Atomic force microscopy and the mechanism of fatigue crack growth

Fatigue crack test was performed using a grain-orientated 3% silicon iron under constant amplitude loading. Growth behaviour of the fatigue crack and slip deformation behaviour near the crack tip were observed in detail by using an atomic force microscope. In the lower K region, only one preferential slip system of this material operated and the fatigue crack grew along that slip plane. It was found that constraint of slip deformation due to cyclic strain hardening resulted in crack arrest and crack branching. The fatigue crack grew in a zigzag manner as a result of such successive crack branching and deflection. In the high K region, two preferential slip systems operated simultaneously to an almost identical extent and the fatigue crack grew in a direction perpendicular to the loading axis. The slipping distance in one loading cycle was measured quantitatively by using the image processing technique and the crack growth mechanism is discussed.