Abstract
The smooth specimens of 3% silicon iron polycrystals were fatigued under completely reversed bending. The early propagation of a small fatigue crack was studied based on the etch-pit observation of slip deformation and compared with the propagation behavior of a long fatigue crack. Most of small cracks began to decrease the crack propagation rate, departing from the relation between the rate and the effective stress intensity range for long cracks, either when the growth path changed from the grain boundary or slip band to the transitional or non-crystallographic path. The crack growth rate was found to be uniquely correlated to the highly strained region near the crack revealed by the etch pit method. It is concluded that the crack closure is mainly responsible for the anomalous behavior in the relation between the crack propagation rate and the stress intensity range for small cracks.
