Abstract
It is not clearly known how surface defects or inclusions of a medium carbon steel affect a fatigue strength. In this study, we used SM35C specimens with spheroidized cementite structure to eliminate dependence of micro structure of fatigue crack. The investigation was carried out by behavior of crack closure at non-propagation crack and effect of the fatigue limit according to the artificial defects size. Experimental findings are obtained as follows : (1) Fatigue crack initiation point of medium carbon steel with spheroidized cementite structure is at the surface defects. (2) Non-propagating crack length of smooth specimen is equal to the critical size of defect. (3) Considering the opening and closure behavior of fatigue crack, the defect shape results in various crack opening displacement, while it does not affects the fatigue limit level of medium carbon steel with spheroidized cementite structure. (4) The critical length of the non-propagation crack of smooth specimen is the same as critical size of defect in transient area which determines threshold condition in steel with spheroidized cementite structure.