Crack growth from internal defects and related size-effect in VHCF

It is well-known in the literature that fatigue cracks in VHCF originate from small internal defects. More than 95% of the total VHCF life is consumed in originating the so-called Fine Granular Area (FGA) around the small initial defect. Within the FGA, crack growth takes place even if the Stress Intensity Factor (SIF) is smaller than the threshold value for crack growth. Researchers proposed different explanations for this unexpected phenomenon but they unanimously accept that a weakening mechanism occurs around the initial defect, which permits crack growth below the SIF threshold. In the present paper, crack growth in the VHCF regime is innovatively modeled and a general expression for the fatigue limit is then obtained. The statistical distribution of the fatigue limit is also defined and a model for the fatigue limit as a function of the risk-volume is proposed. Finally, the proposed model is successfully applied to an experimental dataset.