Fatigue Strength Evaluation of Cracked Components.

Abstract

Fatigue crack growth behavior has been investigated using structural carbon steel (JIS S 45 C) 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 found to be given by the constant value of the effective stress intensity range which was equal to the threshold value 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 for ratio of the applied stress to the yield stress and the increment of crack length. The R-cruve 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 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 nonpropagating cracks agreed well with the experimental results.