Atomic force microscopy of fatigue crack growth behavior in the low K region

Fatigue crack growth tests were performed under constant amplitude loading in the low K region using a grain-oriented 3% silicon iron, which had two operative preferential slip systems against the direction perpendicular to the loading axis. Growth behavior of fatigue crack and slip deformation near the crack tip were observed in detail by means of an atomic force microscope (AFM). It was found that the inclined fatigue crack grew along one preferential slip plane, since one of the two slip systems operated predominantly in the low K region where crack driving force was relatively low. The crack growth along one slip plane was then followed by crack deflection or kinking. The microscopic length of crack extension was measured by successive AFM observations as well as the slip deformation at the crack tip by an image processing technique. It was found that the crack growth decelerated and the amount of slip deformation in front of the crack tip became small just before crack deflection took place. It could be concluded that the piled up dislocations were cyclically introduced in front of the crack tip as a consequence of crack growth along one slip plane, which brought about both the reduction of slip deformation and the deceleration of growth rate. Then the other slip system, the direction of which is different from that of the constrained slip, would be activated and resulted in crack deflection or kinking. Fatigue crack growth model in the low K region was successfully proposed in view of AFM microscopy.