An investigation of crack blunting processes under plane strain conditions

The process of crack blunting was investigated in the present work using direct observations in the scanning electron microscope (SEM). It was found that there are two deformation processes at the crack tip which lead to crack blunting. Shear deformation results in a square crack tip and is characteristic of the plane stress condition at the surface whereas plane strain deformation results in a rounded crack tip. It was noted that the crack blunting process was accompanied by a micro-fracture process including formation of voids and local crack initiation as well as microcrack growth along the blunted crack front. It was also observed that the fatigue crack surfaces close to crack tip were subjected to intense deformation, and became transferred into the surface of stretched zone (SZ), i.e. the portion of the fatigue crack which retains the fatigue morphology on its surfaces moved backward during the crack blunting process. A model is suggested to calculate the amount of the backward movement of the fatigue crack front and the strain level of a blunted crack tip.