Numerical Simulation of Temperature-Dependent, Anisotropic Tertiary Creep Damage

Directionally-solidified (DS) Ni-base superalloys are commonly applied as turbine materials to primarily withstand creep conditions manifested in either marine-, airor landbased gas turbines components. The thrust for increased efficiency of these systems, however, translates into the need for these materials to exhibit considerable strength and temperature resistance. Accurate prediction of crack initiation behavior of these hot gas path components is an on-going challenge for turbine designers. Aside from the spectrum of mechanical loading, blades and vanes are subjected to high temperature cycling and thermal gradients. Imposing repeated start-up and shut-down steps leads to creep and fatigue damage. The presence of stress concentrations due to cooling holes, edges, and sites sustain foreign object damage must also be taken into account. These issues and the interaction thereof can be mitigated with the application of high fidelity constitutive models implemented to predict material response under given thermomechanical loading history. In the current study, the classical Kachanov-Rabotnov model for tertiary creep damage is implemented in a general-purpose finite element analysis (FEA) software. The evolution of damage is considered as a vector-valued quantity to account for orientation-dependent damage accumulation. Creep deformation and rupture experiments on samples from a representative DS Ni-base superalloys tested at temperatures between 649 and 982°C and three orientations (longitudinally-, transversely-oriented, and intermediately oriented). The damage model coefficients corresponding to secondary and tertiary creep constants are characterized for temperature and orientation dependence. This advanced formulation can be implemented for modeling full-scale parts containing temperature gradients.