Soutenance de thèse : Louis HÉBRARD

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

Doctorant : Louis HÉBRARD

Laboratoire INSA : MatEiS

Ecole doctorale : ED 34 : Matériaux de Lyon

In Very High Cycle Fatigue regime (fatigue lives longer than $10^8$ cycles), the fatigue cracks tend to initiate internally. This makes the visual detection impossible during maintenance operations. Consequently, engineers need robust fatigue datasets to prevent internal crack initiation to lead to the fracture of a critical part. For that purpose, 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 internally notched specimen in order to collect fatigue datasets. More preciselly, the crack shape of internal fatigue cracks has been monitored in 3D at different instant of their propagation up to the Very High Cycle Fatigue regime. Internal notches have also been linked to the specimen surface and so to the ambient atmosphere in order to study the influence of environment on internal fatigue cracks. This expermental procedure allowed to acquire for the first time, in-situ observations of internal fatigue cracks propagating with and without air in a Ti64 alloy. In this context, 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 crack growth rates.