Internal fatigue crack propagation in a Ti-6Al-4V alloy: An in situ study

Fatigue specimens of a Ti-6Al-4V alloy containing internal artificial defects with controlled and reproducible size and shape have been produced. These defects systematically led to the initiation of a fatigue crack which propagation has been monitored in situ by synchrotron X-ray tomography during R=0.1 uniaxial fatigue tests at 20 Hz. The crack growth curves of the internal cracks have been obtained for 6 samples. Ex situ fatigue tests have been performed on samples submitted to a supplementary heat treatment or containing a defect put into contact with air. The results obtained tend to support the fact that internal fatigue cracks grow from the notch in a vacuum environment. On the fracture surfaces of samples containing an artificial defect not connected to air, two regions have been observed. They correspond to the Rough Area and the Fish Eye regions observed for internal cracks initiated from natural defects. The transition between those two regions takes place when the plastic radius size is equivalent to the grain size.