A high temperature fatigue damage model for single crystal superalloys

This work presents a new formalism for a time-dependent (or incremental) fatigue damage model developed for single-crystal nickel-based superalloys. The proposed coupled damage viscoplastic formulation accounts for the damage associated with microcracks lying on planes parallel to those of the crystallographic slip systems, and for the effect of oxidation on microcrack closure. The latter is introduced in the microcrack damage evolutionary behaviour through an additional oxidation-related damage variable. A simplified approach is relied upon to enable an efficient numerical integration of the single crystal formulation within a cycle, by treating the viscoplasticity and oxidation-related damage in an uncoupled manner, and an extrapolating approach is proposed to deal with periodic fatigue loading conditions. Fatigue tests on the commercial superalloy AM1 are performed at several load amplitudes, frequencies and crystallographic orientations to calibrate the model and to study the effect of fatigue loading on the crack initiation process. Fatigue torsion tests on perforated specimens are then carried out to validate the proposed formulation under local multiaxial mechanical fields. Finally, the accuracy of the proposed model to estimate crack initiation behaviour in the superalloy and its potential for component life prediction are discussed.