THE 2 THRESHOLDS OF FATIGUE BEHAVIOR

Two limiting thresholds to fatigue crack propagation are discussed. Th e first threshold is related to the microstructural texture and this t hreshold may therefore be deemed a material-based threshold. The secon d threshold is mechanically-based, and is related only to the stress s tate at the tip of a substantial defect. The material-based threshold is characterized in terms of Microstructural Fracture Mechanics (MFM) and the mechanically-based threshold is characterized in terms of Line ar Elastic Fracture Mechanics (LEFM). The former condition is importan t when considering the fatigue limit of materials and components, whil e the latter is more applicable to the fatigue limit of structures. Th e different factors which affect the two threshold conditions are brie fly presented. Finally, this paper discusses aspects of MFM relevant t o the fatigue resistance of metals and components.