Fatigue threshold diagrams of crack-like defects predicted based on cyclic R-curve method

Under the assumption that the threshold range of stress intensity factor (SIF) Delta Kth depends only on the current crack length, a simplified method is proposed to determine the shape of cyclic R-curve on the basis of published data. A new equation is applied to cyclic R-curve, expressing the increase in Delta Kth with crack extension from crack initiation. Crack initiation is defined as the onset of Stage II crack propagation, and the initiated crack length is determined by the effective threshold range of SIF, Delta Keffth. The Delta Kth value increases with the amount of crack extension, approaching to the threshold for long cracks, Delta Kthlc. Kitagawa-Takahashi (K-T) diagrams derived from the determined R-curve show the existence of nonpropagating cracks below the fatigue limit, Delta sigma w0, of smooth specimens. The new method is applied successfully to the published data of six materials. The fatigue limit of crack initiation in smooth specimens, Delta sigma w01, is determined to be roughly 95 % of the fatigue limit for fracture Delta sigma w0. An approximate method is proposed for predicting the threshold diagrams, using only three properties, Delta sigma w0, Delta Kthlc, and Delta Keffth. In comparison with Chapetti model, the present model shows better agreement with the experimental data both in K-T diagram and the threshold SIF diagram.