The Growth of Naturally-Generated Small Fatigue Cracks in a Nickel-Base Single-Crystal Superalloy

An experimental and analytical study on the formation and growth small fatigue cracks embedded in a notch in single-crystal superalloy has been investigated. The experimental program consisted of 12 constant amplitude fatigue tests performed on single-edge notch (SEN) fatigue specimens oriented with the loading axis along [010] and with a notch factor of 2.7. The fatigue tests concentrated on one temperature (650°C) and loading condition with the secondary crystallographic orientation---the notch orientation---being the primary variable. Two secondary crystallographic orientations were considered in the present study, [101] and [100]. In the analytical study, the distribution of stresses and strains in the notch region and the stress-intensity factors and the elastic-plastic J-integral for Mode-I semi-elliptical surface cracks embedded at the notch root were investigated using the finite element method (FEM). The anisotropic material properties were shown to have a significant effect on both the stress and strain distribution in the notch region and the crack-tip parameters. The results of the experimental study have shown that fatigue cracks formation occurs via expansion of elliptical subsurface interdendritic pores located at high stress regions in the notch. Once the subsurface crack intersected the notch surface, subsequent crack growth occurred as semi-elliptical surface cracks. The secondary crystallographic orientation had a marked effect on crack-initiation life (the number of cycles to form a crack with a surface length of 760 mum) but no effect on small crack propagation behaviour. Crack initiation life predictions were made using a holistic lifing approach that considers the size, distribution and local stresses acting at the subsurface pores and utilizes the small fatigue crack growth data obtained from the experimental study. The predictions were found to agree reasonably well with the experimental test results and to account for the crack initiation life variability observed in the fatigue experiments.