Abstract
It was found that the surface damage of annealed mild steel subjected to the cyclic straining in the plastic range is almost slip band cracks. A model is proposed to explain the fact that the slip band cracks are formed independently of the magnitude of stress if there are some large and strong obstacles in the microstructure, but only in the range of low stress if there are none such. With repeated cyclings, the screw parts of piled-up dislocations loops on the two neighboring parallel slip planes cross slip each other and annihilae themselves. As the result, an intrusion is formed, and the pile-up is made of edge dislocation loops enough to initiate a crack. Next, the intrusion is connected with the initiated crack, and hence a slip band crack is formed. Under high stress fatigue, no slip band crack is formed, if there is no large and strong obstacle, because the concentrated tensile stress is reduced by the pile-up of edge dislocations of opposite sign and the initiation of crack is thereby arrested.
