A novel TCD framework for fatigue lifetime prediction incorporating defect and stress concentration effects

Defects in powder metallurgy (PM) superalloy tend to become a source of cracks under cyclic loading, reducing the fatigue lifetime of crack initiation and affecting the safety of aero-engines. The mechanism of crack initiation is affected by both the defect type and the gradient stress distribution, resulting in the large difference of fatigue lifetime. To study the relationship between the defect type at the crack source and the fatigue lifetime under the influence of stress concentrations, a series of fatigue experiments of notched round bar specimens were carried out. Then a modified theory of critical distance (TCD) on the basis of concern range of defect was proposed, which accommodates both surface defects and internal defects cracking mechanism. Compared to the experimental results, prediction results of fatigue lifetime fell within a scatter band of ±4. Finally, a concept of inclusion sensitive area (ISA) was proposed to quantify the area of stress concentration that was susceptible to cracking due to inclusion. It was validated by the fracture surface observation, where all inclusions on crack sources in the experiment were within the predicted region. This work will support lifetime prediction and defect detection strategies for aero-engine components.