Abstract
Propagation of microscopic fatigue cracks by periodic overstressing was studied with a low carbon steel. Significant acceleration of crack propagation (more than one hundred times) occurred also in microscopic cracks similarly as in the case of macroscopic cracks previously reported when the understress values were equal to the threshold stresses. There was an appreciable effect of microstructure in microscopic cracks. When the understress values were below the threshold stresses, significant acceleration did not occur in microcracks smaller than 200μm whereas it did in cracks larger than this size. These results indicate that microcracks propagate faster under periodic overstressing than under steady alternating stresses once they started to propagate, and this would be a point that should be remembered in practice.
