Evaluation of gigacycle fatigue limit and life of high-strength steel with interior inclusion-induced failure

The gigacycle fatigue properties with the interior inclusion-induced failure for three kinds of high-strength steels are investigated in this study. Fatigue strength of these steels in the gigacycle regime is highly related to the sizes of inclusions that are present in the effective damage zone under loading condition. The induced stress concentration at the inclusion-matrix interface plays a key role in the small crack growth process within fine granular area, but has little effect on the macroscopic crack growth outside the fine granular area. Considering the effect of stress gradient around the inclusion, new models were developed to evaluate the values of stress intensity factor at the front of the fine granular area and the fish-eye. A nearly constant stress intensity factor value of 4.5MPam for the fine granular area can be regarded as the threshold value controlling interior macroscopic crack growth. A method from the viewpoint of small crack growth was proposed to evaluate the fatigue limit/life of high-strength steel with the interior inclusion-induced failure in the gigacycle regime, which reveals the influences of loading condition, inclusion size, and specimen size. In life prediction, this method is mainly based on the relationship between the fine granular area size and the fatigue life. Because of the maximum inclusion sizes used, the partially predicted results may be somewhat conservative, but they are more satisfied with the requirement of safety design.