Applicability of Neuber’s Rule for Thermomechanical Fatigue

Gas turbine components, such as blades and vanes, are routinely subjected to non-isothermal fatigue conditions. Accurate service life predictions can be made from analyzing transient stress-temperature histories and constitutive modeling. The local stress and strain histories at geometric discontinuities are typically calculated with stress shakedown approaches (i.e., Neuber’s Rule, Molski-Glinka Approach, Calladine Method) based on elastic responses rather than coupled elastic-plastic deformation observed from low cycle fatigue (LCF). For the notched material subjected to thermomechanical fatigue (TMF), there is no widely-accepted method for correlating remotely applied load with notch root behavior. In this study, a notched specimen of the Ni-base superalloy IN939 is modeled by means of Finite Element Analysis (FEA) via the ANSYS general purpose software. Calculations made from the Neuber Rule are compared with numerical simulations of the notch root response. Limitations of this classic stress shakedown approach are identified. Although the candidate material of this study is a generic polycrystalline, dual-phase Ni-base superalloy, the presented techniques are likely to be straightforwardly transferable to other materials. When combined with S-N data from experiments on tensile specimens, the method can be used to correlate notch tip response under TMF with fatigue life.