THE TWO THRESHOLDS OF FATIGUE BEHAVIOUR

TLDR

Fatigue crack propagation is limited by two thresholds: a material‑based one linked to microstructural texture and a mechanically‑based one tied to the stress state at a defect tip, affecting fatigue limits of materials and structures respectively. The study surveys the factors influencing these two thresholds and examines how Microstructural Fracture Mechanics informs the fatigue resistance of metals and components. The material‑based threshold is defined through Microstructural Fracture Mechanics, while the mechanically‑based threshold is characterized using Linear Elastic Fracture Mechanics.

Abstract

Abstract Two limiting thresholds to fatigue crack propagation are discussed. The first threshold is related to the microstructural texture and this threshold may therefore be deemed a material‐based threshold. The second threshold is mechanically‐based, and is related only to the stress state 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 Linear Elastic Fracture Mechanics (LEFM). The former condition is important when considering the fatigue limit of materials and components, while the latter is more applicable to the fatigue limit of structures. The different factors which affect the two threshold conditions are briefly presented. Finally, this paper discusses aspects of MFM relevant to the fatigue resistance of metals and components.