Novel Creep–Fatigue Interaction Model Based on Kitagawa–Takahashi and a Probabilistic Creep Pore Model

Abstract

The Kitagawa–Takahashi (KT) diagram and the El Haddad equation are widely used to predict the allowable stress range $\Delta \sigma$ for an internal defect size $a$. This approach discriminates between regions designating nonpropagation and propagation of short and long cracks. However, the KT diagram is incapable of describing the damage under creep conditions, as in that case, the assumption of a time-independent threshold for fatigue crack propagation is invalid and must be considered as time dependent. The proposed Kitagawa–Takahashi with creep (KTC) method combines pore size distributions predicted by a probabilistic creep pore model with the El Haddad equation. This new approach is suitable to characterize the interaction of creep–fatigue loading. Within this work, modified Wöhler and Haigh diagrams for creep–fatigue at various temperatures are presented and validated with creep–fatigue experiments as well as high-cycle fatigue (HCF) tests on precrept specimens made from the polycrystalline nickel-base superalloy 247.