Modeling of multiaxial creep behavior for Incoloy 800 tubes under internal pressure

In order to validate the long-term behavior of fast breeder reactor steam generator tubes, a creep test monitoring program has been undertaken. A series of over one hundred tests was performed on small specimen tubes which were cut from the as-received tube material of Incoloy 800, similar to those used in generator applications. In contrast to uniaxial conditions in which constant load conditions usually apply; multiaxial experiments were performed assuming constant stress conditions. Thin-walled tube approximation was used to estimate the stresses across the tube wall thickness required for given internal tube pressures. This paper deals with a refined numerical modeling of stresses and strains in the pressurized test specimens, taking into account: internal pressure changes, nonlinear geometrical effects, and creep deformation during all the test procedure cycles. Comparison of the numerical results with the experimental measurements shows a good agreement which validates the multiaxial generalization of the RCC-MR strain creep laws which were used in the calculations. A multiaxial creep rupture criterion is discussed