Fatigue thermique et fatigue isotherme d'un superalliage à base de cobalt : étude métallurgique de l'endommagement et modélisation

par Farhad Rézaï-Aria

Thèse de doctorat en Sciences Physiques

Sous la direction de Luc Rémy.

Soutenue en 1986

à Paris 11 , en partenariat avec Université de Paris-Sud. Faculté des sciences d'Orsay (Essonne) (autre partenaire) .

mots clés

• Interaction fatigue-oxydation
• Fissure courte
• Fragilisation par oxygène
• Cobalt -- Alliages
• Alliages -- Fatigue
• Alliages réfractaires

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Résumé

The aim of this study was to model the behaviour of superalloys for vanes in jet engines. The model was based on thermal fatigue (TF) and high temperature low cycle fatigue (HTLCF) experiments carried out on a cast cobalt superalloy MAR-M509. The temperature-stress-strain history of critical elements at the thin edge of wedge shaped TF specimens was computed. The influence of the specimen geometry and the maximum temperature of the thermal cycle were studied. The strain rates (frequency range from 10-3 to 20Hz), the tensile or the compressive hold times and the environmental effects were studied in HTLCF. Fatigue oxidation interaction was the basic mechanism of crack initiation and propagation for both types of loading. Crack initiation was seen to occur early in the life time in TF and HTLCF. The accumulated and the cyclic creep damage and the "strain range partitioning" models were applied to predict the TF and the HTLCF life times to initiation. Ail models were shown to predict HTLCF experimental data within a factor of about three. However, the accumulated and the cyclic creep damage models overestimated thermal fatigue life time in all cases. A model which describes the fatigue oxidation crack growth interaction was applied to TF and HTLCF data. This model gave a good prediction of HTLCF and especially TF life time. A new model of life prediction, which takes into account the fatigue oxidation interaction, is proposed. This model is based on the local stress rupture reduction by oxygen diffusion ahead of the crack and the load ratio (R=PminPmax) is considered. Local stresses ahead of the crack are estimated from Tracey's finite element analysis of the plane strain small scale yielding hypothesis which is adapted to the cyclic loading situation using Rice's hypothesis. A fairly good prediction of TF and HTLCF life time was obtained. This model predicts the crack propagation within a few millimeters and gives good prediction of the slowdown of the crack observed experimentally in thermal fatigue.