Filipe Da Rocha Chaves's thesis defense

This thesis focuses on the characterization of crack initiation in Ti-6Al-4V and Inconel 718 alloys under fretting fatigue conditions representative of blade/disk contacts in aircraft engines.
Conventional fretting fatigue tests are usually performed under simplified conditions—relative displacement of the contacting parts parallel to the interface, constant normal loads, and cyclic tangential loads—that differ significantly from those encountered during actual flight cycles. To improve test representativeness, a dedicated test bench was developed to conduct an experimental campaign under more complex loading conditions and at temperatures up to 800 °C.
The test rig was equipped with advanced instrumentation, including crack monitoring using the Direct Current Potential Drop (DCPD) method, calibrated to detect in-situ cracks as small as 65 µm, and in-situ displacement field measurement via Digital Image Correlation (DIC) based on Williams series expansion. Applied for the first time to a fretting contact configuration, this method enabled robust estimation of stress intensity factors directly from experimental data, which were then used as nonlocal variables in a lifetime prediction criterion.
The experimental campaign investigated fretting fatigue under constant, two-level sequential, and oscillatory normal load configurations, reproducing variations in contact front position typical of engine environments. Results showed that the displacement of the contact front strongly influences the spatial distribution of damage and crack initiation mechanisms. These effects introduce load history influences that tend to extend lifespan under variable normal loading compared with constant load tests. Additional analyses also revealed the impact of load history and temperature on the friction coefficient.
Finally, a nonlocal fretting fatigue model, previously developed through numerical work, was experimentally validated by using DIC-derived displacement fields as boundary conditions in a 2D digital twin of the fretting fatigue test. This approach accounted for complex boundary conditions, including rotational movement between contacting parts and edge effects, thereby improving prediction accuracy. The three nonlocal lifetime prediction criteria tested—tribological, energy-based, and one inspired by Archard’s wear model—all yielded results consistent with experiments, with the tribological criterion showing slightly better accuracy.

 

Composition du jury :

• M. Siegfried Fouvry , Directeur de Recherche CNRS, Mines ParisTech - Rapporteur
• M. Franck Morel, Professeur des Universités, ENSAM - Rapporteur
• M. Jose Alexander Araujo , Professeur des Universités, Universidade de Brasília (Brésil) - Examinateur
• Mme. Véronique Aubin , Professeure des Universités, LMPS, CentraleSupélec - Examinatrice
• M. Yves Nadot , Professeur des Universités, ENSMA - Examinateur
• Mme. Nathalie Limodin , Professeure des Universités, Université de Lille - Examinatrice