Fatigue Strength Evaluation of Cracked Components.

Abstract

Fatigue crack propagation behavior was investigated using structural carbon steel specimens which have single edge or small surface pre-cracks. The crack closure behavior of small surface cracks was dynamically measured by using an interferometric displacement gage with a laser diode. The threshold condition for crack initiation from pre-cracks was given by the effective stress intensity range obtained for long cracks. The crack opening stress intensity factor increased with crack growth. The threshold crack opening stress intensity factor was formulated as a function of the ratio of the applied stress to the yield stress and the increment of crack length. The cyclic R-curve was constructed in terms of the experimentally determined threshold value 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. The cyclic R-curve method was used to predict the fatigue thresholds of the pre-cracked components. The predicted values of the fatigue limit for crack initiation, the fatigue limit for fracture, and the length of non-propagating cracks agreed well with the experimental results