Relationship Between the Threshold Stress Intensity Factor Ranges of the Material and the Transition From Short to Long Fatigue Crack

A relationship between the threshold stress intensity factor ranges of the material is established for microstructurally short, physically small and long fatigue cracks depending on the microstructure at a symmetrical loading cycle. The threshold stress intensity factor ranges calculated by the proposed concept for titanium alloy VT3-1 in different structural states agree well with those determined experimentally. The criteria are proposed for the transition from a small to a long fatigue crack depending on the level of the applied load amplitude. In the whole range of the load amplitudes, the condition when a reversible plastic zone at the crack tip reaches the grain size is taken as a criterion for the transition from a physically small to a long fatigue crack. In the high cycle fatigue region, the physically small crack growth range is to be divided into two areas because of a change in the mechanisms of the physically small crack growth upon the attainment by the stress intensity factor range of the threshold value for the long crack.