This thesis investigates the roles of crystallography and elastic anisotropy in the fatigue crack initiation of polycrystal nickel alloy René 88DT which contains a large number of annealing twin boundaries. The crystal plasticity finite element method is employed in this work in order to conduct full-field modelling studies. Modelling results show slip localizations and corresponding plastic strain gradients develop near the twin boundaries during cyclic loading, which leads to strongly elevated local stored energy densities reflecting experimental observations of the preferential fatigue crack nucleation sites near the twin boundaries. The nucleation behaviours are demonstrated to be sensitive to the elastic anisotropy and crystallography.
Elastic anisotropy is demonstrated to drive elastic constraint which results in the stress concentration near the twin boundaries, leading to high resolved shear stress on the parallel slip system. The activation of this parallel system generates the sharp slip localizations and nucleates the straight cracks which are aligned with twin boundaries. Crystallography of twin/parent pairs is found to affect the plastic strain fields, the resulting geometrically necessary dislocation density and stored energy density, which reflects the microstructure sensitivity in fatigue crack nucleation. The crystallography ahead of the crack-tip is found to be significant in crack propagation behaviors in which the grain boundaries with increasing twist angle provide increasing resistance to short crack propagation, however, the tilt angle effect is found to be weak unless the adjacent grain crystal orientation is unfavourable for slip. The evolution of the stored energy density captures these phenomena, which demonstrates the stored energy density as a potential driving force in fatigue crack nucleation and short crack propagation.