Fatigue Crack Initiation and Early Propagation in 3% Silicon Iron

The etching susceptibility of dislocations in 3% silicon iron is used to study the initiation and early propagation of small fatigue cracks in smooth specimens subjected to completely reversed cyclic bending. The condition for crack initiation along grain boundaries, which is the dominant mode of crack initiation, is derived from the crystallographic relationship of adjacent grains relative to the stress axis and grain boundary. The propagation behavior of small cracks was compared with that of long through-cracks. Most of the small cracks begin to decelerate, departing from the relation between the rate and the effective stress intensity range for long cracks when the growth path changes from the grain boundary or slip band to the noncrystallographic or transitional path. Some cracks become nonpropagating, while others accelerate again in accordance with the relation between the rate and the stress intensity range for long cracks. Since the crack growth rate is expressed as a unique power function of the size of the highly strained region near the crack, except for slip band cracks, it is concluded that the crack closure is primarily responsible for the anomalous growth behavior of small cracks. Some discussion is given on the influence of the grain size on the fatigue process of the smooth specimen.