Fracture mechanics analysis of the fatigue strength curves of metallic alloys containing defects

The growing emphasis on reliability and safety in mechanical components used in industrial applications has intensified the need for predictive approaches in assessing high cycle fatigue behavior. Recent progress in fracture mechanics has significantly enhanced the ability to estimate fatigue life and endurance limits, particularly in components containing small cracks or manufacturing-induced defects. In manufacturing methods such as additive manufacturing, these inherent defects can often eliminate the crack initiation stage, accelerating the fatigue process. As a result, fatigue damage evolution is predominantly governed by the propagation of cracks from critical defects until the failure.

This study contributes to a deeper comprehension of the essential structure of conventional Δσ-N curves for material or components with small defects larger than the average microstructural dimension, highlighting their shortcomings in capturing certain fatigue phenomena. Existing alternatives proposed in the literature, such as Δσ/Δσ_th vs. N curves and ΔK vs. N/a curves, are critically evaluated, and a novel approach grounded in fracture mechanics principles is introduced, proposing ΔK/ΔK_th vs. N curves. This approach provides a comprehensive framework for quantifying crack growth behavior throughout the entire fatigue life, while also addressing and clarifying the limitations of earlier models.

The proposal also allows for more detailed and reliable analyses to be carried out in the prediction of the minimum strengths associated with complex configurations of defects, loading, and material, which can be complemented by the corresponding statistical analyses related to defect distribution and size effect.