Formulation of a new intergranular creep damage model for austenitic stainless steels

The aim of this study is to propose a physically based intergranular creep damage model for numerical simulations on extrapolated situations. A continuum damage formulation is proposed to evaluate nucleation, growth and coalescence of intergranular creep cavities. Nucleation is based on an empirical law where void fraction growth rate is proportional to the creep strain rate. Void growth rate includes the contribution of: viscoplastic strain rate of surrounding grains (Gurson), and vacancy diffusion along grain boundaries (Hull and Rimmer). Void coalescence is based on a mechanical fracture criterion, where the competition between damage softening and viscoplastic hardening is considered. The identification procedure needs only the results of uniaxial creep tensile tests with a range of time to rupture that enables a sufficient diffusion contribution. The constraint effect is taken into account in the formulation of the model and does not need a specific identification. To illustrate the capacity of the proposed model, applications are presented for an austenitic stainless steel tested at 600 °C. It appears that the constraint effect assessment is in good agreement with experimental results, when we compare time to rupture and intergranular damage localisation on notched specimens, or crack initiation time and crack growth rate on fatigue pre-cracked specimens.