The fatigue limit and non-propagating cracks for smooth and pre-cracked specimens in a carbon steel.

Abstract

The characteristics of fatigue limit and non-propagating cracks for smooth and pre-cracked specimens were examined by varying the stress ratio (R=-1, R=0) and the pre-crack length in a carbon steel, STS 42. The maximum stress at fatigue limit for the specimen with no cracks or with a short pre-crack is lower at R=-1 than that at R=0. As the pre-crack length of the specimen becomes longer, the difference between these two decreases. Judging from the dependence of R and the pre-crack length on the length of the non-propagating crack and from the observation of the crack closure behavior of the non-propagating crack, fretting oxide induced crack closure triggered by the roughness induced crack closure has an important role in determing the length. The fatigue limit estimated from the threshold stress intensity for a long crack is considerably higher compared with those for a smooth specimen and a specimen with a short pre-crack. The growth characteristics for a non-propagating crack are quite different between points near the specimen's surface and those of deepest penetration.