Abstract
Abstract—Fatigue crack growth behaviour under intermittent overstressing was investigated in dry air with a low carbon steel under tension‐compression and tension‐tension loading with different mean stress levels. A very small number of cycles of overstress applied intermittently during a very large number of cycles of understress below threshold caused, in all cases, a significant acceleration in crack growth rate as compared to the case of steady cyclic stress. The acceleration for the case with tensile mean stresses was a little smaller than in the case without mean stress and it was related to a little higher crack closure level in the former case. Stress history in the precracking process had some effect on crack closure and, consequently, on the acceleration. A fairly good correlation was obtained between the acceleration and the effective stress intensity range. The acceleration under multi‐level intermittent overstresses with different mean stresses could be predicted by linear summation from two‐level intermittent overstress test data. No difference was recognized in the fracture surface morphology between the cases with different mean stress level including the cases of multi‐level stresses, indicating that the micromechanism of crack growth acceleration would be the same in all cases.
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