in ordinary fatigue tests. These results suggest that the surrounding microstructures around the initiation site are likely a dominant factor on the internal fracture rather than the potential initiation site itself. The internal crack growth rates were lower than 10⁻¹⁰ m/cycle, and extremely slow rates ranging from 10⁻¹³ to 10⁻¹¹ m/cycle were measured in a lower ΔK regime below 5 MPa√m. The internal crack growth rate closely matched that of surface cracks in a high vacuum, and the reason for the very long life of internal fatigue fractures was believed to result from the vacuum-like environment inside the internal cracks.