Stored energy density research on the fatigue crack initiation at twin boundary and life prediction of Inconel718 superalloy

Abstract

The fatigue crack initiation at twin boundary (TB) and the life prediction of IN718 alloys in the high cycle fatigue (HCF) regime and very high cycle fatigue (VHCF) regime were investigated through crystal plasticity finite element (CPFE) simulations and experiments. The fatigue damage parameter, considering the evolution of geometrically necessary dislocation (GND) density, slip and back stress, known as the back-stress-based stored energy density in this study, Gba, was proposed to precisely characterize the plastic strain distribution and localization, which in turn was engaged in locating fatigue crack initiation site and predicting the fatigue life. The element analysis around TB and the CPFE simulations indicated the coupling of slip activation parallel to the TB and the absence of primarily strengthening phase (γ'') on the both side near the TB was responsible for crack initiation. To predict fatigue life, the varying pattern of the Gba at fracture with fatigue life was studied based on the experimental results in the HCF and VHCF regimes, and the life prediction method was obtained accordingly. Finally, the fatigue life evaluations of the IN718 alloy at three load ratios (R = −0.1, 0.1, and 0.5) were conducted and results showed that the life estimates were basically within ±3 scatted bands.