Significance of initiation, propagation and closure of microcracks in high cycle fatigue of ductile metals

Abstract The initiation, propagation and closure behavior of microcracks were investigated through successive observations of fatigue process on the plain specimen surface in rotating bending tests of a mild steel, an α-brass and Al-alloys. The essential difference of crack initiation process and the behavior of microcracks was recognized between an age-hardened Al-alloy and the other annealed metals. Based on the results, the characteristics in fatigue under various loading conditions, such as 1. (i) fatigue behavior under two-steps loading, 2. (ii) coaxing effect of a low carbon steel and 3. (iii) size effect of a notched specimen and fatigue notch sensitivity, were consistently explained with special reference to the behavior of fatigue microcracks.