Fatigue behaviour of a nickel based superalloy

Abstract: This thesis deals with different aspects of fatigue crack propagation in IN718. The ultimate objective of the work is to gain understanding of the fatigue process, from the macroscopic stress-strain relations to microscopic fatigue crack growth mechanisms. First, the stress-strain relations, which are fundamental in order to understand what the material encounter, are dealt with. The high temperature material model by Bodner and Partom is expressed in an implicit form, suitable for FE-implementation. Furthermore, different methods to extract material parameters for the model are employed. A direct method with strain as the independent variable is chosen. The resulting stress from an applied strain is compared to stress calculated by linear kinematic hardening. The concept of using the effective stress intensity factor range for crack growth expressions is employed for two temperatures. It is found that at 687°C crack growth is dependent upon time whereas at 550°C a cyclic crack propagation expression as a function of the effective stress intensity factor range is successful. An in-situ SEM investigation of fatigue crack mechanisms reveals that crack closure is very dependent upon grain orientation. The local crack growth rate also varies significantly, although no obvious correlation between crack opening stress and crack growth rate is found. The investigation also shows that a crystallographic fracture mode is active well up in the Paris regime, at higher stress intensity factors than anticipated. For some grain orientations, shear is the active mechanism for crack extension, even in the Paris regime. When this is the case, the crack needs not to be visibly open to grow. This is a possible explanation for the recently discovered Donald effect. As different crack growth mechanisms yields different crack growth rates, a need to employ the knowledge from the materials engineering field in close corporation with traditional solid mechanics is thus needed to further improve crack growth rate predictions. Metal fatigue is thus an interdisciplinary field.