Abstract
The propagation and arrest behavior of fatigue cracks under various stress ratios was investigated using structural carbon steel specimens having single edge or small surface pre-cracks with lengths ranging from 0.16 to 4.33 mm. The threshold condition of crack extension from pre-cracks was given by a constant value of the effective stress intensity range irrespective of pre-crack length and stress ratio, and the value was equal to the threshold value obtained for long cracks. Cracks extended from pre-cracks were arrested under low stress amplitudes because of the development of crack closure with crack growth. A cyclic resistance curve was constructed in terms of the maximum stress intensity factor which was the sum of the range of the threshold effective stress intensity and the crack opening stress intensity factor, and was used for predicting of fatigue thresholds. The predicted values of the fatigue limit for crack extension, the fatigue limit for fracture, and the length of non-propagating cracks agreed very well with the experimental results. The cyclic resistance-curve method was further applied to predict the effect of the mean stress on the endurance limit of smooth specimens. Haigh diagrams for the endurance limit were successfully derived from the arrest condition of nucleated small cracks in smooth specimens.
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