Sharply notched specimens of structural low-carbon steel were fatigued under several ratios of maximum to minimum stresses. The growth behavior of a fatigue crack near the notch root was analysed based on the crack closure measurement. A fatigue crack first decelerated with increasing crack length, and then accelerated or became non-propagating depending on the range and ratio of the applied stress. The effective range of the stress intensity factor was found to be the singlevalued parameter in describing the growth rate of small cracks near the notch root, and their relation agreed well with that obtained for long cracks. By considering the increase in crack closure stress with crack length, a quantitative method was proposed for predicting the non-propagating crack length and the fatigue strength reduction of notched specimens as a function of the applied stress and the notch geometry.
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.
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