The crack closure model proposed by Budiansky and Hutchinson for long fatigue cracks was extend to analyze the closure of small fatigue cracks formed in a smooth specimen or near the notch tip. The following results were obtained.
(1) The effective fraction U for small cracks initiated in a smooth specimen increases as the crack length becomes smaller, and approaches to unity as the crack length approaches zero. The variation of U with crack length calculated through the present model was found to agree with the experimental data.
(2) The influence of crack length on the fatigue growth threshold of small cracks results from the following two causes: the effective stress intensity range and the effective fraction. The dependence of the effective fraction on crack length is more predominant in determining the growth threshold of small cracks.
(3) The effective fraction for cracks emanating from the notch tip also depends on crack length. For the cracks larger than the reverse plastic zone size at the crack tip, the value of U is the same as that for small cracks having the same length but without notch.
