Environment effect on internal fatigue crack propagation studied with in-situ X-ray microtomography

In the present study, physically short fatigue cracks initiated from an artificial internal notch under vacuum conditions are observed in smooth specimens made of titanium alloy. The crack initiated from such a notch is not in contact with the surface during the majority of its propagation. Another batch of artificially notched specimens in the same alloy having their internal notch linked with the surface to bring air to the initiated short crack were tested too. All the specimens were ultrasonically cycled with synchrotron micro-computed tomography acquisitions which were regularly acquired from crack initiation detection to final failure without demounting the specimen. The number of cycles to failure, the internal crack growth rate and the crack growth mechanisms of internal cracks are compared for propagation under either air conditions or in the core of the alloy (i.e. under vacuum environment). It is shown that the environment plays a key role in the explanation of the very low crack growth rate of internal short crack loaded at very low stress amplitude leading to the gigacycle regime.