A numerical simulation of creep?fatigue crack growth in nickel-base superalloys

Abstract

The proposed one-dimensional simulation of creep–fatigue crack growth in a nickel-base superalloy, at 750 °C under high vacuum, is based on different cohesive models and on experimental data pertaining to crack growth rate evolution with hold time. Particular focus is placed on the small ΔK range where crack propagation occurs during the reloading phase only, although creep effects play a very important role. In a first step, a cyclic loading without hold time is considered: a scalar damage field related to the time independent plastic stretching (crack opening displacement) is introduced in a finite region adjacent to the crack tip, along the crack path. A Paris-type ΔK–da/dN crack growth curve is predicted. The crack closure effect is also taken into account. Then a second simulation meant to describe a cyclic loading with hold time is proposed. Besides the elements already present in the cyclic approach, this second simulation incorporates a new creep-damage field, defined in another interval on the crack path. This new damage field develops during the hold time, its rate being related to the creep crack opening displacement field. Crack blunting during hold time is also taken into account. Crack propagation is governed by a fracture criterion involving both damage fields. Several hold time durations (up to 1000 s) are investigated. The creep–fatigue interaction is described by the association of the cyclic and creep-damage fields. The predicted cracking curves correctly reflect the influence of hold time duration in the ΔK range where crack propagation occurs during the reloading phase only.