The transition between small and long fatigue crack behavior and its relation to microstructure

The transition point from small to long crack behavior is experimentally studied in a single phase aluminum alloy. It is shown that scatter decreases until reaching a steady state value for long crack growth. This point is defined as the transition from micro-structurally small to long crack growth and is shown to correspond to the point when the growing crack front intersects approximately 15 grains. This transition point is experimentally validated from fatigue crack growth data both from single, corner micro-cracks and multi-site micro-cracks on a smooth surface.