Propagation of microscopic fatigue cracks by periodic overstressing was studied under different overstress and understress conditions using a low carbon steel. When the overstress value was large, a significant acceleration of crack propagation (more than one hundred times) occurred even if the understress value was lower than the threshold stress, similarly as in the case of macroscopic cracks. The lower limit of the understress that caused such a significant acceleration was reduced as the overstress was increased, but it was independent of the crack length as far as the crack was microscopic (less than 50μm). For longer cracks, the lower limit was considerably reduced by increasing the crack length. When the overstress value was low, a significant acceleration in short cracks occurred only when the understress value was nearly equal to the threshold stress, but in longer cracks it took place even the understress value was lower than the threshold stress. The microscopic fracture surface consisted of small facets, which was supposed to be related to crystal structures. Observation of the specimen surface showed that cracks propagated intermittently, hesitating at the boundary of microstructure. These observations indicate that microstructure has a significant effect upon microscopic crack propagation under intermittent overstressing.
