PROPAGATION OF MICROSCOPIC FATIGUE CRACK UNDER PERIODIC OVERSTRESSING (OVERSTRESS AND UNDERSTRESS VALUES AND ACCELERATION OF CRACK PROPAGATION).

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 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. 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.