The Behavior of Short and Long Fatigue Cracks at Threshold and Near-Threshold Levels

The fatigue crack growth behavior of physically short cracks (0.2 ≤ a ≤ 1 mm), intermediate cracks (1 < a ≤ 2 mm), and long cracks (2 < a < 25 mm) at near-threshold and threshold levels was investigated with single-edge-notched-type specimens using four materials with stress ratios R of 0.05, −1, and 0.5. The materials were SAE 0030 cast steel, AISI 1020 and 1045 hot-rolled steels, and 7075-T6 aluminum. Short cracks grew much faster than long cracks based upon applied ΔK. The transition crack lengths at which there was similitude with ΔK were between 1 and 2 mm. Mean stress effects were significant at these low da/dN values for both long and short cracks. Crack closure played an important role in this behavior, but appears not to be the entire cause. Scanning electron microscopic fractographic analysis did not indicate a difference in fatigue crack growth morphology between the long and short cracks.