Study on the effect of the environment on internal fatigue crack propagation in Ti-6Al-4V

Overview & Synopsis

In the Very High Cycle Fatigue regime (fatigue lives longer than 1e7 cycles), fatigue cracks tend to initiate internally. This makes visual detection impossible during maintenance operations. Consequently, engineers need robust fatigue datasets to prevent such internal crack initiations that may lead to the fracture of a critical part. To obtain such data, in-situ experimental techniques are developed. In particular, micro-computed tomography allows to acquire direct images of internal fatigue cracks. In this study, a combination of in-situ micro-tomography and ultrasonic fatigue cycling has been applied to specimens that contain an internal sharp notch to study the propagation of internal fatigue cracks. More precisely, the crack shape of internal fatigue cracks has been monitored in 3D at different instants of their propagation up to the Very High Cycle Fatigue regime. Some internal notches have been linked to the specimen surface given access to the ambient atmosphere. This was done to study the influence of the environment on internal fatigue cracks. This experimental procedure allowed to acquire for the first time, in-situ observations of internal fatigue cracks propagating with and without air in a Ti64 alloy. Temperature measurements were also carried out to evaluate the effect of the environment on the plasticity at the crack tip. Finally, the fatigue data obtained at ultrasonic frequencies allowed to evaluate the influence of the cycling frequency on the crack growth rates.