Creep-fatigue behavior of a new generation Ni-base superalloy for aeroengine usage

Aeroengine components are often affected by high temperature creep-fatigue mechanisms which result in cyclic crack propagation. Some of these components, like turbine and compressor disks, have some geometrical features that act as stress concentration regions with a strong impact in the crack front geometry. Also, recent investigations show that shot peening may have an important influence for crack arresting or even in the delaying of short crack propagation. This investigation presents some experimental results obtained for two types of specimen geometries of a new generation PM Nibase superalloy. A set of fatigue crack growth curves were obtained from high temperature fatigue testing considering different loading frequencies and two surface conditions of the material: as-machined and shot-peened. The results indicate that the compressive residual stress field due to shot peening contributes to the crack initiation stage, which however diminishes as the initial damage increases beyond a critical dimension. In the case of long crack propagation, it was found that the crack front assumed different geometries during the propagation stage. This fact combined with the high stress gradient region inherent to the specimen's stress raisers led to the development of a specific stress intensity solution for this non-standard transitory crack front geometry, which was carried out using a finite element analysis. The results from the computational analysis provide a proper stress intensity factor solution that can be used in some experimental particular cases where an alteration of the crack front geometry is expected.