Investigating and modeling of the multi-axial fatigue and complex deformations of a nibased superalloy for improving disk lifing

The research work in the paper aims to improve turbine disk lifing methodology by enabling viscoplastic constitutive models and utilizing multi-axial fatigue theory. A series of uniaxial and multi-axial (cruciform specimen) LCF tests, therefore, have been conducted under different cyclic loadings at high temperature to investigate the complex deformation and fatigue behaviors for Udimet 720 Li superalloy used in turbine disk. All its deformation behaviors, such as rate dependence, cyclic hardening, fast isotropic softening, mean stress relaxation and creep, are identified and used for viscoplastic modeling. The Bodner-Partom type viscoplastic constitutive model is adopted for that purpose. And it is modified to improve the modeling of the deformation behaviors, especially the behaviors under asymmetric cyclic loadings and dwell time. The constitutive models are implemented in general purpose FE software ABAQUS through UMAT interface combined with a self-adaptive time stepping strategy. The results show that improvements are very satisfactory. By introducing a factor to consider creep damage effect on LCF, combined with the critical plane theory, a new multi-axial fatigue prediction method is proposed and verified based on the multi-axial fatigue results for different loadings conditions. Meanwhile, four common-used multi-axial fatigue theories are used for the comparisons; the predicted results show that the scatter band is in ±2 times, which are much better than the results by other four theories.