Unified viscoplastic models for low cycle fatique behavior of Waspaloy

The deformation behavior of Waspaloy during strain-controlled fatigue tests and creep tests is investigated based on the Chaboche viscoplastic model. Material parameters were determined using the least square fit to uniaxial strain-controlled low-cycle fatique and creep test results. A standard viscoplastic model using nonlinear kinematic and isotropic hardening rules gave a good description for the cyclic hardening or softening observed in symmetric low-cycle fatigue tests, but the application of the deformation model to asymmetric strain conditions resulted in a large overestimation of the mean-stress relaxation. Two modified models are presented as solutions for the overestimation of stress relaxation. A combined nonlinear and linear kinematic hardening rule (NLK+LK model) and a nonlinear kinematic hardening rule with a threshold (2-NLK-TH model) are proposed and their applications are discussed.