Effects of environmemt on the acceleration of fatigue crack growth under periodic overstressing.

Abstract

The acceleration of fatigue crack growth under periodic overstressing was studied with centernotched plate specimens of low-carbon steel under push-pull loading. Tests were carried out in moist air, dry air, nitrogen, and vacuum. A very small number of cycles of overstress applied intermittently during a very large number of cyclings of undersress below the theshold stress intensity range ΔKth caused a significant acceleration of crack growth in the range of relatively large understress ΔK1(ΔKth/ΔK1>0.5) in moist air, dry air, and nitrogen. In the region of relatively small understress, however, acceleration of carck growth in moist air was lesss than that in dry air and nitrogen. It was found by means of Auger spectroscopy that the maximum thickness of excess oxide within the cracks enhanced crack closure and reduced the effective stress intensity range of understress, (ΔK1)eff, resulting in less acceleration of crack growth in moist air. Accelaration of crack growth in vacuum, on the other hand, was lesss than that in other environments in all understress levels. Fractographic study suggested that rewelding in vacuum resulted in the less acceleration. There was no sequential effect of overload on the acceleration under two-level variable stress. The acceleration under multi-level variable stresses was predicted by a method using a linear cumulative concept on the basis of test data under simpler two-level variable stresses.