Viscoplastic constitutive equations of combustion chamber materials including cyclic hardening and dynamic strain aging

Cobalt-base and nickel-base superalloys for aircraft engine combustion chamber applications (e.g. Haynes 188, Haynes 230, Hastelloy X) show a complex viscoplastic behavior. In a large temperature range from 300 to 800 °C, an important cyclic hardening with memory effects combined with a negative strain rate sensitivity are observed. This behavior can be related to the dynamic strain aging phenomenon. The present contribution aims at proposing a set of constitutive equations that are able to predict those experimental observations. A classical unified viscoplastic framework is coupled with a physically motivated macroscopic modeling of dynamic strain aging initially introduced by McCormick and Kubin, Estrin. The complete model has been identified on original multilevel and multirate cyclic experiments and cyclic relaxation experiments on the Haynes 188 alloy. The predictive abilities of the model are demonstrated by simulations of a large set of experimental data including fatigue tests from the literature.