Crack closure during fatigue crack propagation of IN718 superalloy under high positive stress ratios

Crack closure commonly occurs during fatigue crack propagation of IN718 superalloy. The fatigue crack growth rate of IN718 superalloy was evaluated under several positive stress ratios at room temperature and the plastic deformation characteristics of the wake zone were analyzed. Nanocrystalline thin layers were observed in the wake region of all specimens, indicating that the plasticity-induced crack closure effect persists even at higher positive stress ratios. The thickness of these layers decreased with increasing stress ratio, indicating a positive correlation between the thickness of the nanocrystalline layer and the plasticity-induced crack closure effect. Subsequently, a thinner nanocrystalline layer was associated with a higher fatigue crack growth rate and a lower threshold value. Furthermore, the formation mechanism of the nanocrystalline thin layers was elucidated.