Liza Lecarme's PhD Thesis

A collision between a bird and an aircraft engine occurs once in a while during flight, and can cause the immediate fracture of a blade from the outward fan and instantly make the engine out of service. After this fan blade out (FBO) phenomenon, several engine components made out of titanium alloy Ti-6Al-4V can undergo a violent crash. Their resistance in this situation is simulated by numerical tools using very restrictive hypothesis. More realistic material constitutive models would lead to a better engine design, thus to mass reduction.
The elasto-viscoplastic behavior of the alloy Ti-6Al-4V material produced under conditions near representative of a blade has been analyzed, based on the results of mechanical tests. A focus was made on the influence of test temperature, strain rate, material direction, and stress state. A rate dependent strain hardening law, based on the Kocks-Mecking formalism, has been identified. The analysis of the fracture strains and fracture surfaces have shown the influence of the experimental conditions on the damage process and highlighted the impact of the Ti-6Al-4V two phase nature. X-ray microtomography observations during tensile loading provided a quantitative analysis of the nucleation and growth of the voids, with the help of a cavity tracking algorithm. Finally, a viscoplastic Gurson-based damage model has been developed, based on finite element void cell calculations and experimental results, and the predictions have been assessed.
The results obtained through this thesis are however not solely limited to the FBO problem. This work contributes to a better understanding of the ductile damage process in two phase materials in general and in α/β titanium alloys in particular.

Jury:
Prof. Thomas Pardoen (UCL), Promoteur
Prof. Issam Doghri (UCL), Président
Prof. Pascal Jacques (UCL), Secrétaire
Prof. Anne-Marie Habraken (ULg)
Dr. Eric Maire (INSA de Lyon, France)
Dr. Benoît Tanguy (Commissariat à l’Energie Atomique, France)