Low-cycle fatigue tests were carried out on copper alloy and SUS 304 steel, to observe the initiation of fatigue cracks in smooth specimens and the subsequent growth of the micro-cracks. Their surface crack lengths were examined using cellulose acetate replicas. The results obtained were as follows:
(1) Initiation of a main crack which causes the failure of smooth speimens occurred at the early stage of their lives. The crack initiation cycle ratio Nc/Nf was small for SUS 304 steel in the low-cycle regime, but in the high-cycle regime for copper.
(2) The crack initiation site was almost near the grain boundary triple point and the subsequent growth rate was high at the early stage. The initiation at the slip bands was rarely observed and the crack length did not change for a while.
(3) The experimental crack growth rate was not well correlated in fracture mechanics fashion by ΔK or ΔJ. The crack growth rate at the stable stage was propotional to the crack length, so the nondimensional fatigue crack growth rate k=1/ae·da/dN represented the data well in this study.
(4) The nondimensional fatigue crack growth rate k was closely connected with the failure lives Nf of smooth specimens, in such a way as k=(9-10)/Nf.
(5) An expression for the damage in low-cycle fatigue was derived on the basis of micro-crack growth behavior.