Fatigue life prediction of additively manufactured ductile nickel-based superalloys : Constitutive and crack initiation modelling

Sammanfattning: This dissertation was produced at the Division of Solid Mechanics at Linköping University, and is the final result of a project that included mechanical testing and modelling of an additively manufactured ductile nickel-based superalloy.The main objective of the work presented in this thesis was to investigate and model the cyclic behaviour and the fatigue life behaviour of an additively manufactured ductile nickel-based superalloy, with emphasis on modelling the stabilised material behaviour, by which fatigue life predictions can be based upon. The mechanical and fatigue behaviour of additively manufactured alloys have shown to often depend on how the components are manufactured in the 3D-printing machine, meaning that the material is anisotropic. This anisotropic effect is important to account for when predicting the life of components. Therefore, in this work models to predict the mechanical response and the fatigue life of such components have been established. Monotonic tensile tests, creep tests and cyclic fatigue tests at constant temperatures, as well as anisothermal cyclic tests, have been performed to investigate the mechanical and the fatigue behaviour of the material, where specimens built in different orientations have been used to also study the anisotropic behaviour of the material. With the tests as a basis, a constitutive model has progressively been developed and implemented in a finite element context that accounts for the anisotropic behaviour under both elastic and inelastic deformations. In addition, a fatigue crack initiation life model has been developed for the tested low-cycle fatigue and thermomechanical fatigue conditions, which account for both material anisotropy and temperature effects.