Short fatigue cracks are known to have a growth behaviour different from that of long cracks, the latter well predicted by linear elastic fracture mechanics. Short cracks can grow at high rates at load levels well below the threshold value for long cracks, before entering into the long crack region, or arrest and become nonpropagating cracks. The growth behaviour of short cracks is strongly influenced by the microstructure of the material, such as grain boundaries and direction of slip planes within the grains, as well as of local plasticity around the crack tip. Microstructurally short cracks, typically shorter than a few grains, grow in a single shear mechanism along specific slip planes within the grains, cf. Suresh [
], leading to a zigzag crack path, cf. Fig.1.